A better workshop foundation?

[Bill Jobe]

Had a guy all set up to make a compacted gravel base for my new 12x20 Amish shed. Then I had 2 more guys show up to bid on the job. Both told me they thought compacted gravel was a bad idea, even with an 8x8 treated frame made in 3 sections with common sides. Some settling was inevitable they both believed. The first guy was just starting to tell me of a better way using sonotubes and pouring concrete "pads" down 30" and leveled perfectly with on another when a second guy showed up and agreed that he thought that would be far more stable. The first guy said he thought just one in each corner for the skids to sit on would suffice.
I did not care for his suggestion and he soon left as I was discussing with the 2nd guy what he proposed. It was his opinion that 4 along each outer skid and 2 more under separate skids near the center of the building.
I'm convinced now his plan is the way to go and I'd like to get some opinions from members here on that method.
He told me the cost would probably be fairly close to a framed compacted gravel pad, or perhaps a bit less.
Please share your thoughts on this idea.
Also, all the while thinking sound deadening it occurred to me that a hard rubber pad on top of these structures might help isolate sound transmission. Any thoughts on this idea?
The shop will have 4x6 skids notched 1" for each 2x4 joist placed 12" on center and a 3/4" plywood floor.

[Reed Gray]

I did concrete work for 30 years. Gravel will settle. How much depends as much on how it is put in as it does on the type of graveland the base you put it on. Mostly there are a lot of voids in gravel, and the weight of it and what ever goes on top of it will push it down into the dirt and/or sub grade. They do use a fabric type of mesh under some roads now which kind of acts like a big snow shoe. That helps some. You can put a lot of fines (sand and/or dirt) on top and wash that down with a lot of water and it makes it more solid, but even gravel roads get ruts in them from constant use. By the time you do the sono tube route, you may as well go ahead and pour a 4 inch slab. The slab, again, works like a big snow shoe. You may do okay with a smaller slab, but having concrete to sit the shop on is the most solid way to go. Plywood will flex. As far as sound deadening, not much noise from turning wood...
robo hippy

[Bill Jobe]

I did concrete work for 30 years. Gravel will settle. How much depends as much on how it is put in as it does on the type of graveland the base you put it on. Mostly there are a lot of voids in gravel, and the weight of it and what ever goes on top of it will push it down into the dirt and/or sub grade. They do use a fabric type of mesh under some roads now which kind of acts like a big snow shoe. That helps some. You can put a lot of fines (sand and/or dirt) on top and wash that down with a lot of water and it makes it more solid, but even gravel roads get ruts in them from constant use. By the time you do the sono tube route, you may as well go ahead and pour a 4 inch slab. The slab, again, works like a big snow shoe. You may do okay with a smaller slab, but having concrete to sit the shop on is the most solid way to go. Plywood will flex. As far as sound deadening, not much noise from turning wood...
robo hippy

There will be with a 5hp DC.

[Reed Gray]

A 5 hp DC in a 12 by 20 shed??? That size of DC would do fine in a 2000+ sq. ft. shop, maybe even bigger if it is a good system. Lathe on concrete slab. The DC, depending on what type and style (2 stage?) would do okay on some of the 12 X 12 inch pavers, but I would want it anchored to the walls some how. Most of the noise from a DC is from the impellor blades and air flowing through the pipes, not from machine vibration.
robo hippy

[Bill Jobe]

With the shed on a slab wouldn't the noise from just the impellor increase? Wouldn't every noise inside the shed be louder outside with the shop on a slab?

[David L Morse]

With the shed on a slab wouldn't the noise from just the impellor increase? Wouldn't every noise inside the shed be louder outside with the shop on a slab?

The plywood floor will transmit much more sound than concrete. Imagine a piano with a concrete soundboard or an acoustic guitar with a cast concrete body. Coupling sound vibrations to air takes a lot of motion and plywood will flex more easily than concrete.

[Jim Becker]

Pouring a slab may have local zoning implications...check on that before deciding between a slab or the concrete supports. A slab would be "more permanent" than posts. From a cost perspective, the concrete posts "may" be less expensive and you can still put in geo-textile covered with compacted stone to keep things clean under the shed if you want.

[Bill Jobe]

So how many sonotube piers do you think would suffice?
One bidder said 1 in each corner would work what with the 12" on center 2x4 joists. Another recommended 4 on each outer skid and one under each of the 2 inner skids near the center of the 20' span.
I called the Amish builder and he strongly suggested 4 under each outer skid (there are 4 skids) and 3 under the inner most 2 skids.
Just called the guy wanting to use just one sonotube,in each corner. I had called him because after more than a week I was wondering why I had not received his bid. He told me he was waiting on me to make up my mind.
The contractor suggesting 4 on the outer skids,plus 2 more in the middle of the 2 center skids has not gotten back to me either.

Would members here please give me their honest opinions as to making a foundation using sonotubes.
They have not started building my shed yet though I ordered it nearly 5 weeks ago.
I am having a lot of trouble understanding the vast differences between bidders and I'm about to the point of doing the sonotubes myself.
Need advise, please. Bill


The shed will be used to store 2 600lb motorcycles, a Grizzly g0766 lathe, more than likely more woodworking machines in the future such as a good band saw. Also will be using for the rest of my tools, miter saw, heavy bench and vice, small table saw and storage for many other things. May also move my reload bench to the shed which weighs about 250lb with press.
If I go with slab I'll have to deal with additional codes. But I've not ruled that out. I will also be storing sveral hundred pounds of kiln dried wood.

[Reed Gray]

If you go the sono tube route, for a 12 by 20 foot shop, then minimum one in each corner, one in the middle of the 12 foot side, and 2 in between the corners on the 20 foot side. They would need to be pretty much dead level with each other. Easier to pour a strip footing, maybe 12 inches wide, and 6 inches deep. You could get away with a full 4 inches deep. Rebar, one piece minimum around the perimeter. 2 pieces is better. It does spread the load, and keeps it together when it cracks. Only 2 guarantees you can give about concrete is that one it will crack, and two, no one is going to steal it... Look into the cost of a full slab, 2 by 6 outside frame box, slab about 4 inches thick. Rebar 2 ft. on center, not mesh (it always seems to be on top of the slab when you are finishing it, or down in the gravel. With sono tubes, you can get a bunch of bags of fence post mix, and it will work, but it is a lot of work to do. If you get one of the portable mixes on site dispatch trucks, if they can get back to it, best value is a strip footing or a slab. Some even have chutes that will swing back and forth to place the concrete on the ground. Don't try to pour it yourself unless you know concrete or it will work you half way to death, and you are stuck with the final product. From what you are going to put into the shed, it sounds like it is about half the size you will need unless every thing is moveable, and every time you want to go work in the shop you have to move a bunch of stuff out of the way. Out where I am, you can do 200 sq. ft. without having to get a permit. No idea what code is where you are. Some times a car port can be done with slab and no permit. They may require one side to be open and the other 3 can be enclosed. I am limited on my lot to any out building having a maximum size of 10% of the total surface area of the lot. I had to cheat and put in a small 'living area' in the shop. I may rent or use it for office space. Hope this helps some....
robo hippy

[Ric Flanders]

Personally I would go the sonotube route as your contractor suggested. Its the strongest and most cost effective way to go. Having a wood floor is the most comfortable in the long run, but mass does a lot to dampen noise so if sound is a higher priority than cost and/or comfort get a concrete slab. If you do an all wood floor, add another layer of 3/4" plywood with a layer of hardwood floor underlayment (dense roll foam) between the two plywood layers. This added layer adds mass and rigidity. The foam isolates vibrations and also acts as a vapor barrier from the ground. Your back, knees and tools will appreciate this option in the long run!

[Bill Jobe]

Well since it's been 5 weeks since I put down the downpayment i'm sure I could make it longer, but I cannot go beyond 12' wide and still get it to the back yard.
This whole thing has been very stressful and i'm almost to the point of putting in sonotubes myself.

[David Utterback]

A good source for information may be your local permit office. They can let you know how deep to place the sonotubes to avoid heaving from frozen soil, if that is likely to happen in your area. They may also help with load bearing calculations to address the number needed and locations.

[Peter Christensen]

If available in your area look into helical piles, sometimes called screw piles. Machine comes and winds them into the ground until the correct resistance is achieved. Then they cut the tops off and attach a fitting with a plate or saddle on top for the beams to sit on. They are in and out in an hour or two. Not the cheapest but they will be ready when your shop gets to the site.

[Bill Jobe]

Thanks for the idea, Peter. Still have time to decide.

[Curt Harms]

Maybe I missed it but where are you? Cold where frost heaving is a consideration?

[John K Jordan]

Well since it's been 5 weeks since I put down the downpayment i'm sure I could make it longer, but I cannot go beyond 12' wide and still get it to the back yard.
This whole thing has been very stressful and i'm almost to the point of putting in sonotubes myself.

I forgot what you said earlier, but if possible I would consider giving up on the portable building, pour a slab, and build a building from scratch. You won't be restricted by the access clearance and can make it wider. And building a shop is great fun! When I built my first building (20x20, for a shop) I bought some books to learn how.

Pouring a slab is not all that expensive if you do the forming and prep (rebar, etc.) yourself and pay someone to pour and finish. I've done several buildings here, the shop 24x62 and a recent one for a peacock house 12x24. It took me less than a day to prepare the peacock house slab for concrete.

If the site is not level, you can put in concrete piers and build your own wood-floored building, either on posts or as a portable building - exactly as the Amish building you've ordered. (Go to any place that sells these and look underneath and see how simple they are made.) BTW, if using piers to support a wooden floor building for heavy shop equipment I would put piers in the middle as well as around the perimeter to better take the weight - but a slab would be far better. If you know where the equipment will be placed you can even position piers appropriately at the expense of future flexibility.

Building a small structure on a slab is easy if you have mobility, can design and measure, and are handy with tools - and most people who play with wood can handle the tools. I've built a number of buildings, sheds, and decks here and at other places where we lived. There is also a tremendous satisfaction in building and working in a shop you built yourself! It is usually easy to find someone to help with things like trusses and setting large poles. I've developed ways to do most of it by myself using big C-clamps and braces, even when building with timbers so heavy I had to drag them to move them.

Alternatively, there are many people who will come in and put up a small building for you. If the site is prepped the cost may be less than you think and it can go up quickly (a day or two, depending on the design). Several times I've hired a builder friend from our church to build farm buildings and extensions or to help put up a roof - he brings a helper and they are very quick. He is willing to do small jobs like this by the hour instead of by the job which can save a lot of money since a contractor will often double or triple a job estimate to protect himself.

JKJ

[Bill Jobe]

Frost depth is 30", that's what I've always been told, but the contractor who wanted to do just one concrete column in each corner told me it's 42".
I'm along I80 in Illinois.

[Bill Jobe]

JKJ, I'd do it myself but I have a very impatient wife.
Hired out ground prep and prebuilt dropped is about all she can deal with.
That's not a sexist statement nor a slam. It's just the plain and simple truth.
We're all put together different. About 15 years ago I suggested we have a larger living room built on the back of the house since grandkids were coming fast .Though she was very excited and shocked that I would suggest such a thing, that was perhaps the longest 6 or so weeks in my entire life.

[Malcolm McLeod]

Frost depth is 30", that's what I've always been told, but the contractor who wanted to do just one concrete column in each corner told me it's 42".
I'm along I80 in Illinois.

Soil conditions and drainage both make a huge difference too. Sandy/dry drop the shed on a flat spot and it will last; clay/wet and your floor will be like a day-hike in the Himalayas.

Once upon a time, I did a shop on 12-piers. Never again! Never. ...I did say "never"? Didn't I? But on the other hand, never again too.

[Bill Jobe]

How so Malcolm?

[Malcolm McLeod]

How so Malcolm?

I was hoping no one would ask...:o:o

For a typical, small, open-floor-plan building, sitting on piers, the overwhelming majority of the weight is the roof and it's related structure. This is transferred to the ground thru the walls - - and they are ALL at the perimeter. So, the perimeter piers bear this weight. If you put piers in the middle (I did), they have relatively little weight on them. Even with a shop full of tools, the combined weight (live load) is generally much less than the structure (dead load). (....I'm talking averages here - - not the guy with a 24' 152,000lb hammer forge in his detached garage.) This weight bias towards the perimeter is compounded if you put your tools on wheels and roll them out of the way. ...On the wall, right?

So how is this bad? My clay soils and 12 ea. 24" deep 18"x18" piers allowed the perimeter to sink. Yes, 'that' perimeter - - the one with all the weight on it. The center piers never moved. Welcome to the Himalayas, and a close personal relationship with the friendly neighborhood foundation repairman. ...And you thought mistresses were expensive? ROFL

Have your eyes glazed over yet...?

To go the whole 9-yards as I understand it, you need to know the compressive strength of the soil. How many psi will it bear? Estimate the total weight of the building (or have a Civil Engr SWAG it based on his books and 30-yrs experience). Let's say the soil will bear 10psi and the building weighs 10,000 lbs. So, you need 1000 sq inches of something in contact with the soil to evenly distribute the load. Plus some factor of safety for when you roll all the big iron to one end.

If it's not evenly distributed then there's another set of equations - to try to keep the independent movement of the piers to a minimum. And they will move.

IF you use piers, my humble suggestion would be to use them at the perimeter only, and make sure that a beam, properly sized for the expected load, spans between each pier. Not perfect, but you won't need ropes and a climbing harness to retrieve a dropped socket from the corner. ... I mentioned never, didn't I?

I will use a slab.

[Ric Flanders]

Malcolm mentions some very important lessons in working within your expertise. It also points out the reason behind building permits and inspectionss. Your contractor (if experienced in your region) knows what the frost heave and ground conditions are and makes adjustments accordingly. The building inspector should know this as well. Piers are usually sized based on the loads that they are to carry. Hence larger piers and bases for those piers placed on the corners and perimeter. Where my lake home is, the soil is far different from my other residence. Even though the frost depth is the same. Pier sizes and depth would be different even if the same home were built in both locations due to the soil conditions alone. Simply putting in a properly sized Sonotube into a properly sized hole in the ground does not mean things are done properly. The base for the pier has to be made properly too. Sadly even the WI highway department didn't do this right when they built the Leo Frigo bridge in Green Bay. Two piers sunk years later about a foot closing the bridge for quite a time until proper footings were remade.

[Malcolm McLeod]

... the reason behind building permits and inspections.
... The building inspector should know this as well. ...


Careful...nobody's perfect. My former broke-back dusty little respite room WAS permitted and inspected at each stage of construction.

Inspectors may miss things, or be flat out incompetent? Ideally they can and should be an asset and your ally, not an 'enemy' to be endured.

And "meets code requirements" doesn't necessarily mean "meets site requirements".

[Bill Jobe]

Well perhaps I should take another look at compacted gravel....with a wooden frame, of course.

[Perry Hilbert Jr]

Concrete plants in this area put left over concrete into molds that make giant Lego type blocks. The large ones are 60 inches x 24 x24. They weight about 4,000 lbs. Many folks use them for retaining walls, foundations, etc. I have seen them used under mobile homes and large sheds. I checked on the price five years ago. They were 4t dollars a piece back then. There is a smaller size that weighs 2500 pounds. And costs less. I would use those for a shed base

[Osvaldo Cristo]

It is interesting to read about your construction techniques... here downward under Capricornium tropic it is quites simple: gravel for draining, 2" to 4" steel rod reinforced concrete topped with epoxi flooring. Voilá!

Of course we never have freezen temperature...

[Bill Jobe]

I've hired a guy I believe will do the best job. His bid was also lower than my second choice. Also he is putting in 12x48" tubes why my second choice was only going down below 30".
According to the guy in hired you pay for 2 cu. yards regardless of whether or not you need that much. Minimum deliver. He and I both figured the cubic yards needed for 14 12x48 tunes and it came out to less than 1.5 yards, so why not.
That gives the 2 outermost skids 4 pads ea. And the 2 inner skids 3 each.

Have a question about sound reduction....has anyone tried filling the open space that a shed on skids would have with foam insulation. Seems to me that would not only quiet down sound transmitted thru the plywood floor as well as make the floor warmer in our harsh upper Midwest winters.

Also, the building inspector told me the setback using a slab would be more strictly enforced because of its nature while tubed would allow for some leniency, should a neighbor complain about a few inches. I can always move the shed over a little whereas a slab would have to be broken up and redone.
Also, cheaper than slab, which would take over 5 cubic yards at 6", if my math is correct.
In my situation tubed make the most sense.

Any comments would be appreciated, particularly regarding foam insulation under the floor. That would also provide better protection from moisture than the planned plastic underlayment.

I considered building on site but setbacks would still limit the size. I would have loved a larger shop, but anything bigger and i'd run into location problems. 12x20 is the biggest I can go where I want the shed. Any longer would put it against a large oak, any wider would make the patio much to small.

[Reed Gray]

A concrete slab for that type of shed, not engineered would be a 4 inch thick slab with 'thickened' edges, so use a 2 by 6 for outside thickness, and one piece of 1/2 inch rebar around the outside, suspend mid way between top and bottom of concrete, and 3 or so inches in from the side. You can just lift it up as you pour, and an experienced concrete finisher can do this, or a brick underneath. If you want to put anchor bolts into it, you can dig holes for the bolts, or just put them in a rebar bender and put a 90 degree bend in them. Fine for a 'garden shed' but maybe minimal for tornado proof. Concrete costs can vary. The big redimix trucks that carry 9 or more yards are more expensive than the portable mix it on the spot type trucks. The 2 yard minimum price is kind of a standard, but they also take in 'standing time' which is how long it takes you to get it off the truck. If you can back the truck up and just dump it easy and cheaper than if you have to wheel barrow it around the back side of the house. A lot of the old trucks also need to be able to wash out the chutes, so gravel drive is nice, street is not... If your concrete company knows you, and/or the contractor, you may get preference for delivery. If they don't, you can get what is left on the other trucks at the end of the day, which may be barely good enough for footings or sono tubes, but not a working surface slab. If it comes and looks like thin soup, send it back.... Your shop will be cold. Foam insulation to keep wind and weather out would be great. Foam down into the ground would help insulate some as well. A vapor barrier does not provide insulation. Cheapest way is black plastic which does not rot out like the clear stuff does. However, if you put gravel on top of that, you will poke holes in it, so the moisture comes through. Half inch thick foam sheets would help that a lot. I never liked pouring directly on plastic because the concrete does not set up right, Water needs to seep out the bottom. What happens is the water all rises to the top. The concrete sets up hard underneath the surface water, and then you end up with a lot of grit on top of the slab as you are trying to finish it. Not too bad if you have a troweling machine, but that doesn't happen on a small slab. You get an okay surface. If you want the slab and shop to be some what insulated for winter, both outside and bottom need to be insulated from the elements. Here in Western Oregon, it seldom gets below freezing, so that type of insulation is generally not needed for a shop. Some thing to consider for the future if you are going to sell eventually and not take the shop with you, it will have more 'resale' value if it is on a slab. As far as property easements, there is not a neighborhood in the US that does not have multiple 'violations' of this code. Here it is 5 feet clearance from any fence. Most of the time that is only enforced if a neighbor complains or the fire marshal comes through. This is some thing that can also effect resale values... Hope I haven't confused you more.....

robo hippy

[Bill Jobe]

No, you haven't confused me. The inspector highly recommended that I use sono tubes, pointing out that a slab would come under greater scrutiny thus making allowances for setback more difficult for him to approve it. A slab would cause more problems should a neighbor complain, as well. With 12" columns the shop can anyways be moved over a bit but you can't slide a slab.
I point this out because the outer skids are set in a few inches so the tubes would also be set in whereas a slab would more than likely require a 12x20.
Make any sense?
Anyway, the job is set and I'm going with tubes. The inspector will be happy, I may save on concrete cost and going 48" deep, my shop, once leveled, should never move.
That freeze and thaw cycle here in upper Illinois is real hard on everything. Particularly concrete. If a crack developed and went unnoticed under the shed it would eventually cause it to move out of level.

Are for the foam insulation underneath it, I was thinking spray....while doing the walls. Seems to me that would be ideal for both insulation reasons and moisture problems from under the wooden floor.

[Ric Flanders]

Bill,
Spray foam is great! I have it the walls of my home. It is expensive though so its not often used in the floor. Keep in mind that the foam (or any floor insulation) will need to be covered somehow to prevent insects and rodents from making their home in it. Maybe have your contractor install plywood under the joists, then fill the joists with insulation before you put on your flooring.

[Lee Schierer]

You don't indicate what part of the country you live in. Years ago when I built my pole barn.
370784
I was advised to put in a compacted gravel floor. It worked great the first summer and winter, however, in the spring when the ground and gravel thawed it became spongy and the boat trailer left ruts. When I paved the driveway, I had the garage floor cemented and have had no further issues.

You might want to consult your local building codes to see what is permitted in your area. You will likely have to pull a building permit to make your shop.

[Bill Jobe]

This thing has been one pain in the butt after another. Now the guy I hired to do the sonotubes has backed out.
I'm considering a hail Mary and hauling in gravel, compacting it myself, then framing it out and pouring a slab. Just too late to do anything else. My second choice for the job must have been upset that I didn't hire him because he isn't answering my phone calls.
So, having never poured a slab, how deep do I need for a gravel base? Also, with night time temps dipping into the sub-freezing numbers i'm wondering if I still have time for a slab. And there's nothing but rain forcast through the next several days. I understand they can mix something in to heat the concrete but know nothing about it.
I honestly do not know what to do at this point. I've decided that i'm fine with the 5' setback. I'll lose some patio footage but I can live with it.

Open to ideas of any kind.

[Bill Jobe]

The next bid in line wants to pour a 5" slab on compacted gravel with no footings around the edge.
Bad idea?
If I raise the slab a couple more inches than planned the gravel base should drain well. Will that do?
Also, I'm going to tell him not to add more water when the concrete arrives, nor any lime. But i'm trying to decide whether to have the concrete source to add fiberglass or airiated (can't recall the term used) .
Which do you think would be better?
It's supposed to get back up in the 50s next week and that's the time he planned to pour.
Any other things i've missed?

[Reed Gray]

Okay..... Personally, I would have a 4 inch thick slab in the center, and 6 inch deep by maybe 1 foot wide on the exterior perimeter. Yes, you could do a solid 5 inch thick slab if you want. The 'fiber mesh' in the mix do work similar to rebar for holding the slab together after it is poured. You can end up with hair balls on top of the slab which can be burned off with a torch. I would do the rebar, #4 or half inch. One piece around the outside, and then 2 ft. grid on the inside. Best to suspend it rather than pick it up. If you can find some knock outs from the cinder blocks, or broken cinder block, that works as well as the little cubes that they sell with the feed sack wires in them. You do need to watch to make sure the concrete does not push it up against the forms as you pour. The cubes can be pushed over as you pour. You want a 4 to 5 inch slump for your concrete, and most dispatch companies will send it out at a 4 inch slump which is a bit difficult to push around, but you can add a little extra water. You don't want it soup mix slump, which can happen if you pour in the afternoon and they are trying to get rid of some thing that no one else wants. It is common to add 'air entranment' in areas that get a lot of freeze thaw type weather. Works better than standard mixes. We use it some times here in Oregon. Not really necessary for interior slabs because they don't get down to below freezing temps when on the inside. It does make it a bit more difficult to work as the mix gets kind of sticky. Most of the time in the winter, we would add hot water to accelerate the set up time. It is a little help, but cold ground and cold water and cold aggregate can cancel most of that. Salts (calcium chloride) is a common additive to accelerate the set up time. Never did like that stuff. I had heard that it can rot out the rebar sooner as salt and metal don't mix well... You will need to plan to insulate the slab for up to a month. Black plastic and straw work well, or they have fancy insulating blankets. Concrete is supposed to reach 90% of PSI strength in 10 days, and be fully cured in 30 days. A bit faster in warm/hot weather. If the building is lifted by crane onto the slab, then safe at 10 days, or even a week. If you are sliding it on, then wait 2 weeks at least, depending a bit on weather. You can drill in anchor bolts (one brand was 'red heads') which work okay. Just letting it sit will be okay unless there is a tornado, and you are in tornado country.

Only 2 guarantees you can give about concrete, It will crack, and no one is gonna steal it.....

robo hippy

[John K Jordan]

The next bid in line wants to pour a 5" slab on compacted gravel with no footings around the edge.
Bad idea?
If I raise the slab a couple more inches than planned the gravel base should drain well. Will that do?
Also, I'm going to tell him not to add more water when the concrete arrives, nor any lime. But i'm trying to decide whether to have the concrete source to add fiberglass or airiated (can't recall the term used) .
Which do you think would be better?
It's supposed to get back up in the 50s next week and that's the time he planned to pour.
Any other things i've missed?

I personally don't like a slab of any size without at least a turn-down on the edges. A lot depends on the slope and drainage. I've seen too many quickly poured things like sidewalks wash out under the edges or corners.

The fiber does work but I really prefer to add rebar even with a small slab, at least around the edges. For a larger slab I get serious with the rebar and weld every intersection. 1/2" Rebar is cheap. (I was an inspector and I've seen too much.) I run an extra piece of rebar near the bottom of the turn down. You cannot put too much steel in concrete. Enough rebar will hold the slab together even with some ground settling. If you use rebar make SURE it is not pushed to the bottom where it is useless - lots of guys doing the pour don't care. I like to block it up to the right height with broken pieces of cinder block - the plastic "chairs" can break down when walked on. This is the end of my shop with the slab designed to support my tractor and bobcat:

371204

If wiring a sub-panel in the building you can clamp a heavy solid ground wire to one of the pieces of rebar and bring it out the side of the slab as a secondary ground.

Make sure the edges are rodded. I always cover the green concrete with plastic and let it cure for seven days. In freezing temperatures we put insulation on the top.

Reed, you sound like you were in the concrete business! I was an inspector at one time, the guy who showed up at the job site with a slump cone and forms for test cylinders.

JKJ

[Bill Jobe]

Thanks everyone.
Robo, your comment on no one is going to steal it reminds me of when I removed the chain link fence and put up a wood privacy fense.
The metal posts were buried in concrete and some of them broke off right at the top of the concrete. I put an add in the paper for the fense, posts and top rails. A carload of guys showed up and said they'd take it all.
Thinking, dang those posts that broke off. Now i'll never get rid of the posts or the ton of concrete stuck to the bottom.
But they meant, literally, all of it. They loaded up huge concrete chunks with the end of a broken post and all. Having just a car they tied all of the top rails ( about 200 ft.) to the roof of their car with bailing twine and away they went, rails splaying every direction. I was so tickled that they wanted everything that I suggested they load the posts - broken and unbroken-- into my truck and I'd haul them home for them.
They were tickled to death and offered me money, although I had advertised them for free.
I just couldn't take the money.
What they ever did with all that mess I'll never know.

[John K Jordan]

Only 2 guarantees you can give about concrete, It will crack, and no one is gonna steal it.....


My concrete guy gives three guarantees:
- It will set up
- It will crack
- No one will steal it.


My 24x62 shop floor has shallow relief cuts every 12'. Four years later we had to get down on the floor with a bright light but we found the cracks - in the grooves but so tight they were almost invisible.

JKJ

[Bill Jobe]

My concrete guy gives three guarantees:
- It will set up
- It will crack
- No one will steal it.


My 24x62 shop floor has shallow relief cuts every 12'. Four years later we had to get down on the floor with a bright light but we found the cracks - in the grooves but so tight they were almost invisible.

JKJ

You cut instead of pressing a groove? Do you think it's the best way? I've never messed with concrete.
So, 1 cut across the middle cross ways should do it for my 12x20 or should I give it 2?
What width and depth do you think is best, John?

[Bill Jobe]

I don't think fiberglass is very expensive. Would adding it to the mix as well as rebar give a stronger bond? This guy hadn't intended to do either.
Also, I have a few lengths of 1/2" rebar left over from making a koi pond cover frame and some trelleses and flower pot hangers. It is considerably rusty. Can it still be used if fully suspended in the concrete?

[John K Jordan]

The concrete installers I know wait a week or so then run a big concrete saw over the surface but it should be done sooner in hot weather. (Most of mine has been in cold weather.) I didn't measure the grooves, but maybe 1/8" wide or so. I think the cut is supposed to be 1/4 of the way through the slab but I found some very shallow and they still worked. Concrete shrinks as it cures and dries so the saw cuts create weaker areas where the cracks can form instead of cracking at random. That said, a small 10x20 slab may be fine without a saw cut. A 12x24 slab for my new peacock shelter back in the spring of this year didn't get any grooves and there are no cracks. I did keep it covered with plastic for about three weeks and occasionally hosed water under the plastic to keep it sopping wet. (I use overlapped 4' wide strips.)

As with most things, there is controversy about rust on rebar. As long as the rust is on the surface and the iron is not corroded significantly I personally don't think the rust matters. Nearly all rebar I've seen at big constructions sites was left outside in the weather and had some rust on it. I've used very rusty rebar pieces to rod wet concrete near the forms and the concrete stuck to it so well it was difficult to break off with a hammer. The ribs on the rebar are what the concrete grips.

Some people don't put steel in a small slab. Lots of people just use welded steel mesh made for reinforcing concrete, available at any building supply. (Lay it right on the gravel, pour the concrete over it, then use a hook to pull the mesh up into the wet concrete. On small slabs around the farm I sometimes use left over pieces of galvanized fencing and add what pieces of rebar I have on hand around the slab, maybe a foot from the edges. For a slab that a portable building sits on the concrete I don't thing the concrete is too critical - most of them sit on gravel or cinder blocks. However, I'd want it done the best way possible - the slab might be used another way someday.

BTW, I like to slope a slab slightly for drainage, sloped more if it will be exposed to rain or need to be hosed down occasionally. This shouldn't matter for yours since it's going to be covered. It's still best to put good drainage around the edges (e.g., french drain if needed), especially if the ground is sloped so water won't freeze under the slab and break it. I just dug a ditch around the back of my peacock house.

I think it's worth getting someone with experience to pour and surface the concrete. And if this needs to be inspected, ask the building inspector about all this. Nothing here needs or gets inspected.

I am certainly no expert on making concrete slabs, all I know is what I've done which has worked for me. I hope to put one this winter down one side of my barn where the llamas and donkeys like to hang out, maybe 10'x40' and another 12x24 for a shelter.

JKJ

[Frank Pratt]

The construction company my brother works for has developed a system for concrete slabs with no control joints at all. The biggest they've done is somewhere in the 10s of thousands of square feet. Can't remember all the details of how they do it.

[Bill Jobe]

The construction company my brother works for has developed a system for concrete slabs with no control joints at all. The biggest they've done is somewhere in the 10s of thousands of square feet. Can't remember all the details of how they do it.

If you think of it please post how they manage to do that.

[Frank Pratt]

I know they start with a meticulously prep'd base that is perfectly flat & heavy layer of poly under the pour. The idea is that the entire slab shrinks as it cures & that base allows it to move freely. As far as mix & such, much of that is proprietary. I seem to recall he said they use glass fibers for reinforcement. It's not always a sure thing though, & sometimes there are small cracks that appear.

[Bill Jobe]

I know they start with a meticulously prep'd base that is perfectly flat & heavy layer of poly under the pour. The idea is that the entire slab shrinks as it cures & that base allows it to move freely. As far as mix & such, much of that is proprietary. I seem to recall he said they use glass fibers for reinforcement. It's not always a sure thing though, & sometimes there are small cracks that appear.

Doesn't the gravel base have to be sloped as well, to keep water from pooling up under the slab?

[Frank Pratt]

Doesn't the gravel base have to be sloped as well, to keep water from pooling up under the slab?

Don't believe so. They use a pretty dry mix so I doubt that water under the slab is an issue. I'm sure drainage of the base gravel is addressed though.

[Mike Heidrick]

I used a mesh riser called mesh-up i found online out of Louisiana. I installed them every 12". Pex was tied to mesh. I wanted pex in middle of 5" slab. They dont break when they walked on them. Mesh acted like a box spring and no bringing up the mesh etc. Pretty sweet. My concrete guy never heard of them before and now he resells them.

[John K Jordan]

I used a mesh riser called mesh-up i found online out of Louisiana. I installed them every 12". Pex was tied to mesh. I wanted pex in middle of 5" slab. They dont break when they walked on them. Mesh acted like a box spring and no bringing up the mesh etc. Pretty sweet. My concrete guy never heard of them before and now he resells them.

That sounds promising. I'll tell my architect son. A concrete guy told me the architects and builders would have a fit if they knew how often plastic chairs broke when they stepped on them.

JKJ

[Mike Heidrick]

Watch the Mesh Ups by Lotel video on youtube. I bellieve your concrete guy because I went to the building stores and stood and broke every chair I tried . Only one that worked was the mesh up. It was funny to see ythe workers faces. I went back and looked and we did 18-24 inches not every 12".

https://youtu.be/jUpOVVUt3Bg

http://i39.photobucket.com/albums/e169/BloomingtonMike/Pole%20Barn/floor44_zps16b70bfa.jpg

[Bill Jobe]

After sending the new guy an email a couple of days ago I heard back from him today. He's backed out.

[Mike Heidrick]

Wait till spring an option?

[Peter Christensen]

You're running out of time and may need to reconsider the screw piles I suggested last month or wait until spring. It will get you to a point where you can build the structure and then decide if you want to concrete the floor inside with temporary heat or build a pressure treated wood floor hung on the beams. The floor could be completed in the spring.

[Bill Jobe]

You're running out of time and may need to reconsider the screw piles I suggested last month or wait until spring. It will get you to a point where you can build the structure and then decide if you want to concrete the floor inside with temporary heat or build a pressure treated wood floor hung on the beams. The floor could be completed in the spring.

Help me out, Peter. What are screw piles?
I'm considering building a frame out of 6x6s ,filling them with gravel and compacting it. But I don't know if I can do a good enough job to provide proper support.
I'm on my own at this point. I just lost who I thought was my only friend in the world. Turns out he was harboring deep resentments for me, so I'm now a one man show. And my marriage is falling appart as I am typing this.
Ever wish you'd never been born?

Bill

[Peter Christensen]

Bill I mentioned Screw Piles or Helical Piles in post 13. They are basically a pipe or bar with a plate on the bottom that has a spiral shape. The installer brings a machine in (often a skid steer) and screws it into the ground. When done they cut them all off at the height needed and then bolt or weld a fitting (plate, saddle, etc.) on top. They come in different sizes to suit the loads and application. I was looking into the costs this past summer and 2 3/4" inch ones to hold up a deck were about $400Can each installed. That would be a little over $300 in your money. When you factor in that you don't have to dig holes, use Sono tubes or make forms of some kind, cost of concrete, access etc., and the fact that they can be installed in a morning, they can make sense.

A couple links to get a start at understanding them.
https://en.wikipedia.org/wiki/Screw_piles
https://www.technometalpost.com/en-CA/advantages/helical-piles/
Look at images when you do a search for helical piles to see what can be done with them.

You don't have a location in your info so I can't look up to see if there are any installers near you.

As for the personal things you are dealing with. I don't even come close to offering good advice to you other than to say go to a professional counsellor to talk to.

[Bill Jobe]

Bill I mentioned Screw Piles or Helical Piles in post 13. They are basically a pipe or bar with a plate on the bottom that has a spiral shape. The installer brings a machine in (often a skid steer) and screws it into the ground. When done they cut them all off at the height needed and then bolt or weld a fitting (plate, saddle, etc.) on top. They come in different sizes to suit the loads and application. I was looking into the costs this past summer and 2 3/4" inch ones to hold up a deck were about $400Can each installed. That would be a little over $300 in your money. When you factor in that you don't have to dig holes, use Sono tubes or make forms of some kind, cost of concrete, access etc., and the fact that they can be installed in a morning, they can make sense.

A couple links to get a start at understanding them.
https://en.wikipedia.org/wiki/Screw_piles
https://www.technometalpost.com/en-CA/advantages/helical-piles/
Look at images when you do a search for helical piles to see what can be done with them.

You don't have a location in your info so I can't look up to see if there are any installers near you.

As for the personal things you are dealing with. I don't even come close to offering good advice to you other than to say go to a professional counsellor to talk to.


Thank you,Peter. I will look into them.

[Dan Rude]

You might also look if anyone does this system: http://www.startribune.com/a-new-type-of-deck-footing-diamond-piers/211348291/ I have seen them in JLC quite a bit. Dan

[Peter Christensen]

Those were used on This Old House last weekend.

[Bill Graham]

Let's go back to the get-go and talk about gravity, that thing that tries to put the roof into the basement(or the ground in your case).

The first thing we have to know is how much weight will be carried on the outside walls. First to consider is the live load, like what your average 200-lb shop gorilla produces stomping around on the floor and the dead load(what the floor materials weigh). International Residential Code specifies for anything other than a sleeping space a floor must support 40 lb/sft live load and 10 lb/sft dead load. That covers the gorilla but doesn't consider anything other than furniture and rugs, definitely not stationary tools.

Next we have to support the walls, a typical wall weight for 2x4 studs with cement siding and 5/8" gyp board is 12 lb/sft. For wood siding and 1/2" gyp call it 8 lb/sft. So an 8' wall weighs approximately 64 lb. for every linear foot if you use wood siding and 1/2 gyp..

Then there's the ceiling and roof. The IRC specs ceilings without inhabitable attics and limited storage at 20/10(live load/dead load). Same specs for the roof unless you live in an area with snow load. I live in Georgia so snow load isn't something we have to deal with, you can do some research and modify the numbers if you do.

Now we get down to arithmetic: how much weight do the footings have to carry? Assuming your 12x20 building has floor joists,walls, ceiling joists and rafters running the short dimension, it's just a matter of punching #'s into your calculator. The long side of the building is carrying half of the total load so....

Floor load per square foot is 6'x50 lb/sft=300 lb/lft, wall load is 64 lb/lft, , ceiling load is 6'x30 lb/sft=180 lb/lft, roof load is the same so add in another 180 for roof load and the total load on your footing is 660 lb/lft. That's pretty much moot since ready-mix concrete is rated at a minimum of 2000 lb/sft and even the most miserable clay soil(like we have here in Georgia is rated at 1500 lb/sft. So far, so good.

But you have another issue, the frost line. I think I read your frost line is 32", if that's the case a continuous footing would have to be 32" deep at the slab perimeter. That's a lot of concrete for your building, it would be 60 lft at 12" wide, 32" deep. About 6 yd, according to my calculator. Add in your slab at 4" thick and there's another 3 yd. So you're talking a full truck of ready-mix. Piers supporting beams start sounding like a reasonable alternative. But the question is how many piers?

A 4x6 SYP beam will carry 560 lb/lft on a 4' span so you would need 5 piers on each long side carrying 1240 lb./pier. A 12" pier would carry that load so you would need 5 piers at 12"x32" for a total of 3 yards of concrete for both sides. So you save 6 yd. of concrete by using piers but you get a gravel floor and you'll pay a delivery charge on less than a truckload of concrete.

That's not considering tool weight that will go far beyond the 10 lb/sft dead weight these calcs are based on. Never mind that the 2x4 joists spanning 12' won't support anything more than a lawnmower, IRC doesn't bother rating them in their span tables. I'd be OK with walking on that floor but not if it's loaded.

My suggestion is that you pour a slab with turn-down footings beyond the frost line and build your shop on top of it.

Just so you know, I've been building houses for the last 25 years and I haven't had any structural problems. My roofs stay out of my basements.;)

[Bill Jobe]

The weight will be spread more evenly with an Amish building. The weight will be on 4 20 foot long 4xs.

I recall quite a number of years ago when Billy Myers built a new drag strip in Texas. It's been said it was pre-stressed concrete, something that meant a lot when you are trying to apply 10,000 hp and get it to stick.
If memory serves that is where the first 300mph run was made.
Not official according to NHRA, but 300 just the same. Eddy Hill, I believe.
Could that maybe be what Frank was referring to?

[Andrew Seemann]

If you went the slab route, you probably would not need to go all 32 inches down to the frost line. Here in MN, for detached buildings like garages and sheds, we normally use a what is called a floating slab foundation. 4 inches thick in the middle and the outside is 12" x 12" tapered up to the 4" middle. The increased thickness on the perimeter is to support the weight of the walls. Any internal support posts would also require a thickening of the slab under them. The main reason for them here is that they work, and we don't need to go down 42 - 60 inches to get below the frost line for a simple outbuilding. Main thing is that you can't have a direct connection to a building with frost footings (like a house), because they may move at different rates in the spring thaw.

http://www.minneapolismn.gov/www/groups/public/@regservices/documents/webcontent/convert_264724.pdf

[Bill Jobe]

Yes, I think I could see the problem with that.
I'm just really bummed out that everyone I hired to do the job backed out so now i'll be without my lathe for the most part, til spring.

[Bill Jobe]

Thanks for the info on codes. Our frost level here is 30" i've been told.
I'll need to touch base with the inspector since I now plan on a slab.

The ONLY concrete I've ever done was a 4x4 slab for an up flow sand and gravel filter for a koi pond I used to have. For the filter to backflush properly it had to be level. That was about 12 or 13 years ago, the pond and filter are long gone, but that slab is still perfectly level, if I never pour another one. Concrete just has never been a priority for me til now.

I think it was John that mentioned a very slight slope so water wouldn't stand under the shed.
I assume you meant sloping to one end of the shed, regardless of the surrounding landscape? I think water would get trapped if it sloped to one side what with the 4 runners the shed sits on.
I have plenty of time to study up on concrete. Perhaps I'll gain the confidence to do it myself. I'm not a genius, but nor am I an idiot.