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Thread: A better workshop foundation?

  1. #16
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    Do it yourself?

    Quote Originally Posted by Bill Jobe View Post
    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

  2. #17
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    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.

  3. #18
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    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.

  4. #19
    Quote Originally Posted by Bill Jobe View Post
    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.

  5. #20
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    How so Malcolm?

  6. #21
    Quote Originally Posted by Bill Jobe View Post
    How so Malcolm?
    I was hoping no one would ask...

    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.
    Last edited by Malcolm McLeod; 10-19-2017 at 5:48 PM. Reason: I moved. Anyone need a climbing harness?

  7. #22
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    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.

  8. #23
    Quote Originally Posted by Ric Flanders View Post
    ... 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".

  9. #24
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    Well perhaps I should take another look at compacted gravel....with a wooden frame, of course.

  10. #25
    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

  11. #26
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    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...

  12. #27
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    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.

  13. #28
    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

  14. #29
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    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.

  15. #30
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    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.

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