Portable building foundation questions

[Zachary Hoyt]

I have found that I am not going to be able to build a 2 car garage sized workshop building at my new location as I had intended, because a town water line runs right across the area and I can't put in a foundation or cover it. What I will have room for is a 14 or 16 foot wide building, and I am thinking of making it 28 or 32 feet long. Something like those portable garages on skids that companies and Amish people sell, but without the big door. My question is how the base or foundation is built. My impression from some reading is that pressure treated skids are used, running lengthwise, and then 2x6 treated joists on 12" centers are run across them, (treated?) plywood is used for the floor and from there it's an ordinary stud frame. What I don't know is how they go about joining lengths of wood to make the skids, and how many skids and of what size are needed for a given size of building. I prefer to overbuild rather than underbuild, usually. I can imagine using 4x4s with 2x4s on top to join them together, but that seems like it would be weak, or using 2x6 or 2x8 standing on edge in 3 or 4 layers and staggering the joints, but again I don't know if that would be enough. I'll be very grateful for any insight into how this is normally done.

[Ron Selzer]

Around here there are places selling portable storage buildings. I suggest you look around as there probably are ones around you also. You can look at and compare different ones and how they build them.
May find you can buy for what materials cost you if you put any value on your time.
I would start with solid patio blocks or solid 4"x8"x16" block on side. Then 4"x6" runners length of building. then at least 2x6 on top then 3/4" t&g flooring plywood. I would scarf the 4"x6" runners or use heavy steel fish plates. from the 2x6 drop a 2x4 down to the 4x6 to fasten together.
definitly some gravel under all blocks, no wood within 2" of ground and not in a low spot where water collects
Good luck
Ron

[Lisa Starr]

Seeing your location, how are you going to handle the ground movement from the ground freezing each winter? If you're thinking of putting equipment that is sensitive to twisting in it, I'd be extremely concerned. These types of building can and do heave unevenly every year. Is there a different location you could construct a permanent building? Could you alter the footprint of the building in such a way you could miss the water line?

[Zachary Hoyt]

Yes, I should go and look at how the folks who sell them do it. I am hesitant to waste their time by asking questions, but I'll try to find a polite way to go about it. I was planning to have some gravel dumped where the building will be and to spread it out maybe 3-4" deep, then to put solid blocks on top of that. I figured that would keep water from migrating up from the ground since it can't jump from one rock to the one above it. The spot is well drained and flat, with a slope away on the south side.

This is the only flat spot on the lot where I can put a building easily, otherwise I'd have to build one side up quite a bit and I don't want to put the kind of money into it that would be required to build a real foundation in that kind of situation. I'm not planning to stay here permanently, but I need a place where I can work for a few years. I don't think twisting and frost heaving should cause me a huge problem, if it happens I'll just have to adjust the building once the snow melts if it doesn't come down flat. My equipment is just ordinary hobby size stuff, a Shopsmith, 18" bandsaw, floor drill press, contractor table saw, 6" jointer, lunchbox planer etc. I guess at worst I might have to put a wedge under one corner to keep things from wobbling. The old concrete floor in my current shop is flat overall but has some unevenness for sure.

[Tom M King]

I use solid 8" cinder blocks, six on each end corner, with leveling treated wood on top where needed, to set shipping containers on. The blocks are just sitting right on the ground. Two floor jacks under one set of corners will lift one side, or end of a 40' container, with the jacks on 5/8 plywood. One side, or end is leveled, then the opposite corners.

[Jim Becker]

I watched the delivery of that type of setup on the 'Tube awhile back. The building came in two sections that were joined onsite and provided something like a 16x40 or so building. (I forget the exact size. The building was placed on top of a leveled and tamped crushed stone pad that had PT wood bordered. And yes, the building components were on PT skids...the same way that smaller utility buildings are made and delivered. It was kinda the same concept as a "double wide" home, but obviously simplified without plumbing, etc. The buyer did the interior finishing himself.

[Jaromir Svoboda]

I just got my 12'x20' shed delivered last Monday. I build gravel pad about 6" deep. It sits on four 4x6 skids.

[Zachary Hoyt]

Thank you all very much, that gives me a lot to work with. I'm hoping to make this building movable so that I can take it with me when (if) I build a house someday.

[Jim Becker]

Thank you all very much, that gives me a lot to work with. I'm hoping to make this building movable so that I can take it with me when (if) I build a house someday.
They actually are movable, but keep that in mind if you get a two piece unit so that whatever you do in the interior can be split down the middle without too much hassle. While finishes is the obvious, making it so you can split the electrical, too, is important. Surface mount makes that a lot easier, IMHO.

[Zachary Hoyt]

Yes, surface mount electrical does seem like the best approach. Around here a new 14x28 or 14x32 can be delivered whole from the manufacturer, but I don't know who may be able or willing to move this building if and when the time comes, or what their limitations might be in terms of size.

[Jim Becker]

I've moved to surface mount for shop wiring completely at this point. Easier to install and easier to change over time. But yes, a little added cost for conduit or raceway. Worth it, IMHO.

Relative to the future moving of such a building, make that part of your conversation with the maker. Ask them specifically if they can provide assistance with that and what the caveats are, especially since a 14' wide unit requires special handling.

[Zachary Hoyt]

I'm to be the maker of this building, and I am not always on speaking terms with myself but I'll try to start negotiations off on the right foot. I have a conduit bender and have used it to make bicycle trailer parts, but never till now to bend conduit to actually put wiring in, so that will be exciting. I'll have to see if I can learn to do it neatly.

[Charlie Velasquez]

I, too, had to go skids; city assessments and code issues.
Not as big, 12x16.
Slight slope.
Used 4”x8”x16” solid blocks
In Iowa.


Laid out the corners and block locations (4 rows of three)
Dug about a 2’x2’x6” hole at each location, The hole at the high corner was deeper so that block set only about an inch above grade.
Lined each hole with a couple of layers of 8 mil visqueen.
Filled each hole with pea gravel.
The plastic keeps the pea gravel from migrating into the soil. The pea gravel acts to spread the pressure over the entire exterior of the hole (https://www.stevespanglerscience.com/lab/experiments/magic-tube-strong-tissue-paper/)

Then the 4x8x16 blocks. The high corner got one block, the diagonal corner got three.
Covered all with another layer of plastic, extending about a foot, cutting out for the blocks (to keep down grasses)
Covered that with about an inch of pea gravel (to secure plastic, slow rain runoff, “lock” the bottom block, and help keep critters out)
4x4 skids.

I planned to insulate the floor with styrofoam and needed to protect the insulation from critters and insects. Intended to use screening or hardware cloth, but at the time 1/2” treated ply was cheaper, so,...
1/2” ply nailed to the skids
2x6 treated rim joists, 2x6 joists @ 16” o.c. on the interior
cavities filled with three layers of 1.5” extruded polystyrene insulation with 1” gap between layers.
3/4” Avantech as the finished floor.
Drove 3/4” rebar about every 6’ or so aside the rim joist and secured.

We have had some pretty wicked winters with freeze/thaws. After three years, a soccer ball in the center stays put.

[David Hostetler]

I am doing something similar, but quite a bit smaller as I am limited to 10x20 (200 sq / ft max).

The site will be prepped via excavation / flattening / compacting the soil, creating a full french drain surrounding the foundation to a leech field allowing saturation / runoff to the bayou behind the house / property.

Then a layer of compacted sand, and gravel, and finally concreate pavers.

The build is being done of recycled / reclaimed large pallet wood, with any needed additions. I am using pressure treated 4x4x16, and 4x4x10, doing timber frame shiplap joints that will be crosswise, glued, and lag bolted so I end up with 4x4x20 skids. And 2x4x10 PT stringers every 16". Foundation fully boxed to preclude pests and limit air movement. Rigid foam insulation, and reclaimed pallet lumber milled to T&G for the flooring, 2 layers at 90 degree orientations. From there, standard walls / roof.

For ME and MY LOCATION I need to keep moisture at bay underneath hence the gravel / sand pad underneath, and the french drain. ALSO I will be anchoring using hurricane anchors and straps. Pretty standard gulf coast fare for portable buildings to keep it from getting blown into the next county when a storm comes through...

In order to skirt HOA rules, and city permits, the 200sq ft limit is key, as is keeping overall height under 9', and it must be a "portable building". so no poured slab, skid construction is considered valid...

And yes, it will likely over the decades flex itself to bits, but by the time that happens, I will be flower food.

[Lisa Starr]

Ideal has a PDF document that is downloadable from Lowes that provides the measurements and "how-to" for producing all the different EMT bends. It makes it much easier for someone that doesn't do it everyday.

[Kris Cook]

This video for conduit bending was helpful to me.

https://www.youtube.com/watch?v=5L5Kujr98Cg

[Steve Rozmiarek]

I didn't see a specific answer on making long skids from short lumber, apologies if someone already said this. Build them of from shorter pressure treated 2x, with joints staggered and PL400 adhesive. Minimum of three layers with at least 16" of distance between seams.

Back in the 1920's when my great grandparents settled in this area, most buildings were built this way on their ranches. They would move them periodically to reconfigure the homestead for whatever was needed at the time. Some of them still exist, most have rotted into the ground. Back then, they moved them with horses. The buildings that survived were used as a shed to keep whatever feeding equipment that needed to be used daily, out of the winter weather. After the war when my grandparents took over, they quit using horses and switched to little John Deere "caterpillars". Those little tractors were still in use on the ranches into the 90's, and their use for feeding cows remained the same throughout their 50 plus year service life. During that caterpillar era, the little shed became the "cat house" that the caterpillars were stored in so they would start better in our rather extreme winter. The sheds would still get drug around when I was a kid to whatever valley cows were being fed in. One thing that struck me was the number of varmints under their floors when that shed started to move away. Snakes, mice, moles, possums, coons, etc. I never saw rattlers, but grandpa had a story about moving one and coming across hundreds of them under one. Point is, make sure the creepy crawlies can't make a comfortable home under it.

[Zachary Hoyt]

Thank you very much, that's very helpful. I have built and moved small buildings on skids here over the years for similar reasons to what you describe, but I haven't had to move them that far. I have never done this with anything bigger than a small shed, so the skids were always made from full length boards. I had been figuring on 3 layers of 2x6 per skid, but I hadn't thought about the PL400, that is wise. I'd figured I could stagger the joints 4 feet, and use deck screws to hold the boards together. I'm still wondering if I should make 4 or 5 skids for a 14 foot wide building. I'm always tempted to overbuild, but on the other hand it's more money and time.

[Jim Becker]

I didn't see a specific answer on making long skids from short lumber, apologies if someone already said this. Build them of from shorter pressure treated 2x, with joints staggered and PL400 adhesive. Minimum of three layers with at least 16" of distance between seams.

I agree with this. In fact, this is the preferred method for the posts in modern post frame construction to be made, the only difference being that in that case, just the lowest portion is made from PT. Laminated skids will be strong, resilient and likely much straighter than trying to source really long ones milled directly from a tree.

[Clark Hussey]

I recently had a Tuff Shed, 16X30 shed built. With a barn style roof. They built the floor with galvanized Steel 2X6s 16” on center. The roof height is 15’5” giving me plenty of storage space

.469689

[Zachary Hoyt]

Jim Becker, that's true, and it's especially hard to get overlength PT lumber since it has to be able to fit in the tank or whatever they use to treat it, I imagine.

Clark Hussey, that looks very nice in there. I've thought about a taller roof like that, and have not yet decided if the potential ability to move the building in the future is a good trade off for not having much upper storage space. Luckily I've got all winter to think before having to decide.

[Clark Hussey]

Zachary, nothing like those lake effect snows you get out there. We have had about a foot here so far and it’s all melted. I love the extra height just to be able to stand projects up and move them.. storage is an added bonus.

[james glenn]

Since you want to keep this long term, I will offer the following:

You should use galvanized bolts rated for pt wood and thru bolt the laminations with the PL400 on the skids. Screws will pull and move eventually with water saturation, not to mention if you move it, the screws are weak in general unless they are structural Timerlocs.

Use pressure treated plywood on the floor as well, it's worth not having to deal with the aggravation later to replace.

Make sure the gravel doesn't have any large stones in it, and make it deep (10inches plus). Pack the gravel hard in 3" lifts as well with a tamper and make it wide enough so the edges don't erode, or use something to keep them in place. You only need a few inches raised above grade though.

I live in NY too, and the frost will move the building trust me. That size will then allow it to sink slowly over time during thawing, so the more gravel and harder it is, the better. I have to lift one of mine every 4-5yrs because I was in a hurry, but it's only for farm tools so doesn't matter.

[Zachary Hoyt]

Thanks, that's helpful. I can see the advantage of bolts. I will have to see if the frost bends or tilts the building. If it just goes up and down I don't mind that. It's all an experiment in some ways, as this is bigger than anything I've built before on this kind of (non)foundation.