Garden shed foundation
Trevor_Lambert
| Posted in General Questions on
My wife has purchased a 12×12′ wooden garden shed, including assembly on-site. We’re responsible for ground preparation, which they specified to be removal of top soil and 3″ of levelled, crushed stone.
It seems to me this leaves it susceptible to frost heaving, so instead I was going to pour four concrete piles at the corners, down to below the frost line, 42-48″. The shed builder thinks the piles actually create the frost heaving problem, whereas just plopping it on crushed stone won’t have any issues. This makes no sense to me. Who’s right here? This is in the cold edge of zone 5, if it matters.
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It's common for these structures to sit on grade and the risk of the ground moving isn't really too much of a problem for the structure.
If it's not meant to be supported only on 4 corners, your footing work might not be useful. For example, does it just have joists that sit directly on the crushed stone? If so, the designer is expecting the support to be uniformly distributed, not only at the corners.
Seems likely this would be sitting on "skids" and needing support all around. I would want the gravel base and not bother with any digging.
In either case, the shed is resting on skids. In the case of the crushed stone base, those skids would be on concrete deck piers. The piles would replace the piers.
So the 'skids' are load beams? Carrying the 12' load shed? Like a girder?
I usually think of skids as resting on grade or being supported every couple feet. If you are redirecting all the load to only the 4 corners (which seems unconventional and a bit odd) and only touching ground at those spots, I think your take on the frost issue would be accurate. If the skids touch ground between the piles/piers though you could have differential movement which would be worse than just letting the whole unit rise/fall with the frost.
I build a 12x16 garden shed several years ago in Maine. I used PT framing and just put it on a dozen 4" thick solid concrete blocks, each on a few inches of crushed stone. I didn't and don't care if frost moves it a little, but as far as I can tell, it hasn't.
In most location any building without a foundation is consider temporary and is not taxed and not subject to most building codes.
If you want water, electric or concrete it can become very complex.
If you want this building to avoid rotting away you should elevate it above the gravel bed you described by 8 inches or so. This is because when rain water hits the ground it will splash back onto the building getting it wet every time it rains. If you elevate the building, you will need a ramp or steps to enter.
Preparing the bed, you described is often a bigger job than one might imagine.
The location you chouse may appear flat and level to your eye but it is almost certainly not as the rain water does not puddle there. You need put a level on an 8-foot straight board to know but my wild guess is one corner will be 6 inches above the lowest corner. My wild guess is you will hall away 12-15 full wheelbarrows of soil. All of that will be replaced with gravel.
You can buy piers like this photo at one of the box stores.
Ground movement is only a problem if the shed is somehow attached to something that does not move like a house.
Walta
I'd go with a stone base for a building that size. Preferably in a spot or prepared so that it has some drainage away from the building. If there's ever any intolerable building movement it should be a quick fix with a jack and some shim.
I'm not sure about the critter situation in your area, but they love it under sheds. It may be worth considering some critter prevention options (stainless mesh?)
You are correct, piers that extend below the frost line should not move when the ground freezes. I say "should" because frost can actually extend lower than the nominal frost line, soils can be expansive, and frost can grip the side of the pier, especially if it breaks due to lack of reinforcement. But in general, a shed on piers will not move seasonally, while a shed on pads will move with frost.
I do not think anyone of us can tell from afar what is best in your location. Check with your building official about what they require. Also what is their experience with this type of building. Get some clients from the builder in your area that you can give you their experiences.
A lot depends on your soil. Does it drain or hold water. Understand how the building is designed. Specifically wood grades and supporting structure.