Going to build a nice shed soon. Maybe 10 by 15. Has to go next to a fence row. Walnut, cherry, locust. Big trees and probably can’t dig piers where I want them. Pretty windy, like 50 MPH wind fairly often. Floor will be concrete pavers so its flush and I can drive the mowers in easier. No access for a ready-mix truck, so any concrete has to get mixed from bags.
Was thinking of either heavy timber sills, or poured concrete grade beams. If I do concrete, how big and how much rebar? is 6 by 6 with two #3 rebar big enough? This does not have to meet any codes but I want it to look nice, last a while and not blow away too easy.
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What's your climate like, Bob? Is frost an issue? Is this a well-drained site? Do you have termites?
Al Mollitor, Sharon MA
Yes. Frost depth is 18", plenty of termites, and site is fairly well drained and level. If we get a ton of rain like last year, can get a bit swampy for a couple days. I didn't want to dig a lot and disturb the roots, plus digging would be pretty difficult. I could possibly find a couple spots along each wall for piers, but they wouldn't be spaced very well. There are several 3-foot diameter trees within a couple feet each direction.
If I could get a truck up in there, I'd just pour a floating slab with upturned edges. I've never done a grade beam and have no idea how big it needs to be for reasonable strength and stability. I don't want it crack in a month but don't want to mix 100 bags by hand either. This will be a nice shed, dormers, window boxes, a little porch.
With trees that big so close to the structure, I'd be concerned that the growth of big roots and wind movement might disturb a beam (concrete or wood) on the surface. If you have to be that close, it might be a good idea to dig holes to solid mineral soil 2 feet down. Alternatively, clean up the area and put down a nice bed of gravel to float everything on, but you'd still want a truck for that. To keep things dry and termite-free, the foundation should raise any wood at leats 8" off the ground.
Al Mollitor, Sharon MA
A lot of good thoughts. MAybe I'll get out a hoe and see what the root situation is right where I want the shed. I could compromise and backfill a trench with rock up to grade, then pour on top of that. Could manage to find a couple places for screw-in ground anchors and cast the ends into the beam. Keep it from blowing over and reduce the size of the beams a bit. Maybe 8" tall by 6 wide. Still a lot of wheelbarrow work, but possible. The trees belong to the neighbor. If the shed heaves a little, that's life. If I do something that hurts the tree and the neighbor can point fingers at me when it falls, thats not so good. I could do a floor with joists and make the whole thing like a sled, but it would be too high off the ground. Also have to be at least 2x decking to hold a ton or two of lawn equipment. I like the idea of having the floor right on grade.
A lot of older homes were built similarly to what I described all over Florida and much of the south. Mostly minus the anchors. House are a bit heavier per unit volume. Up off the ground helps with rising damp, water tables in the rainy season, bugs and other critters. It also makes a lot of the homes movable. Ones built on slabs pretty much have to stay where you plant them and if the water rises they get wet. Besides a little bit off the ground gives somewhere for the dogs and kids to hide in the heat of the summer.
Round here, Florida, such foundations are pretty common. No freeze problems though. At least not yet. You never know given the general level of abuse to the environment being meted out.
Swamps and clay swell are more an issue. Usually these sheds are built on skids or piers and the building, sometimes with the help of a loader, has to be dragged to a flatter, drier spot or the piers reset. The later being relatively easy as these are just just concrete blocks or tetrahedrons placed on small beds of gravel dug in just a few inches.
To keep the whole thing from going walk about in a wind storm large screw in anchors, same ones used for mobile homes, are put in and the entire structure tied down and anchored to eliminate any sliding. This is best planned into the structure so that reinforcement straps are hidden and anchor placement is not unsightly or inconvenient. One guy had one foot square sections of the interior floor in each corner removable and straps in place to allow the anchors to be put in under the structure and tensioned.
For best results these anchors need to be screwed in for their full length, 2' to 6' depending on soil conditions and loads, not post holed into place and the dirt returned. Screwed into undisturbed soil and top plates hammered in these units can resist thousands of pound of uplift and sliding force.