Concrete foundation for shed

Old 09-13-19, 03:10 PM
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Concrete foundation for shed

I'm pouring a slab for a 12x16 shed. My question is on the sill plate and foundation. If I pour the slab inside a 4x4 form (leveled and squared to exactly 12x16 on the outside of the form, can I just leave the 4x4s in place and nail the sill plate into the 4x4s? I would reinforce the corners and put some lag screws in horizontally before the pour to keep them snug against the concrete. I'm trying to avoid drilling into the concrete to bolt the sill plate to the concrete.
Any thoughts on whether this will work?
Old 09-13-19, 04:34 PM
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Welcome. Of course you can, it's your shed, but you're trying to reinvent the past however many years of conventional wisdom, and that's not always the best approach. The first problem you will have is finding 4x4's that are going to be as straight as conventional forming, and not that they can't be had but they would be through an independent lumberyard, not a big box. Secondly, concrete shrinks as it cures, not that you would otherwise would notice on that size slab, but enough that your forms aren't going to be as tight as you're thinking. And lastly, so now you're going to set the walls on top of the 4x4's, so the only thing carrying the load of the structure are the screws you put into the side of the pour. No, pour the slab, and either borrow or rent a hammer drill to anchor the walls, or figure where the studs will land and sink some J bolts in the concrete to anchor the walls. Trust me, you'll be much happier.
Old 09-13-19, 04:44 PM
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No, the shed should be built on top of the concrete not the 4x4 that were used to form the slab, eventually they will rot away!

Actually, you should build a block wall on top ot the slab, 6 or 8" high to get the wood walls off the ground.

You should have added anchors into the concrete to hold down the walls but if not then you can drill holes into the slab and add expansion bolts.

That is how I did my shed this summer, it was an existing slab from a prior swim spa!
Old 09-13-19, 07:08 PM
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I agree. Its not a good idea. You need to use 2X4's if you're wanting a 4" (3 1/2") slab. Pour the slab & while the concrete is still soft enough, put anchor bolts in the concrete along the edge so they will go through the bottom plate of the wall. Then just put on nuts & washers.
If you're going to finish the inside of the shed, your 4X8 sheets wont fit on the walls if you dont set the walls on top of the slab. Your walls are going to be 1 3/4" short.
I could go on, but I think you see there are many reasons not to do it the way you asked about.

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