Foundation for pre-built coop - treated lumber vs gravel - northern Wisconsin

fanofdmb84

Chirping
8 Years
Jul 2, 2017
12
10
94
NE Wisconsin
I'm planning on buying a coop from a builder about an hour away. They will deliver it. They recommend a gravel pad to set it atop. After doing some research, the 'best practice' seems to be to build a frame of treated wood, level, then add 4-6" gravel. Easier than a slab, but... it means a lot of digging by hand, a good chance of killing nearby trees by hitting roots, and possibly hauling gravel by the bucket-full, renting a compactor, etc etc, because we don't have any sort of heavy farm equipment. Our site is situated towards the center of a square 5 acre parcel with lots of young trees that I would hate to mash into oblivion driving a dump truck through the yard.

Anyway. Let's say I'm comfortable with perhaps a little less than "best" and I just toss down a 4" layer of gravel and level it out and call it a day. Or further yet, plop the thing directly on the ground. It has treated lumber joists and our soil is mostly compacted sand and all the weed roots holding it together. A little bit of topsoil but generally very dry even in the spring thaw. It's pretty flat already - was an old potato field 35+ years ago.

Would I be making a monumental mistake cheaping out on the site prep? Will the treated lumber joists rot away being in contact with the ground? I know the pressure-treated wood is supposed to last longer, but what exactly is "longer"?

For reference, the coop is 10x10 and is a ground-level, walk-in type. Once it's down it's unlikely we'll be able to move it.

Thank you for humoring my over-thinking.
 
Can you post pictures of the site?

What is the floor of the coop made of? If it's sitting on gravel, moisture will still rise from the ground below.
 
The floor is 3/4" plywood but there are floor joists below it. This roughly where it will be located (where the dogs are in the 2nd Pic)
 

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I can tell you what we did for a 8x12 shed we are using as a llama run-in shed, built from a shed kit. Bottom of floor is 2x4 NOT PT joists running across the shed (8’ direction). Following one of the suggested foundations for the shed, we got 3 12’ 4x6s (that’s an upgrade we made, the instructions called for two 4x4s but we used 3 to shorten the span/stiffen the floor and the larger size because it’s what was available in the needed length) and laid them down, one centered side to side under the shed and the other two 2/3 of the way to each edge. Where necessary because the ground had a bit of slope diagonal to the shed we put cinderblocks under the front ends. It has stayed stable for two years now.

Now our soil and topography is different than yours. Like you we can get significant depth of frost (traditionally plan on 4 to 4.5 feet to stay below though winters are trending significantly warmer). But this is undisturbed forest soil, in a flattish spot on a hillside, with glacial sand/gravel under what is probably a thin layer of humus. I figure it should be quite stable even as frost goes into the ground.
 
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I just made a correction in the post above: directions for shed foundation were two 4 x 4s, not 2 x 4s!

Which as written I upgraded to three spaced across its 8’ width.

Here is a photo of the shed two years ago when it was newly erected.

68937651834__730BB5A3-67BC-45CD-86F0-6AF896194BF3.jpeg
 
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