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I'm building a 16' x 7' shed and chicken coop in Seattle. I want it to have a dirt floor and be safe from predators crawling underneath. I've been struggling with how best to do the "foundation". I saw a post where someone built one like the picture below. Basically an on-grade footer 6" wide, 6" deep, that the bottom plates get bolted to. I suppose this would float with any frost heave(?) I like the idea of something like this to prevent critters from burrowing under-neath and it seems easy enough to do. I wouldn't do it directly on grade, but a few inches below.

My frost line is 18" deep. If I do this with some 2x8 molds and put a piece of rebar in the molds do I need to worry about frost heave causing problems? After I dig the trench and place the molds should I put down a few inches of gravel before the pour? Would it help to have the molds a few inches above the bottom of the trench and let a bunch of concrete come out underneath? Any better ideas?

A lot of other people seem to do something like this with CMUs, but that seems like it would settle unevenly. I know I'm probably way over-thinking this given its intended purpose, but want it to last.

Thanks,

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  • Note that in the craft they are referred to as "forms" as opposed to "molds". – Michael Karas Feb 24 '16 at 6:54
  • 6x6 isn't enough. Concrete cracks easily, and that size doesn't have enough cross-sectional area to remain stable afterward, even with rebar. I'd go 12x8, at least. – isherwood Feb 24 '16 at 13:26
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Yes, the poured, floated & sealed afterward slab is the best way to go, if you're up to it, & it's the best anchor. Definitely do at least a 4-inch bed of gravel under everything, this is what moves instead of a slab or blocks or pads.

However, absolutely consult your Building Dept. first & foremost. They have plans for their requirements & you'll need a permit & likely 3 inspections. They have to approve your plan & that approval will hinge on if you can do anymore "paving" on the property, if the shed/coop placement meets setbacks & if the planned structure is sound or even big enough for your vision.

Other than that stuff, going with blocks filled with concrete is pretty solid & much easier. If anchoring the building is done like putting in fence posts then just concrete pads halfway in & halfway out around the perimeter would accomplish the same overall desire & be the easiest by far.

Oh I initially forgot, if the back, friend & truck's good you can possibly use parking lot Wheel Stops (6x6's) for the foundation or more. There shouldn't be any monolithic requirement & they're higher density concrete that's rebarred. It's slightly rednecky, but saves an awful lot of bag hauling, temporary lung damage, form work & drying time...you'll love the pinning holes too.

  • I have used 2X4" and just poured that thickness with wire mesh. At the edges made a small trench aprox 4 inches deeper. that was ~20 years ago and the 100 sqft shed is still there and no cracks all hand mixed. on small hand jobs like this a uniform thickness is the biggest thing to prevent cracking that I have found. the small trench around the outside keeps the edges from cracking (I park my small tractor in the shed). In oregon under 100 square feet no permit is required. Check your local code. – Ed Beal Feb 24 '16 at 14:29
  • I like it & great job! I like the no permit too, around me they only skip the permit if it's a non-vehicle traffic replacement job but all new stuff has to be Impervious Surface Ratio checked. – Iggy Feb 24 '16 at 14:50

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