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 2×8 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,
Best Answer
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.