How deep is the frost line for footers in Michigan? I want to put in a small shed in the back yard. 200 square feet large.
How deep is the frost line for footers in Michigan
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Personally, I would dig the topsoil out from under the blocks—preferably down to mineral soil so that almost all of the concrete is buried. This makes less of a step up for wheeled equipment and less space for critters and weeds to grow underneath. I put mine very close to the ground on concrete blocks with slots for 2 inch wide lumber and concrete screws through the joist into the block to keep the wind from lifting it up:
The kind of concrete block you are using is plenty strong, though I don't see anything to keep your shed from moving, for example from strong wind, heavy rain, or leaning livestock.
The strength of the joists is the next weakest link in this design. Some more support in the middle could be overkill, but it is unlikely to be so.
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.
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Historically, the frost line has been 42" in Leelanau county, in northern Michigan (http://www.leelanau.cc/howdoia.asp?rid=12). But with the severe cold spells in the past couple of years, where even southerly Kalamazoo, frost has reached down 5 feet in some places, MI municipal code officials may adjust that number downward one day soon.