Concrete – gravel under concrete slab? is it absolutely required


so, I have a post-frame shed, 12×16. there are posts at 6' on the short sides and every 4' on the long side. posts are set ~18" below grade, on a 2" cap of cement over a few inches of well compacted gravel, holes were all about 27" or so, 2×6 girder runs level at the top (very level all the way around) around base,
ok, so, basically I have ~5.5" to grade from bottom of 2×6 girder to top of board, and the Ive got posts set in concrete, at 24" from the top of that board
so I want to fill that in with concrete, I could put gravel in, say 2" and 3.5" of concrete over that.

Its about 3.75 cu yards of crete, or like $400
It will cost $100 for gravel to fill in 2", but my crete is still only a little cheaper, and it will be a thinner slab.

I could put gravel in, say 2" and 3.5" of concrete over that
or just full 5.5" of crete
or I could do lot of gravel and a 2" top coat over that with a bunch of steel and fibers,

Its just a shed, so Im not too worried about it, I'll put steel mesh in and probably fiberglass to control cracks and run a groover down the center.

Point is, can I get away without using gravel and just lay plastic down and pour a slab, or is the clay soil going to destroy it if I dont have a couple inches of gravel. I am not, going to dig this down deeper, or pour higher that the girder.

Its 5.5" of space, with a few low spots I can rake out. It much faster to do a one day pour so I can put my books back in and my tools to keep them dry, so full 5.5" concrete is good and fast, but a couple inches of gravel would be nice, but is a total extra set of work for little gain

(yes, I should have done this in a different order, it was not an option at the time)

Best Answer

Is gravel absolutely required NO but it is a good idea. Most the time i see slabs poured directly on earth the owners are doing it themselves and the base is not level or has holes. If a slab is not a uniform thickness this can cause cracks even with reinforcement of fiberglass, mesh or rebar.