Q. I have a client who wants me to build a 30x40-foot unheated barn-garage with a slab floor. The area where the building will sit is very flat, with no lower terrain nearby, so it's not feasible to build a typical 4-foot stem wall with a daylight drain. Frost depth is 48 inches around here (northern Vermont). I wonder if a shallow frost-protected slab would work in this application. Do I need to thicken the edges? And do I need a rebar grid in the slab?