A.Alan Gibson, co-owner of G-O Logic Homes, a design-build company in Belfast, Maine, responds: The usual approach to preparing the subgrade for an insulated slab is to build it up in a series of 6-inch lifts of material, tamping each with a plate compactor or vibratory roller. After checking the resulting surface with a long straightedge, you can use a rake to shave off any high areas and move the material into the low spots, then compact it again as needed.
How successful you are in doing that will depend largely on the fill you have to work with. Fine sandy material is easy to scrape smooth but may not compact hard enough. A bank-run gravel may compact extremely well but tends to resist accurate leveling because of the larger stones it contains; if that's what you have to work with, you should use...
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