How Flat Does a Subgrade Need to Be?

Q. When you're preparing the subgrade for an insulated slab foundation, how flat does the surface need to be before you lay out the rigid foam? Is "pretty flat" good enough, or do you have to eliminate every minor hollow and high spot?

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 a finer, more evenly graded material for the topmost few inches.

If you're using 2 inches of foam or less, that tamp-and-scrape approach will probably work fine, because the weight of the fresh concrete will press the foam down tightly enough to eliminate any minor voids. Slabs enclosed by a frost wall are also a little more forgiving, because there's generally no structural loading on the slab.

But with thicker layers of foam, any long-term creeping or settling of the foam after the slab has cured could allow the slab to crack or cause other structural problems. We take that possibility seriously, because we often build on slabs-on-grade in a climate that may call for 5 inches or more of foam under the slab.

Rather than spending a lot of time trying to level a subslab by trial and error, I now prefer to top the compacted subgrade with a reliably flat and level 1 1/2-inch layer of flowable fill. This involves ripping 2x4s down to 2x2s, staking them in position around the perimeter of the slab and in rows that divide the area into 10-foot-wide sections, and leveling them with a laser level. The flowable fill is a thin concrete mix with a sand aggregate and an aerating additive that gives the material a foamy quality. It's about half the price of regular concrete and can be easily screeded to a perfectly flat surface with a 12-foot magnesium screed. Once the flowable fill has cured for a few days, the slab itself can be insulated, formed, and poured as usual.