Q: When building a grade-level deck, is it necessary to remove the sod underneath the deck’s footprint prior to construction? Are there any other special considerations for prepping the site?
A: Mike Guertin, a Rhode Island contractor, frequent contributor to PDB, and author of “Ground Rules for Grade-Level Decks,” responds: I can’t think of any requirement that the grass should be removed from beneath a deck, but it’s always a good idea to follow the installation recommendations from the manufacturers of the products that you will be using. For example, some brands of composite decking require a certain clearance above the ground in order to provide airflow beneath the deck. In some cases, it may be necessary to remove the sod in order to provide that clearance.
In addition, some pressure-treated lumber that is used to frame decks is rated for ground contact, while some is rated for above-ground use only. You can tell the difference by checking the end tag stapled to each piece of PT lumber, which indicates the particular preservative that’s been used during treatment, the preservative retention level, and the AWPA-certified use category. PT lumber that is approved for ground contact will have an AWPA UC4A tag, while PT framing lumber that is only approved for above-ground use will have a UC3B tag and require a minimum 6-inch ground clearance.
If the decking and framing meet these clearance recommendations, I don’t think that leaving the sod in place is a problem. Sure, some grass might grow up through the gaps between decking boards, but that could happen even if the sod is removed and some weed seeds started growing. And removing the grass won’t appreciably lower the moisture content underneath the deck, as airflow tends to equalize humidity levels between the space beneath the deck joists and the atmosphere above.
Will leaving the grass make the space more appealing to mice, rats, chipmunks, and other critters? Not in my experience, but that might be a consideration in your area. Other things to consider below a grade-level deck are the grade itself (be sure there is positive drainage away from the building), exhaust ports (if any) for gas-fired equipment, and emergency egress, which needs to be maintained with either headroom beneath the deck or a hatch to the top of the deck surface.