Q:

I am planning to install 3/4-in. quartersawn maple flooring over a radiant-floor heating system. I ran 2×4 sleepers on the flat every 18 in. over the 3/4-in. plywood subfloor and poured a 11/2-in.- thick radiant slab between the sleepers. Can I nail the maple directly to the sleepers? Do I need a moisture retarder?

Michael Amos, Fayetteville, AR

A:

Charles Peterson, a hardwood-flooring expert and author of the upcoming book The Art of Wood Flooring (The Taunton Press, 2009), replies: You got one thing right: Quartersawn or engineered wood is the best choice over radiant-floor heat. But I have to caution you against using maple here. Wood-flooring experts widely agree that maple is a poor choice for radiant applications because it is extremely sensitive to moisture changes. I installed quartersawn maple, along with several other woods, in custom-made medallions in my own home. Of all the woods I used, the maple performed the worst over radiant heat.

That said, the 2×4 sleeper spacing you mentioned won’t be nearly adequate. The industry standard sets the ideal spacing at 6 in. on center, never to exceed 10 in. It sounds close, but this spacing is necessary to nail the flooring adequately to minimize movement over radiant heat.

If you’re stuck with spacing that exceeds 10 in., you need to install a layer of 3/4-in. plywood over the sleepers. Fasten the plywood to the sleepers with 11/2-in. galvanized ring-shank nails, and trowel on MVP (www.bostik-us.com) moisture retarder. Don’t use an asphalt-based building paper as a vapor retarder, however. It can produce an asphalt smell when used over radiant-floor heat. When you install the flooring, nail it to the plywood every 8 in. to 10 in. and within 3 in. of the board ends.