Maple flooring over radiant heat?

I am planning to install 3/4-in. quartersawn maple flooring over a radiant-floor heating system. I ran 2x4 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?

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 2x4 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 the sleeper spacing is greater than 10 in. on center, install an additional layer of 3/4-in. plywood to broaden the nailing area. Flooring can be nailed directly to sleepers as long as they are spaced 6 in. to 10 in. on center.

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.