Hardwood over radiant heated concrete

Q: How can I attach some birch hardwood flooring to a heated concrete floor? The floor is 6" thick, with 1/2” diameter piping for distributing the hot water set 3” below the surface. Right now I have 3/4-inch-thick spruce flooring nailed to 1/2-inch thick wooden strapping. I’d prefer direct contact between wood & concrete, with no air space like I have now.

A: There’s a much better choice for your flooring than what you've got, and it’s probably not what you imagined. Some people might suggest that you glue your new wood flooring to the concrete directly, but I'd be concerned that the glue wouldn’t hold properly long term, especially given the heat involved. Instead, fasten sheets of 5/8-inch-thick plywood to the concrete with Tapcon screws driven into predrilled holes in the concrete every six inches in each direction. Use construction adhesive underneath the ply, too. Next, fasten your flooring to this plywood in the usual way, using finishing nails driven through the tongue of each board at an angle. You'll have more than an inch of wood above your heated concrete with this system, but from what I've seen and heard this won't be a problem. You'll still get pretty good heat transfer to the room. Just understand that solid hardwood sometimes develops board-to-board gaps when used over a heated floor. Engineered hardwood flooring is more stable because of its laminated internal construction, and is a safer choice for a situation like yours.