Hardwood over radiant heat

Q: How can I attach some birch hardwood flooring to a heated concrete floor? The concrete is 6" thick, with 1/2” diameter piping for distributing the hot water. Right now I have 3/4-inch-thick spruce flooring nailed to 1/2-inch thick wood 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 very good heat transfer to the room. There’s one important thing you've got to realize, though. More than the usual amount of seasonal gaps will open up between floorboards when they’re installed over radiant in-floor heating. The added heat from the warm concrete will probably cause larger than usual cracks during winter. As long as that’s okay with you, go for the hardwood.