Concrete is an entirely appropriate base for solid wood (and engineered wood) flooring, provided you have a separating layer between the finish flooring and the concrete. If you have any kind of known “issues” with the concrete slab (moisture, out of level, etc.), you’ll probably want to install a system of sleepers that separate your underlayment from the concrete. But first we have to ask…
Does Your Concrete Slab Have a Vapor Barrier?
Moisture is a problem with any kind of concrete slab. While concrete may seem impermeable and fortress-like, moisture does wick upward and can ruin your flooring. If poured correctly, you will have a vapor barrier above the sand/gravel bed and below the concrete slab, like so:
Install Buffer System
A better bet than solid hardwood is engineered wood flooring. This has better dimensional stability in case of moisture problems.
If your slab has a vapor barrier, then bond down 1″x2″ buffers (sleepers or whatever you like to call them) directly on the concrete.
Make sure that your concrete is absolutely dry before trying to bond the buffer boards down.
These buffer/sleepers essentially are another system of “joists” for your underlayment to rest on. Since you’ve got concrete slab–and not a wooden raised foundation–these become your “joists.” Install the sleepers in a network so that they are no more than 12 inches apart. Make sure that they rest perpendicular to your intended finish flooring direction.
Extra Moisture Protection
Whether or not your slab has a vapor barrier, it’s a good idea to give yourself a vapor barrier of 6-mil or greater poly plastic. You can lay this vapor barrier over the sleepers, installing the underlayment on top. Or you can install the vapor barrier below the sleepers, bonding it directly to the concrete.