Screeds Under Hardwood Floors

For the past 75+ years, the most common type of subfloor used in South Texas, for solid hardwoods installed over a concrete slab, is the tar & screed system.
In Houston, solid hardwood floors cannot be installed directly onto a concrete slab. The moisture content in and under the slab is too high, and the solid floor boards will collect that moisture and either cup and buckle, or simply loosen from the concrete.
Therefore, some type of new subfloor must be constructed on the slab, and the solid hardwoods installed over this subfloor. In South Texas, this is typically a tar & screed subfloor.
Buckets of black, oil-based mastic (tar) are brought to the jobsite, and heated with a propane burner outside the home. The hot tar is then poured onto the concrete slab, and spread out to approximately 1/4 inch thickness. Into this hot, sticky mass of tar the screeds are inserted, and the tar allowed to harden around them.
Screeds are short lengths (2 feet) of pine 1×4’s or 2×4’s. Installed properly, these screeds will stay tight in the tar for decades (barring water intrusion from above – flooding or water leaks, etc.). The tar not only holds the screeds in place, but acts as a moisture barrier to prevent moisture penetration from the concrete.
The solid wood flooring is then nailed to these screeds. Thus the solid floor boards are not only protected from moisture by the tar, but are actually not in contact with the concrete, as they are laying on the screeds.