Q. Before installing a thin concrete or gypcrete floor over a plywood subfloor, what kind of membrane should be installed over the plywood?

A.Doug Mossbrook, president of Eagle Mountain HVAC, responds: None. When gypcrete or concrete underlayment is applied over a standard plywood or OSB subfloor, I don’t recommend using a membrane, which would prohibit the underlayment from adhering to the subfloor.

As the underlayment is poured, it tends to bond to the small cracks and crevices in the surface, providing an anchor. The bond allows the underlayment to become an integral part of the floor system, strengthening the underlayment and keeping it from cracking and shifting.

The only product we apply over the plywood subfloor is a water-based latex primer. The primer keeps the plywood from absorbing water from the underlayment, which might cause it to dry too rapidly, lowering its hardness and strength.