Question: Can an underlayment be installed over a wood subfloor?
Answer: Yes. There’s more prep work than with a new concrete subfloor and, even then, it’s common for cement-based toppings installed over wood to exhibit minor cracking. However, this usually isn’t a concern because the underlayment will be covered with another flooring material.
First, to increase flexibility, replace a percentage of the water in the underlayment mix with a liquid acrylic modifier.
Ideally, to decrease deflection and add stability, the underlayment should be placed over ¾-inch plywood that’s fastened with screws every 12 to 16 inches.
For even better rigidity, some manufacturers recommend applying the underlayment over two layers of ¾-inch plywood. Once the plywood is properly secured, applying bonding primer to the substrate seals off the wood.
When the primer has dried, attach galvanized metal lath to the subfloor as a reinforcing material. I recommend screwing the lath down perpendicular to the direction of the plywood roughly on 6-inch centers, overlapping the sheets approximately 2 inches. Be sure to use enough fasteners to ensure flat, uniform installation.
Some manufacturers suggest applying the underlayment on top of the second metal lath, but some recommend first applying a repair-type mortar as a body coat over the lath. I like to use a polymer additive combined with cement, which provides some flexibility. Make sure whatever product you use is approved by the underlayment manufacturer.
After the repair mortar cures (if you use it), install the underlayment to provide a smooth, level surface. When placing an underlayment over metal lath, be sure it’s at least ½ inch thick so the lath pattern doesn’t ghost through to the surface.
If you’re going over an elevated wood subfloor that requires building thicker lifts, consider bringing in a structural engineer to make sure the floor will support the additional weight.
Priming the substrate
Manufacturers may recommend priming the plywood subfloor to help seal it and reduce the chance for air vapor emitted from the substrate to form bubbles in the underlayment. Some manufacturers require a different primer over wood than concrete, so check the technical guidelines on primer recommendations.
If you’re applying the underlayment to a wood subfloor in an area susceptible to moisture exposure, such as a food prep facility, apply a flexible waterproofing membrane over the plywood to provide additional protection. A variety of waterproofing systems are available.
I’ve had good success using one that consists of pure acrylic and cement mixed at a one-to-one ratio. After mixing, I trowel the material onto the substrate and embed a fabric mesh in it.
