Q: What is the best method to deal with a wavy subfloor?
We stripped the carpeting out of several rooms in our house in preparation for laying engineered wood flooring. We were horrified to discover that the subfloor—½-inch particleboard over ½-inch plywood—is nowhere near flat. It has hills that are easily 1 inch high. We've been told just to cover the whole mess with ¼-inch plywood to provide a new, flat surface. But we've also heard that we might be able to seal the particleboard and then use leveling compound to even out the bumps. Which is the best method?
— Shawn Elizabeth, St. Louis, MO
Tom Silva replies: Neither solution makes much sense to me: They just conceal the problem, which will probably resurface later on. Particleboard tends to be sensitive to moisture, so I'm not surprised that your floor behaved the way it did. But covering warped particleboard with plywood or sealing the top surface won't keep moisture away from the underside. Frankly, I don't think ½-inch particleboard has much merit as a subfloor material, even though it's commonly used as an inexpensive underlayment beneath carpeting.
The real solution here is to remove all the particleboard. Then you can cover the existing,½-inch plywood subfloor with builder's felt or rosin paper and fasten a new layer of ½-inch AC plywood specifically made for use as an underlayment, with the best side facing up. Make sure to offset all seams between the two layers and leave a 1/8-inch gap between panels on all four sides to allow for expansion. The felt or rosin paper will prevent the two layers of plywood from rubbing against each other, causing squeaks. Once your subfloor is flat and solid, lay the engineered flooring according to the manufacturer's instructions.