A plywood subfloor supports the finished floor on the joists. In areas with high humidity, which will also be hot areas, treating the plywood subfloor will help it last longer.
Wood is a porous substance. In high humidity areas, moisture will enter the plywood subfloor. Over time, it will cause the plywood to come apart and disintegrate. That will mean tearing up the entire floor to replace the subfloor, a lengthy and expensive process.
Where there is high humidity, there can also be mildew. Once the moisture collects in the wood, it can easily turn to mold. By treating the plywood subfloor before it’s laid, you stop moisture from entering the plywood, so it will last much longer.
Treating a plywood subfloor is a step most people forget or ignore, but it’s an important one. An investment of an hour and a few dollars can save a great deal of work later and also save health, as mold can cause illness.
When to Treat
For the best protection, treat the plywood subfloor after cutting but before you put it down. This means the cut edges will be treated, too, preventing moisture from entering any part of the plywood. Allow time for the treating agent to dry in the wood before laying the subfloor.