Moisture Resistance: For areas prone to moisture, such as bathrooms or basements, it is crucial to use plywood with higher moisture resistance. Marine-grade plywood or pressure-treated plywood can be excellent choices in these scenarios.
You should not use pressure treated plywood in your bathroom because: it is usually cdx grade, it doesn't lay truly flat, and the high moisture content makes it unstable. Your bathroom subfloor shouldnt qualify as a damp environment. Most likely your toilet wax ring is leaking under the finished floor.
Plywood is suitable for bathrooms — however pressure treated 3/4″ plywood subfloor is the best subfloor material for bathrooms. Pressure treated plywood is water resistant and will withstand the humid, damp bathroom environment much better than untreated plywood or OSB.
Cement board is best suited for: Ceramic and Porcelain Tile: Cement board is specifically designed to be a moisture-resistant substrate for tile in high-moisture areas like bathrooms and kitchens.
Using marine grade plywood can offer a different aesthetic than the standard materials used in bathrooms. It will also withstand the high moisture environment that comes from the shower and sink.
Pressure treated plywood can be used on its own as it is water resistant and will withstand the humid temperatures that are common in bathrooms.
The National Construction Code (NCC) requires all wet areas in a home, including bathrooms and laundries, to be waterproofed.
Both plywood and OSB are effective choices for your subfloor needs. Plywood is a better option for projects that require a lot of cutting and molding, while OSB is a good option for those who need to cover areas where moisture exposure may be an issue.
The best flooring options for bathrooms are floors that can withstand moisture and humidity, aren't slippery when wet, are easy to clean, and comfortable underfoot. In terms of withstanding moisture and humidity, luxury vinyl tiles (LVT), vinyl roll, and porcelain or ceramic tile are the best bathroom flooring options.
To waterproof plywood, an epoxy, waterproof paint, oil, varnish, latex or PVA can seal the wood for both indoor and outdoor use. These can come in either a paint-on or spray form, and they all bond easily to the wood to seal it and protect it from moisture. Simply sand down, apply and leave to dry.
Mostly concrete subfloors transmit moisture. Concrete is porous and allows moisture to pass through to your basement/room level. So if you have a concrete slab/subfloor, then read on. If you don't, you won't have to worry about a separate moisture barrier as plywood subfloor doesn't transmit that much moisture.
Flooring grade, 15-18mm plywood should be fixed to the existing subfloor in a brick bond or staggered fashion so the joints are not uniform, screwed down at 300mm centres, ensuring the screw heads are flush with the boards surface.
For subfloors, CDX plywood is often recommended. The “C” and “D” grades refer to the front and back faces of the plywood, respectively, with “X” indicating that the plywood is intended for exterior use and has some moisture resistance.
Direct Contact with a Source of Moisture
Use pressure treated wood in any situation where there's direct contact between the wood and anything that could supply moisture.
Sealant is a popular choice for waterproofing wood, offering long-lasting protection by forming a waterproof barrier on the wood's surface. Sealant is one of the closest options for how to permanently waterproof wood. Unlike oil, sealants provide both strength and protection.
Underlayment is the material placed on top of the subfloor that assists in the installation of the tile or other type of flooring.
You can seal a subfloor using a sealant specifically designed for subfloors, such as a penetrating sealer or a moisture barrier. Penetrating sealers are typically made from a wax or oil-based formula that soaks into the wood, protecting it from moisture and rot.
Assuming your subfloor is in good shape and level, your next step is waterproofing. Use a layer of unmodified thin-set mortar on your substrate, then add an uncoupling membrane, like Schluter®-DITRA & DITRA-XL. Then add another layer of thin-set mortar on top of the membrane, and finally, you place your tiles.
OSB can be manufactured into panels that are larger than plywood. OSB is more uniform, so there are fewer soft spots, such as those that can occur in plywood. OSB is less expensive than plywood. To build a typical 2,400-square foot home, OSB may cost $700 less than plywood.
Specifically, ceramic and porcelain tiles are great options for bathrooms. Tile flooring is resilient, waterproof, available in a variety of colors and designs, and generally less expensive than other hard surface options. Porcelain tiles absorb less water than any other ceramic product.
Subfloor Material
Therefore, because concrete is porous, it's a good idea to install a vapor barrier over your concrete subfloor before installing your flooring. This way you can stop the vapor drive of water upward to your floors, protecting them from any damage (source).
In general, you need to apply three layers of waterproofing product which can be waterproofing polyurethane or resin. Remember, it is extremely important to make sure every layer is absolutely dry before you apply the next one. Make sure to allow enough time for every layer to cure.