Plywood is composed of layers of wood glued together, and it's used often in home and commercial roof assemblies. "Fire rated" and "fire retardant" are two different classifications, so it's important to know the distinctions. Fire rated means the plywood is pressure-treated to prevent fire ignition over a period of time--30 or 90 minutes. Fire-retardant materials will catch fire but are treated to slow down the spread of flames and limit smoke. Quality is rated by the American Plywood Association (APA), which is accepted by most building codes, so look for the APA stamp when buying plywood.
Purchase an aqueous chemical solution for treating plywood. The formula for the solution depends on the rating, thickness and composition of the plywood, so chemical treatments are made individually for consumers.
Hire a company that specialises in pressure-treating wood. The liquid chemical solution needs to be infused into the plywood with high pressure and at a high temperature to be considered fire rated.
Dry the plywood in a kiln before using it in projects, especially construction that must meet building codes. The pressure-treating company will probably take care of this step, but it's a good idea to ask about it first.
Purchase fire-retardant chemicals for home projects that aren't required to be fire rated. You can apply these chemicals yourself using a paint roller or brush.
Apply one coat of the chemical solution and let it dry. Repeat this step twice more for a total of three coats. Make sure the plywood is completely dry before you use it.
You can purchase plywood that's already fire rated from a number of lumber companies and building suppliers.
If you're applying fire-retardant chemicals, make sure you do it outside or in a well-ventilated room.