You may never see the plywood that lies underneath your flooring, but the type you choose can make a big difference. Plywood is a popular subfloor material for laying down carpeting, tile, hardwood and other types of flooring. In some cases plywood is used as an underlayment and placed on on top of the subfloor to create a smooth durable surface for installing the final flooring material. As a result, a floor might have three or four layers -- the concrete foundation, a plywood substrate, a plywood underlayment and, finally, the surface flooring material (carpeting, tile, hardwood, etc.).
Plywood panels are given two alphabetical ratings from A to D. The first letter rates the veneer quality of the front facing side of the panel, and the second letter rates the quality of the back. The letter "A" is the highest grade available and is used for panels that are free of small knots and defects. The letter "B" is the second highest grade, and it is given to panels that might have small knots or nicks. A "C" grade might have open knots, holes, splits or discolorations. And a "D" grade panel might have multiple knots. However, a panel might have an "A" face and a "D" back. The letter "X" means that the panel was manufactured with scrap wood for the center plies.
Interior plywood is a good subflooring choice for rooms that do not get a lot of exposure to water and moisture. These would include rooms such as bedrooms, dining rooms and hallways. Interior plywood is also a good subflooring choice for carpeting, tiles, vinyl, engineered wood laminate flooring and hardwood flooring. Common grades for interior plywood are A-C and B-D.
Exterior plywood is a better choice for rooms that do get a lot of exposure to moisture. Even though it is called "exterior plywood", it has interior applications for use in rooms such as kitchens, bathrooms and basements. Unlike interior plywood, the glues and adhesives used to bond the wood veneer layers together are stronger. This makes the wood hold up to moisture better and helps prevent warping. The most common grades for exterior plywood are A-C, B-C and CDX.
Structural plywood is extra strong and is only surpassed by Marine plywood in strength and durability. This is the type of plywood that works best in rooms that have heavy furniture, including living rooms, bedrooms and dining rooms. It also works well in high-traffic areas such as entryways and upstairs hallways. Structural plywood has a rough texture and is usually available in grades C-D or D-D.
Marine plywood is a good choice for bathrooms and basements. It is the strongest and toughest of all plywood on the market. Marine plywood is bonded with high-quality glues to make the plies structurally solid and to resist moisture. It is usually only available in A-A grade. This means that you can use either side to show on the surface. Because of its strength and water resistance properties, marine plywood is a very good choice as a substrate material for laying hardwood, engineered flooring and tiles.