A subfloor is the structural layer of a building’s floor system, providing a stable platform over the floor joists for the finish flooring. This layer is typically made from engineered wood panels like plywood or Oriented Strand Board (OSB) rated for structural performance. Pressure-treated plywood is wood infused with chemical preservatives, often copper-based, using a high-pressure process. This treatment enhances the wood’s resistance to decay, rot, and insect damage. The question of using this material for a subfloor involves understanding its structural performance and chemical composition in an interior environment.
Suitability of Treated Plywood for Subfloors
For standard indoor construction areas, pressure-treated plywood is generally not the preferred choice for a subfloor. Standard construction practices recommend using structural wood panels rated specifically for subflooring applications, such as APA Rated Sturd-I-Floor or CDX plywood, which are designed for dimensional stability.
Dimensional stability prevents movement that can compromise the finished floor installation. Untreated engineered panels are manufactured to specific moisture content levels that minimize shrinking and swelling. Using standard, non-treated materials also helps manage project costs and simplifies the installation process. Structural panels of at least 3/4-inch thickness with tongue-and-groove edges are typically used to create a strong, continuous surface.
Key Drawbacks of Pressure-Treated Materials
One significant issue with using treated plywood as a subfloor is its high initial moisture content and dimensional instability. The pressure-treatment process saturates the wood with liquid preservatives, causing the panel to swell significantly. When this damp wood is installed and covered, it cannot dry out easily, leading to excessive shrinking and warping as it acclimates. This movement creates an uneven surface that can cause squeaks in wood floors or lead to failure in rigid finishes like tile, potentially cracking the grout lines or the tiles themselves.
The chemical composition of modern preservatives, which often contain copper compounds like Alkaline Copper Quaternary (ACQ) or Copper Azole (CA), presents a major corrosion risk. These chemicals are highly corrosive to standard steel fasteners, including common nails and screws. Using standard fasteners with treated plywood can lead to premature structural failure as the metal rapidly degrades. Builders must instead use specialized, more expensive fasteners, such as hot-dip galvanized or stainless steel, to prevent this chemical reaction.
Finished flooring materials, especially those relying on a rigid bond, are negatively affected by the presence of the chemicals and moisture. While some modern construction adhesives are formulated to tolerate the dampness of treated lumber, bonding rigid finishes remains problematic. The chemicals in the wood can also interfere with the curing process of some adhesives and sealants. For ceramic or stone tile, the movement of the damp wood makes it difficult for thin-set mortar to maintain a long-term bond, often requiring the addition of a cement backer board.
Specialized Applications Requiring Treatment
Despite the drawbacks for general interior use, treated plywood and lumber are legally mandated for specific applications where the need for rot and insect resistance outweighs the stability concerns. Building codes, such as the International Residential Code (IRC), require the use of preservative-treated wood in areas exposed to high levels of moisture or in direct contact with concrete or earth.
One common scenario is the installation of floor systems directly over concrete slabs, such as in basements or lower-level rooms, often utilizing a sleeper system. In these cases, the wood is highly susceptible to moisture wicking up from the slab, making treated material necessary to prevent mold and rot that would quickly destroy untreated lumber. Similarly, applications in damp crawl spaces or as sill plates, where the wood sits directly on the foundation, require the use of treated material.