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Does a Subfloor Need to Be Pressure Treated?

A subfloor is a wooden structure created to support durable finished flooring.  It is the foundation below which finish flooring materials are attached. The subfloor offers a continuous structural surface to attach the floor joists. 

Pressure-treated wood is used to build sections that need protection from moisture, mold, and decay, but can’t be painted. Plywood is currently the most common subfloor material in use today as well as the preferred material in damp floors, decks, outdoor furniture, and outdoor building projects.

Pressured-treated wood can be plywood, railings, timbers, and lumber. It comprises at least three thin layers or veneers of wood held together using glue. Each layer is filled with grains running in opposite directions from the previous layer, which helps increase the plywood’s strength and stability. 

In addition to plywood, other common forms of subflooring include wood plank subflooring, OSB subflooring, and concrete subflooring.

Types of plywood

Plywood is made from various materials and using different manufacturing methods depending on the intended application. Common types of plywood for subflooring include:

1. Oriented Strand Board (OSB)

OSB is an engineered structural panel made from wafers held together using a binder. Unlike traditional plywood filled with grains, OSB is not particle board but instead made from shredded wood strands held together using glue or wax.

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2. CDX Plywood

CDX plywood is the most common “veneer” plywood in use today. Compared to other laminate plywoods, it has cosmetically rough outer laminate layers of grades “C” and “D.” This makes CDX plywood much cheaper and desirable compared to other laminate plywoods. It is suitable for use in outdoor environments because it can withstand exposure.  

Tongue and Groove

Commonly called “T and G” in construction circles, tongue and Groove is similar to a mortise and tenon and used many types of woodworks. It is made with a protruding ridge on the surface that fits into another slot in the adjoining surface.

Tongue and Groove is used in subflooring is because the protruding edges hold the plywood in place and prevent movements during stress and temperature changes. The tongue-and-groove joints are joined together using construction adhesives. 

Plywood Ratings

Plywood comes in five ratings depending on where and how it is used.

  • Exterior – this plywood is waterproofed and can be used in exterior environments due to its ability to withstand harsh conditions. They are used in areas that are constantly exposed to water.
  • Exposure 1 – these are also waterproof panels that can withstand weather elements.  They are, however, not suitable in places that are constantly exposed to water. 
  • Exposure 2 – these plywood panels are held together using glue but not fully waterproofed. They are mainly intended for use in the interior as they get easily spoiled in wet conditions.
  • Interior – these are not waterproofed and are mainly used in the interior. They are not ideal for wet and moist conditions. 
  • Structural 1 – these types of plywood are mainly used in structures that need to withstand earthquakes. 

Plywood Grades 

Plywood grade refers to the quality and appearance of the surface and back veneers. There are four plywood grades A, B, C, and D, with A having the highest quality. It is also the most expensive of the four. Grade D is the lowest quality and the cheapest. 

  • A-grade – it has smooth and sanded surfaces without any wood defects.  Its veneers can also be painted. 
  • B-grade – like A-grade, B-grade is also smooth with its surface sanded.  B-grade may, however, have some unrepaired defects. 
  • C-grade – this plywood grade is unsanded and usually has several defects that need repaired. It is the common grade used in subfloors and garages as well as other areas where decoration and appearance are not important. 
  • D-grade – it is also unsanded and has major unrepaired defects, discoloration, knot holes up to 2 1/2-inches, and sanding defects. 

In addition to the standard plywood grades above, there are other two-letter classifications like BC, which are a mixture of two individual grades. 

Plywood Sizes 

Plywood mainly comes in 4- x 8-feet and 5- x 5-feet sizes. The 4 x 8 feet sheets are the standard sizes. There is a variety of other sizes, including the specialty sizes.

Other sizes include 2 x 2 feet, 2 x 4 feet, and 4 x 4 feet. There are also longer utility plywood sheets measuring 4 x 10 feet.

The largest utility plywood has a width of 5 feet and comes in various lengths like 6, 8, and 10 feet.

In terms of thickness, there are also several variations ranging from 1/8 inch to 1 ¼ inches. Standard plywood thicknesses are 1 ¼, 1 1/8, ¾, 5/8, ½, 3/8, ¼, and 1/8 inches. These measurements take into account the sanding effects. 

Advantages of pressure-treated subfloors 

Pressure-treatment of wood involves using chemical preservatives to avoid fungal damage, termite and insect damage, bacteria weather damage and make the wood more durable.  It is called pressure treatment because the chemicals are forced into the wood through intense pressure.  

Below are unique advantages of pressure-treated subfloors:

1. Moisture Resistance

When left in moist and damp conditions for long, wood becomes soft and easily becomes a breeding ground for fungus, bacteria, and other microorganisms. These microorganisms increase the decomposition of the wood.  Pressure-treated wood contains biocides that protect from microorganisms and prevent wood decay even in moist conditions. 

2. Fire Resistance

Wood-based buildings are always at greater risk in the event of a fire outbreak. In addition, to inject insecticides and fungicides, pressure-treated wood is also injected with fire-retardant chemicals that help keep away fire. 

3. Durability

Pressure-treated wood is more durable compared to natural wood. It can be in moist and wet environments without the risk of weakening or being attacked by pests and insects. Pressure-treated wood does not absorb water and also has chemicals that protect it from burning in case of fire. 

4. Ease of Use

Pressure-treated wood is easy to cut in different sizes using a saw for different applications.  Furthermore, they are also easy to fasten together using nails and other fasteners. They are also easy to bore holes using drills and also very easy to create various shapes. 

Other subflooring materials 

In addition to pressure-treated wood, there are a number of other materials used for subflooring:

  • Concrete Slab – this is made from poured concrete and usually has a thickness of between 4 to 6 inches. However, concrete is susceptible to moisture when poured in wet and moist places.
    Because of this, it is advisable to apply a few sheets of plastic at several sections of the slab to prevent water from seeping in from the outdoor. 
  • Plank subflooring – these are usually 3/4″ thick and 4-8″ wide. These boards are installed by nailing them to the floor joists. They are also fastened using 2-1/2″ deck screws to prevent becoming lose as the building ages and through the forces of expansion and contraction.
  • High-Performance Panels – these are engineered panels and offer almost similar benefits to plywood and OSB. They are designed to be moisture-resistant and made with special resins that reduce water absorption and minimize swelling.  High-performance panels have added density and resin technology that make them stiffer, stronger, and durable. 
  • Mixed subflooring – these are composite subfloors made from concrete mixed with OSB or plywood. One way to make mixed subfloors is fastening 2″ sleepers on the concrete and adding a plywood subfloor.
    Another option is to first lay down floating subfloor made from tongue-and-groove OSB panels attaching it to base layer made from rigid foam insulation or plastic. The base layer prevents moisture intake and keeps the concrete from becoming damp. 


Pressure-treated wood is a common trend in subflooring due to its unique features and advantages.  It is resistant to moisture, durable, and fire-resistant.

Plywood remains the most common pressure-treated subflooring material and comes in different sizes, both standard and customized.