What Type of Plywood is used for Roofing? Size and Thickness Guide

A roofing system consists of a variety of materials and parts, which  are interconnected to protect the inside of your house from harsh weather conditions and elements.

The materials used may vary, depending on the location of the building to the shape of the roof and even the surrounding climate. The materials can range anywhere from asphalt, wood, metal, clay or even glass.

On more modern homes, roof decks are more often seen with laminated panels of plywood, or non-laminated oriented strand boards (OSB).

There are many reasons why these materials are preferred for roofing, such as the following:

Why use Plywood

Plywood is a low-cost material that is water-resistant and has structural stability. This makes it a favorite amongst builders as a material used on the exterior portions of houses.

Though the structure of the roof takes the brunt of it, the strength of plywood is also relied upon to support the rest of the roofing system. The material gets its strength from the way it is built.

Plywood is made up of layers of wood that are glued together, with the grains in each layer running in alternate directions.

This also ensures that the strength of the wood is evenly distributed. This method of layering also minimizes the amount the wood shrinks and expands because of temperature changes and makes it less likely to split when it is nailed into.

Plywood is also known to perform well when exposed to moisture over long periods of time. Unlike other wood, because of the way it is made, plywood expands evenly and dries down fast, returning to its original size all over rather than in some areas.

Types of Plywood used for Roofing

There are multiple types of plywood available in the market like shuttering plywood, general plywood or structural plywood.

Shuttering plywood is used to hold concrete, while general plywood is used for non-structural roles. Like its name, structural plywood is used for structural purposes.

Structural plywood is put through tests to prove structural integrity and meets building regulations, while general plywood doesn’t go through the same process.

CDX is the grade of plywood most commonly used for roofing. The wood is graded on each side from A to D, depending on the level of imperfections, (A being free of all imperfections and knots and D being un-sanded and considerably defected).

One side of the plywood could be A grade with the other side a D grade. This works if the lower graded side is used on the inside of the roof, while the high grade faces the outside.

The C in CDX represents the grade given to the front of the plywood, the D represents the grade given to the back of the plywood and the X represents the type of glue used to bond the layers together.

There is one more type of plywood that has been known to be used in roofing, which is Fire-Retardant Plywood or FRT Plywood. This kind of plywood can be life-saving in the time of a crisis and is sometimes required in certain areas.

Required Dimensions of Plywood for Roofing

Plywood typically comes in 4×8 ft. sheet with varying thickness. The minimum thickness of plywood used for roofing is 3/8^th of an inch.

This kind of plywood can be used on roofs with minimal load; steeper rather than flatter roofs will have lesser load per sq. foot and with a maximum of spacing of 16 inches between rafters.

The ideal thickness for the standard rafter spacing of 24 inches is 5/8^th of an inch. This carries enough structural integrity for most roof loads.

However, 3/4^th inch plywood is also used for a roofing system that has a heavier load; snowfall and ice accumulation can affect the load put on a roof.

For the less standard 20-inch apart rafters, plywood with ½ inch or 5/8^th inch thickness can be used.

Plywood vs. Oriented Strand Board

Strand Board is gaining popularity as a material used for roofing. OSB is a type of wood made up of strands in varying orientations compressed together with adhesive to form a board. This type of wood is typically a cheaper alternative to plywood, but it meets most standards of its alternative.

There are a few disadvantages to using this cheaper alternative to plywood. The most notable ones are the inability of OSB to hold nails as effectively as plywood.

This means that the structure of the roof is compromised ever so slightly. The second disadvantage is the way OSB absorbs moisture.

While it is true that OSB takes a longer time to absorb moisture than plywood, it also takes that much longer to dry. While plywood dries down evenly, OSB tends to absorb moisture more around the edges, making it swell and dry down with a ring around the edges.

Florida Dade County has prohibited the use of OSB for roof decks because of the effects that Hurricane Andrew had on homes that used the material for roofing.