06 Mar Plywood: The Unseen Building Block of the Modern Roof
Here at Peak Roofing we have talked a lot about various kinds of roofing materials: shingles, slate, tile, shake, solar shingles, and green roof systems. One of the essential materials that we have not discussed is plywood. The reason that we have not discussed this yet is because if your roof has not begun leaking (causing rot) then it is usually not necessary to replace the plywood. Most roofs consist of ceiling joints, rafter boards, and plywood overlay.
There are two major types that are used on roofs right now. Oriented Strand Board (OSB) which is like normal plywood except the “strands” used to make the plywood are all oriented in the same direction. Plywood used to look like this but most people use OSB now which looks like this. Most estimates have OSB holding 75% market share over other types of plywood because of how much cheaper it is than traditional ply. So what is the difference and what is your roof made of? Well, if your house was built before 1963 the odds are that it did not use OSB because it wasn’t invented until 1963. Interestingly enough, while OSB was invented in California the number one exporter of OSB in the world is Canada.
The other type is commonly known as CDX. The letters “C” and “D” in this acronym represent the quality of the surfaces- back and front. If you assumed that “A” was the best level of plywood you can buy (without any noticeable knots, holes, or defects that have been repaired), then C and D indicate a lower quality. When you are shopping for plywood you can recognize CDX plywood by the football-shaped patches and small indentations on the front and back. The reason that CDX is not as commonly used anymore is because it does not have the same flexibility that OSB has combined with the fact that that OSB sells for about $3 less per sheet.
Of course, all this information is useless unless we describe the difference for your roof. The primary difference between CDX and OSB is how they react when they are exposed to moisture. CDX has an advantage over OSB when water and snow are present because it swells consistently across the entire sheet and returns to its normal size when it dries out. OSB is more water resistant but when it does get “soaked” it holds water for longer which can result in faster rotting. Also, when OSB gets wet it tends to swell along the edges and those edges can cause what we refer to in the industry as ghost lines through asphalt shingles.
The plywood you use in your roof does have an impact on the quality of your roof, but a good shingle installation should prevent water from sitting on your wood sheathing and minimize the differences between CDX and OSB plywood.