Components of a Roof – Plywood - Series Part 2

Roofing company workers

In a previous blog topic, I identified what a sheathing or roof deck is, and identified that it is commonly plywood or oriented strand board (OSB).  I am going to delve a bit deeper; what is plywood and why is it superior to wood planks? Which do roofing companies prefer to use?

 Have you ever seen a Martial Arts demonstration that included breaking wood planks?  Perhaps using feet, hands, or even their head?  Have you ever inspected the planks themselves?  It is easier to split a board with the grain than it is across the grain.  Had those boards been cut differently, the Martial Arts demonstration might look somewhat different.  Wood structures and wood furniture take advantage of this strength as the any loads or forces acting on the wood support the load itself.  However for sheathing or roof decking, what is the direction of the force acting on the wood planks?

Plywood is made up of many thin “plies” of wood veneer that are glued together.  Each “ply” is laid out so that the grain is oriented in a different direction.  This is called “cross-graining”, and among other things it contributes to dimensional stability, reduced potential to split when nailed close to the edges, and makes the strength of the panel consistent across multiple directions.  The thin layers result in a product that is stronger than a single layer of wood, even if the single layer of wood may be thicker.  These factors taken together are why plywood is superior to wood planks as a sheathing or roof deck material.

 Plywood is graded based on appearance and defects in the plies.  For sheathing and roof deck plywood, since it will be covered by roofing materials to the outside and not visible to the inside, the important issue is structural and not appearance.  There is no need to purchase Grade A (surface veneers free from all defects) and instead we use CDX (face has knots, defects, not appearance grade, not sanded).  CDX is some pretty rough appearance plywood, but for a structural purpose it is more than sufficient.  The important factor for sheathing or roof deck uses is that the plywood be exterior grade.

Now then, next time I see a Martial Arts demonstration that involves breaking wood planks, I want to slip in a plank or two made of plywood.  That will result in quite the headache…

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