When choosing a new roof, don’t forget to consider what surface the roof will nailed onto. I recommend that all roofdecks, especially in moist climates like Seattle, are CDX plywood and not OSB.
What’s a roofdeck? The roof deck is the surface underneath your roof and roofing underlayment. It is usually made of 4′ by 8′ sheets that should be 1/2″ thick.
What is CDX? CDX is veneer plywood. It is made of multiple layers of thinly cut, solid sheets of wood that are pressed together.
What is OSB? OSB is a cheap imitation of CDX. It is made of wood chips and glue.
How do CDX and OSB differ in performance? OSB does not do well when it gets wet. When OSB comes into contact with moisture, it swells and soaks up water. Then it can grow mold or simply just fall apart. It works fine for interior walls but I don’t recommend it as a roof deck at all. Many home builders construct houses with OSB for the entire roofdeck or sometimes they use CDX around the perimeter edges and OSB in the center. As a roofing contractor in Seattle, I strongly recommend that OSB is never used as a roofdeck. Always use CDX.
What if my house currently has an OSB roofdeck? Consider having the OSB replaced with CDX when your roof is replaced.
Below is a close up of OSB:
Below is a close up of CDX: