What is a roof deck in construction?
Short answer. In construction, the roof deck is the structural surface fastened over the rafters or trusses that the roofing materials attach to. On homes it is usually plywood or OSB sheathing; older houses may have board planks, and commercial buildings often use steel or concrete decks.
What the deck does
- Spans between framing members and carries the weight of roofing, people and wind or snow loads.
- Stiffens the roof and the whole house against wind as part of the structure.
- Gives nails a solid base. Shingle wind ratings assume nails driven into sound decking.
Common residential deck types
- OSB: the most common today, often 7/16 inch over framing about 24 inches apart.
- Plywood: often 1/2 inch, handles occasional wetting a bit better.
- Plank or board decking: on older Georgia homes, 1 by 6 or 1 by 8 boards, sometimes with gaps that need to be covered with new sheathing before shingles.
More detail in OSB vs plywood roof decking.
How it is installed
Panels run perpendicular to the framing with staggered joints, a small gap at edges for expansion, and H clips at unsupported edges when the span requires it. Nailing schedules come from the code and the panel's span rating.
When it gets replaced
During a reroof, the crew tears off the old shingles and inspects the deck. Soft, rotten or delaminated sheets are cut out and replaced. Estimates usually list a per sheet price because no one can see the deck until the shingles are off. Long term leaks, poor attic ventilation and bathroom fans venting into the attic are the usual causes of rot. See roof anatomy explained.