Roof Sheathing (Roof Deck)

Roof sheathing, also called roof decking or the roof deck, is the continuous or spaced layer fixed across rafters or trusses. It stiffens the roof frame, spreads loads to the framing, and provides the nailing surface for underlayment and the roof covering. In many codes the deck also acts as a structural diaphragm that helps the building resist wind and earthquake forces.

Common materials

• Plywood: cross-laminated wood veneers glued into panels.
• Oriented strand board (OSB): wood strands bonded with resin in oriented layers; widely used in North American house construction.
• Composite panels: combine veneers with reconstituted wood layers.
• Solid boards (plank decking): the traditional deck, still found on older houses and used under some heritage roofs.
• Tongue-and-groove decking: thick boards or panels spanning widely spaced beams, often left exposed on the ceiling.

In North America, structural panels carry an APA (or similar) stamp with a span rating: the first number gives the maximum recommended spacing of roof supports, the second the maximum floor span.

Installation points

• Gaps: APA recommends leaving about 1/8 in (3 mm) between panel edges and ends so panels can expand with moisture without buckling.
• Edge support: panel edge clips (often called H-clips), blocking, or tongue-and-groove edges support unsupported edges between rafters.
• Fastening: an InterNACHI guide notes common schedules of 6 in (150 mm) at panel edges and 12 in (300 mm) at intermediate supports, with closer spacing near ridges, eaves and gable ends in very high wind zones. Local codes govern.
• Thickness: an InterNACHI table gives 5/8 in (16 mm) as a minimum for 24 in (610 mm) support spacing.

Regional terms

In Scotland and parts of northern Europe, boards fixed over rafters beneath slates are called sarking or sarking boards; "sarking" is also used loosely for underlay. In Australia and New Zealand, "sarking" usually means a reflective or breathable foil membrane rather than boards.

Related decks

Some roof coverings, such as wood shakes and some tiles, are traditionally laid on spaced boards or battens instead of a solid deck.