Roof sheathing is the coverage over the rafters or roof joists and usually consists of nominal 1 incg boards for both pitched and flat roofs. Where flat roofs are to be used for a deck or balcony, thicker boards or double sheathing may be required, especially where joists are spaced more than 16 inches center on center, or where diagonal sheathing is employed.
Sheathing offers a nailing base for roof covering and gives rigidity and strength to the roof framing. Where greater strength and resistance to high winds are desirable, the roof boards are nailed diagonally to the rafters rather than horizonatally.
Plywood, which is sometimes used for roof sheathing, also adds rigidity and bracing to the roof structure.
Boards used in roof sheathing are typically number 3 Common or better, of Pine, Douglas-fir, Redwood, Hemlock, Larch, Fir or Spruce.
It is important that thorougly seasoned wood product be used with asphalt shingle roofing. Unseasoned wood will dry out and shrink in width, and casue buckling or lifting of shingles.