Roof Deck Inspection and Replacement

The roof deck is the structural surface that the roofing system is fastened to. It may be timber boards, plywood, oriented strand board (OSB), profiled steel or concrete. Because the roof covering can only perform as well as the surface beneath it, the deck is inspected before any new roofing is installed, and damaged areas are repaired or replaced.

Canadian guidance says the deck should be visually inspected before roofing starts, deficiencies recorded and reported, and work held until they are corrected. Structural adequacy is a matter for a professional engineer, but roofers are expected to flag weak areas.

What is checked

• Moisture and rot. The deck must be clean and thoroughly dry. Soft, stained or rotted boards and delaminated panels are replaced.
• Deflection. Sagging, bounce or unusual movement can indicate undersized or damaged framing.
• Fastening. Panels should be fastened at their edges and over supports. US Building America guidance for retrofits recommends re-nailing the deck to current requirements during a tear-off.
• Gaps and knotholes. For board decks, the Canadian Roofing Reference Manual sets a maximum gap of 6 mm (1/4 in); larger gaps or knotholes are covered with sheet metal.
• Openings and curbs. Openings should be framed so they do not deflect, and curbs should be in place before roofing.

Deck types

Deck	Key checks (Canadian guidance)
Plywood	Approved grade; typically at least 15.9 mm (5/8 in) for most low-slope uses; all edges supported
Board	At least 19 mm (3/4 in) thick, no wider than 150 mm (6 in), fastened on both edges
Steel	Fastened or welded to supports, flutes clean and dry, no concentrated loads
Concrete	Smooth, dry surface; precast joints grouted; level differences treated

Replacement

During a tear-off, damaged sheathing is cut back to the nearest supports and replaced with panels of matching thickness so the new roof lies flat. Some programmes for high-wind regions also recommend sealing deck seams with tape or a self-adhered membrane before underlayment is laid. On homes with spaced board decks under old wood shakes, solid sheathing is often added before asphalt or metal roofing is installed.

Some building departments do not judge whether sheathing is structurally sound; Bloomington, Minnesota, for example, leaves that decision to the contractor, engineer or homeowner.