Roof Deck and Substrate Preparation
This domain tests the condition the deck must be in before any membrane or shingle goes down: structural adequacy, thickness, fastening pattern, expansion gaps, slope, priming and the separation layers that protect the assembly.
Deck types and fastening
Wood decks include board (nominal 1 in. boards with gaps controlled), plywood and OSB. Plywood roof sheathing is commonly a minimum 12.5 mm (15/32 in.) for shingles, with thicker tongue-and-groove panels for hot-applied built-up roofs. Panels are fastened with 8d common nails, typically 150 mm on centre at supported edges and 300 mm in the field, driven flush, not countersunk. Stagger panel joints, support every edge on blocking or a framing member, and leave a 2 to 3 mm gap between panels for expansion. Other decks include structural concrete and steel deck, each with its own primer and fastener requirements.
Slope and surface preparation
Shingle roofs generally need at least 4:12 unless the manufacturer permits less with a special underlayment. Low-slope roofs need positive slope to drains of at least about 1:50 (1/4 in. per foot). Sweep the deck clean, replace delaminated or rotten sheathing, and prime concrete or wood before hot asphalt. Underlayment, vapour retarders and sheathing paper go down in the correct order so laps shed water.
Worked example
A 9.6 m long roof drains to a single scupper. Minimum slope of 1:50 requires a fall of 9600 ÷ 50 = 192 mm from the high point to the drain. The as-built framing gives only 150 mm, which is 150 ÷ 9600 = 1:64, flatter than the specified minimum. Ponding will result, so the fix is tapered insulation or reframing to restore 192 mm, not simply adding membrane.
Traps that cost marks
Nailing through the gap between panels or missing the framing. Overdriving nails and breaking the panel face. Butting panels tight with no expansion gap. Installing a new membrane over a wet or delaminated deck. Forgetting primer before hot asphalt, which prevents adhesion. Ignoring the requirement to replace damaged sheathing because it is hidden under new material.