Flashing, Drainage, and Waterproofing Details
This domain tests the details that fail first on real roofs: base and counterflashing, step flashing, crickets, pitch pans, drains, scuppers, gutters and expansion joints. The exam expects you to know the minimum dimensions and why each layer overlaps the one below.
Flashing principles
Water must always lap over the layer below and run downhill, never uphill. Base flashing typically extends at least 200 mm up the wall or curb; counterflashing overlaps the base flashing by at least 100 mm and is set into a reglet or over a termination bar. Step flashing is woven with each shingle course so the exposure matches the shingle exposure, and it is never face-nailed through the exposed face. A cricket is required behind a chimney or curb that is wide enough to collect water, commonly 750 mm or more. Pitch pans need a compatible sealant and a rigid retainer.
Drainage
Primary drains sit at the low point of the slope; secondary or overflow drainage is required and is set above the primary so it becomes active only when the primary is blocked. Gutters are sized to the roof area they serve, and downspouts discharge away from the wall. Expansion joints accommodate movement in large roofs and must not be bridged by membrane.
Worked example
A sidewall runs 6.0 m along the eave direction with step flashing at 125 mm exposure. Divide the wall length by the exposure: 6000 ÷ 125 = 48 pieces. Add corner pieces and one extra per 10 for cutting loss, giving about 52 pieces. Reason: step flashing quantity follows the shingle exposure, so a larger exposure means fewer pieces, and the exposure is fixed by the shingle, not chosen for convenience.
Traps that cost marks
Face-nailing flashing through its exposed face. Omitting the cricket and letting water sit against the chimney. Installing base flashing without counterflashing. Using a termination bar with no sealant bead above it. Mixing copper and galvanized or aluminum in contact, which causes galvanic corrosion. Setting a drain away from the true low point, or omitting secondary drainage entirely.