Roofing and Waterproofing
Slope and membrane selection
Steep-slope roofs, above 4:12, use shingles, tile, and metal. Low-slope roofs from about 1/4:12 to 3:12 use single-ply membranes such as TPO, EPDM, or PVC, modified bitumen, or built-up roofing. Asphalt shingles may be applied down to 2:12 with double underlayment. Membrane selection follows slope, traffic, and exposure, not preference.
Flashing details
Base flashing, counterflashing, step flashing at sidewalls, and crickets at curbs wider than about 24 inches. Membrane laps are offset and seams staggered, with fasteners placed per the manufacturer's wind-uplift rating. Ice-dam protection is required at eaves in many jurisdictions. Penetrations are flashed on the uphill side and curbs get a cricket.
Waterproofing
Below-grade systems include fluid-applied and sheet membranes, bentonite, and drainage board with protection. Plaza decks and wet areas use bonded membranes, usually flood tested for 24 hours before finishes go down. Dampproofing resists vapor; waterproofing resists hydrostatic head - they are not interchangeable.
Traps
Specifying shingles below the minimum slope; confusing dampproofing with waterproofing; omitting flood tests; and terminating membrane at a wall without a reglet or counterflashing detail.
Rule of thumb: minimum slope for positive drainage is 1/4 inch per foot, or about 2 percent.