Flashing, Penetrations, and Waterproofing Details
Wall and curb flashing
Base flashing turns up at least 8 in. above the roof surface against a wall or curb, and counterflashing laps over it so water cannot get behind. Terminations bury the counterflashing in a reglet or masonry joint, sealed and caulked. At siding, step flashing must interweave with each shingle course - one piece per course, roughly 4 in. by 4 in., overlapped about 2 in. - and a kick-out flashing at the bottom must divert water into the gutter rather than behind the siding.
Valleys and edges
Open valleys use metal lining, commonly about 24 in. wide, centered and crimped, over underlayment. At eaves, drip edge goes under the underlayment; at rakes it goes over, so water sheds to the outside. Without a drip edge, water curls back under the shingles and rots the fascia.
Penetrations
Pipe boots need a flange lapped over the underlayment and sealed at the top, never at the low side only. Chimneys wider than 30 in. need a cricket behind them. Skylights, vents, and RTUs get curbs with flashing and a minimum curb height well above the finished roof surface. Deck and balcony waterproofing must turn up at least 2 in. above the walking surface and drain over, not through, the wall.
Waterproofing logic
Waterproofing depends on shingle-lap sequencing: upper layers shed onto lower ones. Reverse a lap and the detail leaks no matter how much sealant is added.
Traps
Sealing the low side of a penetration, skipping the kick-out, and burying a step flashing in caulk instead of lapping shingles are favorite distractors.
Worked example
A 40 in. brick chimney: install a cricket and step flashing, counterflash into a reglet, and check the cricket drains to the main roof.