Flashing, Valleys, and Penetration Detailing
Flashing, Valleys, and Penetration Detailing
This domain tests the details that keep water out at walls, valleys, chimneys, pipes, and roof edges-where most leaks begin.
Core rules
- Step flashing at sidewalls must be installed with each shingle course, not every other course. Use pieces with at least 2-in. legs and 2-in. overlap; the base goes under the shingle and the counterflashing overlaps the top.
- Open metal valleys are commonly 16–24 in. wide. Install valley underlayment, set the metal, trim shingles about 2 in. back from the valley center, and keep fasteners at least 6 in. from the center. Never nail through the valley center.
- Pipe boots: the flange goes under the shingle above and over the shingle below; sealant is a supplement, not the primary seal. Use a boot sized to the pipe.
- Install kick-out flashing where a sidewall meets an eave to divert water into the gutter. Use crickets behind chimneys wider than 30 in. and apron flashing at headwalls.
Worked example
A roofer installs step flashing only every second course and seals the gaps with caulk. Why does this fail? Water driven sideways at the wall can enter between courses; each course needs a step flash, lapped 2 in., with the shingle overlapping the vertical leg. Caulk is not a substitute for the missing metal.
Traps
Face-nailing through flashings, omitting kick-out flashing, relying on sealant at pipe boots, and nailing near the valley center are frequent mark-losers.