Water gets in at the joints, so flashing detail decides whether a roof stays dry.
At a glance
- Sealant is a helper, never the flashing itself.
- Step flashing at a wall goes piece by piece, one per course.
- Vent boots crack in the sun and are cheap to replace.
- A chimney needs both base flashing and counter flashing.
Step flashing
Where a sloped roof meets a wall, each course of roofing gets its own bent piece of metal that laps the one below. Done right, water runs from piece to piece and out onto the roof.
The failure is a single long strip of metal run down the whole wall with caulk over it. It works for a few years and then the wall stains.
Counter flashing
Counter flashing is the upper piece set into or under the wall surface that covers the top edge of the base flashing. On a chimney it goes into a cut groove in the mortar.
Without it, the top edge of the base flashing sits open and water runs behind everything.
Valley metal
Valleys carry water from two roof planes. Open valleys use a wide metal channel with the roofing cut back on both sides. Closed valleys weave the material across.
Open metal valleys clear debris better, which matters under the pines and eucalyptus we have all over the county.
Pipe boots and vents
The rubber collar on a plumbing vent boot is the shortest lived part of a roof. In this sun it cracks in eight to twelve years while the roof around it is fine.
They cost very little to replace. On any roof past ten years, replacing every boot is cheap insurance.
Drip edge and rake metal
Metal at the eave and the rake carries water off the edge instead of letting it curl back into the fascia. Older Los Angeles homes often have none at all, which is why so much fascia here is soft.
Adding it during a reroof costs very little and saves the wood.
Common questions
Can flashing be reused on a reroof
Valley and wall metal in good shape sometimes can. Boots and cracked metal should always be new.
What metal is best
Galvanized steel is standard. Copper lasts longest and costs the most. Near the coast, heavier coatings matter.
Why does my chimney leak only in heavy rain
Wind driven rain finds an open counter flashing edge that light rain runs past.