If you ask most homeowners where roof leaks come from, they will say "the shingles." In reality, most water infiltration enters at the transitions — the seams, edges, and penetrations where different planes of the roof meet, or where the roof meets a wall, chimney, skylight, or vent. These vulnerable junctions are protected by flashing, and when flashing fails, the roof leaks regardless of how new the shingles are. If you are seeing early warning signs of water infiltration, the signs you need a new roof guide can help you distinguish a flashing repair from a full system failure.
What Flashing Is and Where It Goes
Flashing is thin sheet metal — typically galvanized steel, aluminum, or copper — installed at every point where water could find a path through the roof assembly. The most common flashing locations are: chimney bases (step flashing along the sides, counter-flashing embedded in the mortar joints); valleys (the V-shaped channels where two roof planes meet); pipe boots (the collars around plumbing vent pipes); skylight perimeters; and anywhere the roof meets a vertical wall, such as a dormer or addition.
Copper is the premium material — it lasts 50+ years and solders cleanly. Aluminum is the most common residential material — durable, affordable, and easy to work with. Galvanized steel is serviceable but can rust eventually. Lead is still used in some regions, particularly around chimney penetrations, but is less common due to environmental concerns.
Step Flashing vs. Continuous Flashing
At a roof-to-wall junction — where a sloped roof meets a vertical wall, like on a dormer — there are two approaches. Step flashing uses individual L-shaped pieces of metal, each woven between courses of shingles as the roof is built. Each piece overlaps the one below, creating a layered waterproof barrier that moves with the structure.
Continuous (or "apron") flashing uses a single long piece of metal running the length of the junction. It is faster to install and works adequately on some details, but it cannot accommodate structural movement as well as step flashing and is more prone to failure over time. A good contractor uses step flashing at roof-to-wall junctions, not continuous flashing.
Signs of Flashing Failure
From the ground, look for rust streaks or visible gaps at chimney bases, valley lines, and roof-to-wall junctions. From inside the attic after rain, look for water stains or active drips near these locations — the entry point is often several feet upslope from where the water appears inside.
Caulk is a warning sign, not a solution. If you see heavy caulk application at flashings, a previous contractor or homeowner patched a leak rather than fixing the underlying flashing. Caulk fails within a few years, especially where metal moves with temperature swings. Proper flashing does not rely on caulk for its primary water seal.
During a roof replacement, all flashing should be inspected and most should be replaced. Reusing flashings from an old roof on a new one is a common shortcut that can create premature leaks. Ask your contractor specifically what they plan to do with the chimney step flashing, valley metal, and pipe boots. For typical repair costs when flashings do fail, see our roof repair cost guide.