Roof Basics

Flashing: Where Roofs Really Leak

Ask most homeowners where a roof leaks and they'll point at the field — the big open area of tile or shingle. But that's rarely where the trouble starts. Roofs leak at the transitions — the spots where the roof meets a wall, a chimney, a vent pipe, a skylight, or where two roof planes come together in a valley. The metal that seals all those transitions is called flashing, and it's where the real craftsmanship — and the real corner-cutting — happens.

Think about it. The flat open part of your roof is the easy part; the covering sheds water straight down and off. But a valley collects water from two directions and funnels it. A wall sticking up out of the roof forces water to change direction. A vent is literally a hole punched through the roof. Every one of those is a place water wants to get in, and flashing is the only thing stopping it.

Good flashing work is deliberate — proper materials, proper laps, water directed where it should go. Bad flashing work is the single most common source of leaks I see, and it's almost always one of two things: reused old flashing, or flashing installed wrong to save time.

That reused-flashing point deserves emphasis. When a roof is torn off and replaced, the flashing should generally be replaced too. But flashing is fiddly and time-consuming, so a contractor under price pressure will sometimes leave the old, tired flashing in place and lay the brand-new roof right around it. You get a shiny new roof with the oldest, most failure-prone parts left untouched.

Reused flashing at a wall transition — brand-new underlayment installed right up against the old, worn flashing that was never replaced.

The other shortcut is what's sometimes called topical flashing — a repair applied over or against the surface at a transition, instead of a proper flashing installation integrated into the roofing system underneath. It's fast and cheap, and it's extremely inferior: it doesn't hold up the way flashing tied into the underlayment and wall does, and it tends to fail at exactly the transitions it was meant to protect.

A short strip of new flashing metal applied at a wall transition without being properly integrated into the surrounding roofing system — Topical flashing — a surface-level patch at a transition rather than a proper flashing installation. Inferior, and prone to failing right where it matters most.

Here's what it looks like done right — flashing genuinely replaced and properly tied into the wall, with the stucco finished around it instead of just caulked over the old material:

Newly replaced roof flashing at a wall transition, integrated with fresh stucco patch work — Properly replaced flashing at a wall transition, with the stucco finished around it — this is what a real flashing replacement looks like.

When you review a bid, look for flashing to be a real line item — not buried, not vague. Ask specifically: are you replacing the flashing at the walls, valleys, and penetrations, or reusing what's there? The answer tells you whether you're buying a roof that's actually new, or just a new covering over the same old leak points.