What Is Roof Flashing and Why Does It Fail?

Q: What is roof flashing and why does it fail?

Flashing is the metal that waterproofs every place a roof is interrupted — valleys, walls, chimneys, skylight curbs, pipes, vents, edges and roof-to-patio transitions. Tile and shingle shed water across the field; flashing handles every joint, and joints are where roofs leak. It fails in four ways: corrosion from the underside, especially in salt air; movement, as thermal cycling and settling work fasteners and laps loose; poor original detailing, where a flashing is too short, lapped in the wrong order, or substituted with sealant; and damage from later trades cutting into it for solar, satellite or stucco work. The important part for a homeowner: flashing is a sequenced assembly, not a caulk joint. A repair that relies on sealant over failed metal has a life of a season or two. A repair that rebuilds the metal in the correct order outlasts the roof around it.

The kinds of flashing on your roof

Valley metal carries the concentrated water where two slopes meet. Step and continuous wall flashing handles roof-to-wall transitions, with counterflashing or a stucco weep detail over the top. Chimneys use a base, steps, counterflashing and a saddle or cricket on the up-slope side.

Then there are the small ones that cause most leaks: pipe boots and vent collars, skylight curb and head flashing, drip edge and rake metal at the perimeter, and the flashing where a patio cover or a second story meets the house.

Corrosion from underneath

Galvanised flashing rusts from the side you cannot see. Water sitting under a valley, or salt-laden moisture wicking into a lap, thins the metal until it perforates while the exposed surface still looks painted and fine.

Debris accelerates it. A valley packed with needles or leaves keeps metal wet for weeks after rain, and any dissimilar-metal contact — a copper fitting on galvanised steel, aluminium against masonry — sets up galvanic corrosion that concentrates in a small area.

Bad detailing and sealant substitution

Flashing works because of overlap sequence: each piece has to shed onto the piece below it, and it has to be tall enough that wind-driven water cannot get over the top. A flashing that stops an inch too low works in drizzle and fails in a real storm.

Sealant used in place of correct sequencing is the most common shortcut in roofing. It is opaque to inspection, it hides the evidence a technician needs, and it lasts a few years at most under Southern California sun. Finding a fat bead of caulk where metal should be is a reliable sign the last repair was cosmetic.

Damage from other trades

Solar mounting, satellite dishes, HVAC installs, stucco patches and painters all interact with flashing, and not all of them respect it. Penetrations drilled through flashing and sealed with caulk are a frequent source of leaks that appear a year or two after the unrelated work was done.

If your leak started within a couple of years of solar, an addition or a stucco job, that is where we look first.

How a flashing repair is done properly

The covering is lifted back far enough to work, the failed metal is removed rather than covered, new metal is formed and installed with correct laps and adequate height, the underlayment is integrated with it, and the tile or shingle is re-laid over the top.

That is why a flashing repair is a real job rather than a tube of sealant, and why most fall in the $1,200–$2,000 range. It is also why they last: done right, a rebuilt flashing is usually the newest and strongest part of the roof.

In San Diego County and along the coast

Flashing is the single biggest reason San Diego County roofs leak, because our climate is far harder on metal than on tile. In the coastal band from Oceanside and Carlsbad through Encinitas, Del Mar, Solana Beach, Coronado and Imperial Beach, salt-laden marine air corrodes valley metal, counterflashing and fasteners from the underside for decades, and it is common to find a roof whose tile is fine and whose metal is finished.

Inland in Escondido, San Marcos, Vista, Poway, Ramona, El Cajon and Santee, thermal cycling is the mechanism instead: metal expands and contracts daily, working laps and fasteners loose and splitting whatever sealant a previous repair depended on. Either way, the repair is the same idea — rebuild the metal correctly rather than seal over it.

How Spot Roof Repair solves this for you

We are a repair-only roofing company based in Carlsbad, working the whole of San Diego County — coastal North County, inland, the city of San Diego, East County, the South Bay and the backcountry. We come out, find the actual cause, photograph it, and hand you a written price before anyone starts work.