Home / Roof Repair & Leak Detection / Roof Flashing Failures
A field of shingles, correctly installed on a sloped surface, is a remarkably reliable system. Each course overlaps the one below, water lands and runs downhill across successive layers, and nothing gets underneath. Left alone, that arrangement works for decades.
Roofs leak where that arrangement stops. At a chimney, a skylight, a wall, a vent pipe, a valley, a dormer — anywhere the continuous shedding surface is interrupted and water has to be redirected by a piece of metal instead. That metal is flashing, and in our experience the overwhelming majority of residential roof leaks originate at one.
Flashing is also the part of a roof most exposed to installer skill. Shingles are largely a matter of following a pattern; anyone can be trained to lay them acceptably in a day. Flashing is judgment, sequencing, and hand-formed metalwork, done differently at every penetration. It is where the difference between a competent crew and a fast crew becomes structural rather than cosmetic — and because flashing is buried under the shingles, that difference is invisible on the day the job is finished and expensive four years later.
This article is a component-by-component account of what each flashing assembly does, how it fails, and how to check yours.
Where a sloped roof runs alongside a vertical wall — a dormer side, a two-story wall meeting a lower roof, the side of a chimney — the junction is protected by step flashing. The correct assembly uses individual L-shaped pieces, one per shingle course, each bent so one leg lies on the roof and the other runs up the wall. Each piece is installed as its course is laid, so it laps over the piece below it and under the shingle above it. The result is a woven stair-step of metal that carries any water reaching the wall back out onto the shingle surface, course by course.
The wrong version, which we find constantly, is a single continuous bent length of metal run up the entire junction, with the shingles butted against it and the top edge sealed with caulk. It is faster to install by a large margin. It also has no capacity to shed water that gets behind it, because there is no lapping — there is one seam, held by adhesive, in the worst possible location.
Two further errors are common even where step flashing exists. First, reused flashing: on a re-roof, the old step flashing is left in place and new shingles are laid around it, which means the flashing is now decades older than the roof and frequently corroded at the fold. Second, sealed rather than woven: pieces are present but were laid on top of the shingles and caulked instead of being interleaved into the courses.
The other half of the assembly is counter flashing, which covers the upper legs of the step pieces so water running down the wall cannot get behind them. On siding, that role is normally played by the siding itself, which should lap over the step flashing rather than butting to it. On masonry, counter flashing must be let into a groove cut in the mortar joint — a reglet — and mechanically secured, then sealed. Surface-mounted counter flashing that is simply caulked to the face of a brick chimney is a maintenance item with a short life, and it is the single most common chimney leak we encounter.
If this article convinces you to check one thing, make it this one.
At the bottom of a roof-to-wall junction, where the roof edge terminates against a wall that continues past it, all the water that has been travelling down that junction arrives at one point. A kickout is a small piece of flashing, bent outward at the base of the step flashing run, that kicks that concentrated stream away from the wall and into the gutter. Without it, that water runs straight down behind the siding.
It is a single small custom-bent piece at one location, easy to overlook in a fast install, and it does not appear on any homeowner's mental list. Many crews simply do not fit them. On homes built before the detail became widely enforced, it is frequently absent entirely.
Nothing, for years. Water enters the wall assembly rather than the attic, so there is no ceiling stain to alert anyone. It wets sheathing, framing, and insulation inside the wall cavity continuously through every wet season. The damage is typically discovered during a siding replacement, a window replacement, or a pre-sale inspection — at which point it involves structural repair rather than a flashing detail.
Stand where a roof edge meets a wall above a gutter. Look at the very bottom of that junction. You should see a small piece of metal angled out toward the gutter. If the step flashing simply stops and the shingles end against the wall with nothing diverting water outward, you have found a real problem. Look also for staining, paint failure, or swollen siding on the wall directly below that point.
A chimney is the most complex flashing situation on a typical house, because it interrupts the roof on all four sides and each side needs a different treatment.
A single continuous piece running across the low side, lapping over the shingles below it. Straightforward, and the least common failure point.
Step flashing, woven course by course, exactly as at a wall, with counter flashing let into the mortar.
This is the one that gets omitted. On the uphill side, a chimney acts as a dam across the slope, and everything running down the roof above it piles into that junction along with a season's worth of conifer needles and leaf litter. A cricket — also called a saddle — is a small peaked structure built behind the chimney that splits water around both sides. Code requires one where the chimney is wider than 30 inches on the upslope side, and it is good practice well below that width on any roof under a tree canopy.
Where a cricket is missing, the back of the chimney becomes a permanent debris trap holding wet organic material against masonry and flashing. In Seattle, under maple and fir, that trap fills every autumn. The resulting failure is not a question of if.
Worth stating because it is frequently misattributed to roofing: a chimney can admit water without any flashing failure at all. Deteriorated mortar joints, a cracked or missing crown at the top, spalling brick, and an uncapped flue all let water into the masonry, where it travels down and emerges at the ceiling adjacent to the chimney — looking exactly like a flashing leak. If your chimney leaks and the flashing checks out, the chimney is the suspect.
A valley collects the runoff of two roof planes and concentrates it into a single channel, so the water volume passing through it is far greater than anywhere else on the roof. Three construction methods are common: open metal, where a metal channel is left exposed with shingles trimmed back on both sides; closed-cut, where shingles from one plane run through and the other side is trimmed to a line; and woven, where courses from both planes interlace.
In this region we favour open metal valleys, and the reason is entirely local. Seattle valleys collect conifer needles, maple seed, and moss continuously. An open metal valley presents a smooth channel that debris can wash through and that can be cleared easily. Closed-cut and woven valleys present a granular surface that catches and holds debris, forming dams that pond water against the assembly through the wet season. Under heavy canopy the difference in service life is significant.
Regardless of method, the valley should have self-adhered membrane beneath it for its full length, and the shingle cut lines should be sealed. Valley terminations at the eave — where the concentrated flow exits — are a common failure point where the metal stops short or is not properly lapped over the drip edge.
Every plumbing stack passes through the roof inside a flashing base with a collar sealing against the pipe. On the common version that collar is a rubber gasket, and rubber has a shorter service life than the shingles around it — typically 10 to 15 years before UV and thermal cycling crack it. This is one of the most frequent single-point leaks in residential roofing and one of the cheapest to fix if caught before the deck beneath softens.
Lead boots last dramatically longer, with the caveat that squirrels and rodents sometimes chew them. Retrofit collars that slip over an existing pipe to renew a cracked seal are an inexpensive interim fix. When we are on a roof for any reason, boots get checked, because replacing one during another visit costs almost nothing and replacing the sheathing under a failed one does not.
Skylights should be flashed with the manufacturer's kit, which comprises separate sill, step, and head pieces designed to integrate with the shingle courses. A skylight sealed in with caulk instead of flashed with a kit is a leak on a timer. Curb-mounted units sit on a built-up curb and are generally more forgiving; deck-mounted units sit lower and depend more heavily on precise flashing.
One important diagnostic caveat: water appearing at the interior frame of a skylight is frequently condensation rather than a leak. A skylight is a cold surface in a moist room, and in a Seattle winter it will collect condensate that runs down the glass and pools at the frame, presenting exactly like a leak. Before flashing gets blamed, that possibility should be ruled out.
A metal angle installed along eaves and rakes that carries water off the roof edge and into the gutter rather than letting it curl back under the shingle edge onto the fascia. It is inexpensive, code-required, and still gets left off, particularly on older homes and budget re-roofs. Its absence produces a slow, cumulative failure: water wicking behind the gutter, saturating fascia, and eventually rotting the board the gutter is fastened to — which is how a gutter ends up pulling off the house during a storm. If you can see the raw edge of the sheathing or the shingles overhanging with no metal beneath them, drip edge is missing.
Flashing material matters more here than in drier regions, because our flashings stay wet for months at a time.
Galvanized steel is the common default: inexpensive and adequate, but its zinc coating eventually depletes and the steel beneath rusts, particularly at cut edges and folds where the coating is thinnest. Aluminium does not rust and is easy to form, but it is soft and reacts badly with masonry and with certain treated lumber, so it is a poor choice in direct contact with mortar. Copper is the premium option: it outlives the roof, forms beautifully, and has the incidental benefit of shedding trace ions that suppress moss and algae on the slope below. Stainless is used where corrosion resistance is critical.
The genuinely important local consideration is galvanic corrosion. When two dissimilar metals are in contact in the presence of moisture — and near Puget Sound, salt-laden moisture — one corrodes preferentially at the junction. Aluminium fasteners through copper flashing, or steel fasteners in contact with copper, will fail at the connection long before either material fails on its own. For homes at Alki, Fauntleroy, Magnolia, Golden Gardens, and the Sound-facing bluffs generally, fastener and flashing metallurgy must be matched deliberately. This is a detail that separates roofers who understand marine exposure from those who buy whatever is on the truck.
A useful amount of this can be checked without walking the roof, which we would generally discourage on a wet Seattle roof anyway.
With binoculars from the ground: Look at every chimney for a cricket on the uphill side. Look at every roof-to-wall junction for a kickout at the base. Look for black roof cement smeared anywhere — it marks a previous failed repair. Look at valleys for debris accumulation and for whether they are open metal or shingled over. Look along eaves for the thin line of drip edge metal.
From a ladder at the eave: Check the condition of the drip edge and whether the gutter sits below the shingle line so water actually lands in it. Look for rust staining running down from flashing points, which indicates galvanized material at end of life.
From inside the attic: Trace the interior locations of the chimney and any skylights and inspect the surrounding sheathing for staining. Flashing failures reveal themselves from below long before they show on a ceiling.
On the exterior walls: Look below every roof-to-wall junction for paint failure, staining, swollen siding, or soft trim. That is kickout evidence, and it is the failure worth catching early because it is the only one on this list that damages your walls rather than your roof.
Individual flashing components can be replaced without touching the rest of the roof, and frequently should be. A failed vent boot, a missing kickout, a chimney counter flashing that has pulled out of its mortar joint — all are discrete repairs, and all are dramatically cheaper than the damage they prevent.
Where flashing replacement genuinely requires roof work is anywhere the flashing is woven into the shingle courses. Step flashing cannot be properly replaced without lifting the courses it is interleaved with, which means that section of roof gets opened up. On a roof with life left, that is still worth doing at the affected junction. On a roof already near end of life, it is usually better to wait and do it correctly during the re-roof.
Which leads to the point worth carrying away: a re-roof is the one opportunity to get every flashing detail right at once, and it is the moment when doing so costs the least. All new step flashing rather than reused. Kickouts fitted at every roof-to-wall termination. A cricket behind any chimney that needs one. Open metal valleys with membrane beneath. New boots. Drip edge everywhere. Correct metallurgy for the exposure.
If a re-roof quote does not specify new flashing, ask whether the existing flashing is being reused. The answer determines whether you are buying a new roof or a new shingle surface over an old roof's weakest components.
Flashing, by a wide margin. A field of correctly installed shingles on a slope is highly reliable. Leaks happen where that continuous surface is interrupted at chimneys, skylights, walls, vent pipes, valleys, and dormers, and water has to be redirected by metal instead.
It is a small piece of flashing at the base of a roof-to-wall junction that diverts concentrated water into the gutter instead of behind the siding. When missing, water enters the wall assembly rather than the attic, so there is no ceiling stain to warn you. Damage accumulates inside the wall for years and is usually discovered during siding or window work.
As individual L-shaped pieces, one per shingle course, each lapping over the piece below and tucked under the shingle above, forming a woven stair-step. A single continuous bent length caulked at the top is faster to install and has no ability to shed water that gets behind it.
Code requires one where the chimney is wider than 30 inches on the upslope side, and it is good practice below that width on any roof under tree canopy. Without one, the uphill side of the chimney becomes a permanent trap for conifer needles and leaf litter held wet against masonry and flashing.
Open metal valleys, because of our tree canopy. They present a smooth channel that debris washes through and that can be cleared easily, while closed-cut and woven valleys present a granular surface that catches needles and moss, forming dams that pond water through the wet season.
The common rubber collar typically fails at 10 to 15 years as UV and thermal cycling crack the elastomer, well before the shingles around it. Lead boots last considerably longer. This is one of the most frequent single-point leaks and one of the cheapest to fix before the deck beneath softens.
Our estimates involve actually walking your roof and inspecting the attic, then a fixed scope and price in writing. Free estimates across Seattle and the greater Puget Sound region.
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