Where do roof leaks actually come from?
Overwhelmingly from the places where the roof plane ends rather than from the middle of it: sidewalls, chimneys, valleys, plumbing and exhaust penetrations, and skylights. At those points water is turned rather than shed, so the defence is metal, and the pieces have names — step flashing up a sidewall, kick-out or diverter flashing where that wall meets the eave, apron flashing at the front of a chimney, counterflashing let into a masonry joint above it, a cricket behind any chimney more than 30 inches wide, valley metal, and a boot at every pipe. Sealant is a finishing detail at the top of counterflashing, never the primary barrier. A quote that says reuse existing flashing or seal with roof cement has priced the leak out of the job.
Why the middle of a roof rarely leaks
The field of a shingle roof is a solved problem. Thousands of overlapping units, gravity, and a slope: water arrives, water leaves. What is not solved by overlap is any place the plane is interrupted — because at an interruption, water has to change direction, and changing direction means it can be pushed somewhere it would not go on its own.
That is what flashing is for. Every flashing component is a piece of formed metal whose job is to carry water across a junction and put it back on top of the roof surface, downslope of the obstruction. Get that geometry right and the junction is permanent. Get it wrong and no amount of sealant will save it, because sealant is a bead of material with a service life measured in a few years, sitting in a location that moves with every thermal cycle.
The rule that explains most flashing failures: water must always be directed onto a surface that is already shedding it, and every lap must be arranged so the upper piece covers the lower. Flashing that relies on adhesion instead of overlap is flashing waiting to fail.
Every component, named
Step flashing (sidewalls)
Individual L-shaped pieces, one per shingle course, woven up a wall as the courses go on: one leg under the shingle, one leg up the wall behind the siding or under the counterflashing.
Fails when: A single long piece of bent metal is used instead of stepped pieces, or the pieces are surface-nailed through the face and sealed. Both put fasteners in the water path.Kick-out / diverter flashing
The piece at the very bottom of a wall-to-roof intersection that kicks water out of the wall channel and into the gutter instead of letting it run behind the siding.
Fails when: Omitted. This is the most commonly missing flashing component on American houses, and it causes some of the most expensive damage: rotted sheathing and framing inside the wall, with no roof leak ever appearing on a ceiling.Apron flashing
The single piece across the downslope face of a chimney, dormer or wall that runs perpendicular to the slope, carrying water around and past the obstruction.
Fails when: Reused from the old roof, or terminated short so water runs behind the ends.Counterflashing (cap flashing)
The upper metal that covers the vertical leg of the base or step flashing. On masonry it is let into a cut mortar joint or a reglet and sealed at that joint only.
Fails when: Surface-caulked flat to the brick. That bead is now the entire water barrier, and it has a few years in it.Chimney cricket (backpan)
A small framed and flashed ridge built on the upslope side of a chimney to split water and drive it around both sides. Required where a chimney is more than 30 inches wide, measured perpendicular to the slope.
Fails when: Replaced with a flat pan and a lot of sealant. Water and debris pool against the masonry until the joint gives.Valley treatment
Either open metal — a formed W or V channel with the shingles cut back either side — or a closed-cut valley where one plane’s shingles run through and the other is trimmed to a line.
Fails when: Fasteners placed inside the water channel, ice-and-water membrane omitted beneath, or a closed-cut valley used on a slope combination that concentrates too much flow.Pipe boots and vent flashing
A metal base flange under the shingles with a collar sealing to the pipe. Plumbing vents, exhaust terminations, gas flues.
Fails when: The collar. A neoprene collar is a rubber part in full sun, and it typically cracks well before the shingles around it are worn out — a very common leak on a roof that still looks new.Skylight flashing
A manufacturer-matched kit — sill, side and head pieces — designed for that unit and that roof pitch, installed with membrane at the curb.
Fails when: Site-fabricated metal and sealant instead of the kit, or a new roof installed around an old skylight whose flashing was never disturbed and never checked.Drip edge
Formed metal at eaves and rakes that carries water clear of the deck edge and fascia. Over the drip edge at eaves, under it at rakes.
Fails when: Reused, omitted, or lapped in the wrong order, which routes water into the edge of the deck.The one component worth its own section
Picture a garage roof, or a bay, whose slope runs into the side of the house. Water travelling down that roof also travels along the base of the wall in the channel formed by the step flashing. At the bottom of that channel it arrives at the eave — and unless something is there to turn it outward, it keeps going in the direction it was already travelling: straight into the wall assembly behind the siding.
A kick-out flashing is the small angled piece that turns it out into the gutter. It costs very little. And when it is missing, the damage is uniquely expensive because it is invisible: no ceiling stain, no drip, nothing you would ever call a roofer about. Just sheathing, framing and insulation getting wet a few times a month for years, behind a wall that looks fine.
Worth checking on your own house today. Stand at any point where a roof slope runs into a wall and look at the very bottom of that intersection, just above the gutter. If there is no small piece of metal angled out into the gutter, there is no kick-out. If the siding immediately below that point is stained, blistered, or soft, get it opened up.
Why "reuse existing flashing" is the tell
Flashing on a roof being torn off is the same age as the shingles being torn off. It has been through the same freeze-thaw cycles, the same UV, the same hail. Galvanised step flashing rusts from the cut edges. Aluminium fatigues at the bends. Neoprene pipe-boot collars are already the oldest-looking component on most roofs.
There is also a geometry problem. Step flashing is woven into the shingle courses as they are installed. To reuse it, you either have to work around metal that is already fastened at a spacing that will not match the new courses, or lift and re-lay it — at which point you have handled every piece anyway and saved nothing but the material cost of new metal, which is a small fraction of the labour.
| Component | On a proper re-roof | What reuse means |
|---|---|---|
| Step flashing | New, woven course by course. | Old spacing fights the new courses; rusted edges stay in the wall. |
| Kick-out flashing | New, and often added where none existed. | The most common omission stays omitted. |
| Apron and counterflashing | New; counterflashing let into the joint, not caulked to the face. | A sealant bead becomes the barrier at a chimney for the next 25 years. |
| Pipe boots | New at every penetration. | The component most likely to fail first is the one you kept. |
| Valley metal | New, over membrane. | Rust and fastener holes stay in the highest-flow part of the roof. |
| Drip edge | New at eaves and rakes. | Correct lap order becomes impossible; fascia keeps wicking. |
| Skylight flashing | Manufacturer kit for that unit. | A 20-year-old detail is buried under a brand-new roof. |
What we specify, and why
Flashing is itemised by component and count, not summarised as a word
You should be able to count the penetrations on your own roof and find the same number in the scope. That is the standard we hold ourselves to on paper.
- New step flashing, woven course by course at every sidewall
- Kick-out flashing at every wall-to-eave intersection, including where none existed before
- New apron and counterflashing at chimneys, counterflashing let into the joint
- A cricket behind any chimney wider than 30 inches
- New pipe boots at every penetration, counted in the scope
- Valley treatment named — open metal or closed-cut — over full-length membrane
- New drip edge at eaves and rakes, lapped correctly at each
- Skylight flashing kits matched to the unit, never site-fabricated
How to tell whether a contractor is cutting this corner
The flashing is the same age as the roof being removed, and reusing step flashing forces the new courses to follow old fastener spacing.
Ask instead: for new step, apron, counter, valley and drip edge as separate line items.Mastic as a primary barrier. It is a legitimate finishing material and a poor substitute for geometry — it has a few years in it, in a location that moves every day.
Ask instead: what metal is carrying the water, and where does it discharge?A chimney needs an apron, step flashing up both sides, counterflashing let into the masonry, and a cricket if it is over 30 inches wide. One phrase covers all four or none.
Ask instead: is a cricket included, and is the counterflashing cut into the mortar joint?Surface-caulked counterflashing makes a sealant bead the entire barrier at the most leak-prone element on the house.
Ask instead: for counterflashing set into a cut joint or reglet.Neoprene collars crack from UV long before the shingles wear out. On a full replacement, boots are a small cost and the most likely first failure.
Ask instead: for new boots at every penetration, with the count in the scope.Burying a decades-old skylight detail under a brand-new roof guarantees the next leak is in the most disruptive possible place to fix.
Ask instead: is a manufacturer flashing kit included, and is the unit being re-flashed?Flashing questions DMV homeowners ask
Where do most roof leaks actually come from?
From the places where the roof plane is interrupted rather than from the field: sidewalls, chimneys, valleys, plumbing and exhaust penetrations, and skylights. In the field of the roof, overlap plus slope plus gravity does the work. At an interruption, water has to change direction, and a change of direction is where it can be driven somewhere it would not go on its own. That is why flashing detailing, not shingle brand, is the strongest predictor of whether a roof stays dry.
What is kick-out flashing and why does it matter so much?
It is the small angled piece at the very bottom of a wall-to-roof intersection that turns water out of the wall channel and into the gutter. Without it, water running down the base of that wall simply carries on in the direction it was already going, which is into the wall assembly behind the siding. The damage is uniquely expensive because it is invisible: sheathing, framing and insulation get wet repeatedly for years with no ceiling stain and no drip to report. It is also one of the cheapest components on the roof, which is why its absence says a lot about a past installation.
Should flashing be replaced when I get a new roof?
Yes, as the default. Flashing on a roof being torn off has lived through the same freeze-thaw cycling, UV and hail as the shingles being removed, and galvanised step flashing rusts from its cut edges where you cannot see it. There is also a geometry reason: step flashing is woven into the shingle courses as they go on, so reused pieces force the new courses to follow old fastener spacing. The material cost of new flashing is small next to the labour of handling old flashing twice.
How long do pipe boots last on a roof?
The metal base flange generally outlives the shingles; the collar around the pipe often does not. A neoprene collar is a rubber component sitting in full sun for its whole life, and cracking or splitting well before the surrounding roof is worn out is normal rather than exceptional. That makes a cracked boot one of the most common leaks on a roof that still looks new, and it is why new boots at every penetration belong in a replacement scope rather than being judged by eye.
Does my chimney need a cricket?
If it is more than 30 inches wide measured perpendicular to the roof slope, the model residential code calls for a cricket or saddle on the upslope side. The reason is simple: a wide chimney is a dam. Water and debris coming down the slope have nowhere to go except along the back of the masonry, so they sit there. A cricket is a small framed and flashed ridge that splits the flow and sends it around both sides. The alternative, a flat pan and a lot of sealant, works until the sealant does not.
Where to go next
Flashing handles the interruptions. The edges and the peak are the other place a roof is attacked — and the other place components get substituted.
Starter strip, hip and ridge components
What a purpose-made starter and cap actually do, and why cut-up 3-tab tabs are the visible tell of a cheap roof.
Read the standard →ShinglesShingle classes and impact ratings
Class 4 impact, wind classes and what the number in a shingle name really means.
Read the standard →ServiceRoof leaks and storm damage repair
Leak diagnosis that starts at the junctions, with photographs of what was found.
See the service →ToolInstant satellite roof quote
Your roof measured from above and priced at current DMV rates.
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