Ask a roofer where a building leaks and the answer is almost never the middle of the roof. It is a valley, a drain, a parapet, a curb, a pipe or a skylight. Those are the flashings, they cover a tiny share of the roof’s area, and they account for the large majority of its leaks.
The reason is straightforward. The field of a roof is machine made sheet, rolled out in long straight runs by people doing a repetitive task. A flashing is cut, bent and fitted by hand, on site, around a shape that was never designed with roofing in mind, often on a cold morning at the end of a shift.
What flashings have to do
Three jobs at once, and they conflict.
Shed water. The obvious one, and the easiest.
Absorb movement. A parapet wall heated by the sun expands on a different schedule from the deck beneath it. The flashing spanning the two takes that difference every day of its life. Over years the material hardens, loses the ability to flex and splits.
Terminate properly. Every flashing ends somewhere, usually against a wall or a curb, and that termination has to be mechanically held rather than relying on adhesive. A termination bar screwed into masonry and then sealed will outlast one that is simply stuck on by a wide margin.
The second of those is what owners underestimate, and it explains the pattern that puzzles people most: flashing failures on roofs whose membrane is in good condition. The field has not moved. The junction has, thousands of times.
The details, in order of risk
- Valleys and drain sumps. Highest water volume and where debris collects. Anything that holds water holds it here.
- Parapet and wall abutments. Highest movement. The upstand, the counterflashing and the termination all have to work together.
- Equipment curbs. Highest traffic. Service contractors stand on these, lift them to run cables, and often do not reinstate them.
- Pipe collars and vent flashings. Seals perish on their own schedule regardless of the roof around them.
- Skylights and hatches. Combination of movement, traffic and condensation, all in one detail.
- Previous repairs. Any of the above that has been patched already is now the most likely place to leak.
That last entry is not a joke. On most commercial roofs, the single best predictor of the next leak is where the last repair was made, because the conditions that caused the first failure are still there and the repair was usually cheaper than the original detail.
Sealant is not a flashing
This is the most common and most expensive misunderstanding in roof maintenance.
Sealant has a legitimate role: a secondary defense behind a mechanically secured termination, or a temporary measure while a proper repair is arranged. It is not a primary water barrier, it cannot carry the movement a junction produces, and it has a much shorter life than the metal or membrane it is being used to replace.
Worse, it makes the eventual repair more expensive in two ways. It hides how far the opening has actually spread, so the repair gets scoped wrongly. And it leaves a residue that new material will not bond to, so it has to be cut out before anything permanent can be done.
A roof carrying years of accumulated mastic is therefore more expensive to put right than one that was left alone, which is the opposite of what the owner was told each time somebody sold them a cheap visit.
What a lasting repair looks like
Four steps, and the second is the one that distinguishes a real repair.
Remove the failed flashing. Not overlay it. A new flashing laid over a failed one seals in whatever water is behind, and the detail fails again from the inside.
Open up and check behind. A flashing failure almost always means wet insulation behind the upstand. That board comes out, and the deck beneath it gets looked at while access is free.
Rebuild to a published drawing. Every manufacturer publishes details for curbs, walls, pipes and drains. The repair should follow the drawing for your system rather than the crew’s habit, and an owner is entitled to ask which drawing was used.
Tie into the field properly. The joint between the new flashing and the existing membrane is welded or bonded in the original system, so it becomes part of the roof. Not sealed.
Then photograph it and mark it on the roof plan, because in three years somebody will be trying to work out what was done here.
A maintenance routine that prevents most of it
Flashing failures are slow, local and cheap for a long time. The whole value of a maintenance program is catching them during that window.
Twice a year, and after any serious storm, somebody walks the roof and checks every detail individually. Clear the drains and the valleys. Look at each upstand for splits, lifted edges and failed terminations. Check each equipment curb for disturbance. Press the pipe collars to see whether the seal has gone hard. Photograph anything found, locate it on the plan, and compare it with last visit’s photographs to see what is moving.
None of that needs equipment and none of it takes long. What it needs is somebody responsible for it on a date, which is the part that usually goes missing. Our page on the drainage items an inspection should cover sets out the full list.
Counterflashing, and the detail owners never hear about
Where a roof meets a wall, two pieces of metal are needed rather than one. The base flashing runs up the wall from the roof, carrying the water. The counterflashing sits above it, tucked into the wall or into a reglet cut in the masonry, and covers the top edge of the base flashing so water running down the wall cannot get behind it.
Leave the counterflashing out and the detail looks finished. Water running down the wall face then arrives at the top edge of the base flashing, finds the sealant there, and goes behind it the moment that sealant ages. The leak appears to come from the roof and is actually coming from the wall.
This is one of the most common omissions on commercial buildings, particularly where a roof abuts a taller neighboring structure or where a parapet has been rebuilt at some point without the roofing being involved. It is worth checking specifically, because the repair is straightforward and the symptom is persistent and baffling until somebody looks.
Where a counterflashing does exist, the question is whether it is held in a reglet or simply surface mounted and sealed. The first lasts; the second is a sealant joint carrying the whole detail.
Where to go next
If water is coming in now and the source is not obvious, tracing a leak properly comes before any flashing work, because on a low slope roof the stain and the opening are rarely in the same place. If several details are failing at once rather than one, that pattern is worth taking seriously as a sign of the roof’s age rather than treating as a series of repairs.
Common questions
What is roof flashing?
Flashing is the material that weathers a junction: where the roof meets a wall, a curb, a pipe, a drain or another roof. It is usually metal or membrane, and it is the only part of a roof that is cut, formed and fitted individually on site rather than rolled out.
- It weathers every junction and penetration
- Metal, membrane or a combination of both
- Formed and fitted by hand on site
- Needs a termination that is mechanically secured
- Counts for a small area and most of the risk
Flashings cover a tiny fraction of a roof's area and account for the large majority of its leaks, and the reason is simply that they are handmade. The field of a roof is a machine made sheet installed in long straight runs. A curb flashing is four corners, two laps and a termination, made by a person on a windy morning around a shape nobody designed for roofing.
Why do flashings fail before the rest of the roof?
Because they take more movement, hold more water and have more joints than anything else. A wall or curb moves differently from the roof deck, so the flashing between them flexes with every temperature change, and movement works on a joint until it opens.
- Differential movement between roof and wall or curb
- More joints and corners than the field has
- Water concentrates at valleys, drains and low points
- Debris collects against upstands and holds moisture
- Sealant used at terminations has a short life
Thermal movement is the mechanism owners underestimate. A parapet wall heated by the sun expands on a different schedule from the roof deck below, and the flashing spanning the two absorbs that difference every single day. Over years it hardens, loses its ability to flex and splits, which is why flashing failures appear on roofs whose membrane is still in good condition. The field has not moved at all; the junction has moved thousands of times.
Is sealant an acceptable flashing repair?
As a temporary measure yes, as a repair no. Sealant has a far shorter life than metal or membrane and it is not designed to carry movement or to be the primary water barrier. Where it is used in place of a proper flashing it fails, usually within a few seasons.
- Sealant is a secondary defense, not a flashing
- It cannot carry the movement a junction produces
- It hides the extent of the opening behind it
- It has to be removed before a genuine repair
- A roof covered in mastic costs more to fix later
The lasting cost of sealant is that it conceals and contaminates. It hides how far the opening has actually spread, so the eventual repair is scoped wrongly, and it leaves a residue that nothing bonds to, so it has to be cut out before new material can go on. A roof that has been sealed repeatedly over years is more expensive to repair than one left alone.
What does a proper flashing repair involve?
Removing the failed flashing rather than covering it, checking what is behind it, and rebuilding the detail to the manufacturer's published drawing with a mechanically secured termination. The repair is then tied into the field membrane with a welded or bonded joint, not a sealed one.
- The old flashing is removed, not overlaid
- Wet insulation behind the detail is cut out
- The detail is rebuilt to a published drawing
- Terminations are mechanically fixed, then sealed
- The joint to the field membrane is welded or bonded
The sequence matters because a flashing failure almost always means water behind it. Overlaying a new flashing onto a wet upstand seals the moisture in, where it keeps working on the insulation and the deck, and the detail will fail again from behind. Opening the detail adds an hour of labor and is the difference between a repair that lasts for years and one that comes back next winter in the same place.
Which flashings should be checked most often?
Valleys and drains, because they carry the most water. Then any wall or parapet abutment, because of the movement. Then curbs under rooftop equipment, because service traffic disturbs them. Pipe collars and vent flashings come next, since their seals perish on their own schedule.
- Valleys and drain sumps take the greatest volume
- Wall and parapet abutments absorb the most movement
- Equipment curbs are disturbed by service visits
- Pipe collars and vent seals perish with age
- Anything previously repaired deserves a second look
Equipment curbs deserve particular attention because the damage there is caused rather than weathered. Mechanical contractors work around those units, stand on the flashing, lift it to run a cable and rarely reinstate it properly. A log of who goes onto the roof, with a photograph of the area before they leave, prevents most of it, and it makes the rest somebody else's insurance claim rather than a line in your maintenance budget.