A roof flashing leak starts where the roof meets something else
A roof flashing leak happens at a junction rather than in the middle of a roof surface. Chimneys, skylights, walls, parapets and valleys are where water concentrates and where two different materials have to move against each other, and those junctions are where the majority of roof leaks on any building actually begin.
The roof surface itself is the simple part. It is a continuous plane doing one job, and it generally does it well. Every leak worth worrying about tends to be at an interruption of that plane, which is why an experienced roofer looks at the edges and the penetrations before looking at the field.
This article covers each of the common flashing details, why they fail faster in this climate than the roof around them, and why the usual response of adding more sealant tends to buy a season rather than a decade.
Flashing is a formed detail, not a seal
This is the distinction that explains almost everything else on this page. Properly done, flashing is shaped metal that directs water back onto the roof surface by its geometry, with laps and upstands arranged so gravity does the work. Sealant is a secondary defence at the edges, not the thing holding water out.
Where that geometry is right, a flashing keeps working even as the sealant ages, because the sealant was never load-bearing. Where the geometry is wrong or missing, the sealant becomes the only barrier, and sealant has a service life measured in a few years in this climate rather than decades.
That is why a detail that has been repaired with mastic repeatedly will keep coming back. Each repair restores the temporary barrier without addressing the reason water is reaching it, so the interval between failures tends to shorten rather than lengthen over time.
Chimneys are the hardest detail on most roofs
A chimney interrupts the roof plane on four sides, each of which needs different treatment, and the water shed by the whole upper slope has to be routed around it. It is the single most demanding junction on a typical house, and it is where a disproportionate number of leaks originate.
A correct chimney flashing is two-part. Base flashing sits on the roof and turns up the chimney, and counter-flashing is let into a mortar joint and laps down over the top of it, so water running down the brickwork is thrown clear of the junction rather than finding the back of the base flashing.
The common failure is a counter-flashing that was never let in properly and is instead sealed to the face of the brick. That joint is exposed to sun and movement, it opens, and water then runs behind the base flashing with nothing to stop it. From the ground, nothing looks wrong at all.
On the uphill side, a cricket or saddle diverts water around a wide chimney instead of letting it pond against the back of it. Where one is missing on a chimney that needs it, you get a reservoir in exactly the worst place, and a chimney flashing leak that only appears in heavy rain.
Skylights usually leak at the curb, not the glass
A skylight leak is rarely the glass or the seal around it, although that is where everyone looks first. The usual source is the curb the unit sits on and the flashing kit that ties that curb into the roof, particularly at the head where water arriving from above has to be diverted around it.
Skylight manufacturers supply flashing kits designed for specific roof coverings, and problems often start with a unit fitted using a generic approach instead. On a tile roof in particular, the interface between a flat flashing kit and a profiled tile is a detail that has to be formed carefully rather than packed with sealant.
Condensation is the other candidate and it is frequently mistaken for a leak. Water appearing around a skylight on a cold morning with no rain is probably condensing on the glass rather than entering past it, and the fix for that is ventilation rather than roofing.
Wall abutments fail at the same point as chimneys
Where a roof runs into a wall, usually at an extension or a stepped elevation, the detail is the same idea as a chimney on one side. The roof flashing turns up the wall and something has to cover the top of that upstand so water running down the wall cannot get behind it.
That covering should be let into the masonry, either into a mortar joint or into a formed channel. Where it has simply been sealed to the wall face, it is on the same clock as any other exposed bead of mastic, and when it opens the water runs straight down behind the upstand and into the structure.
Stepped flashing on a sloping abutment is the version most often done badly, because it is fiddly and time-consuming to do correctly. A continuous strip of metal bent to follow the steps is not the same thing as individual pieces lapped with each course, and it does not perform the same way.
Parapets hold water on top of themselves
Parapet walls are common on the flat and low-slope roofs around the Valley, and they have a vulnerability that pitched details do not. Their top surface is roughly horizontal, so it collects water rather than shedding it, and whatever covers that top surface is the only thing keeping water out of the wall below.
A coping that is cracked, lifted or has open joints lets water into the top of the wall, where it travels down inside the masonry and emerges somewhere that looks nothing like the entry point. These are classic delayed leaks, appearing after the rain has stopped rather than during it.
The junction between the roof membrane and the inside face of the parapet is the second weak spot, because it is a right-angle detail that has to flex with a wall that moves differently from the roof deck. Roof leaks after rain stops covers why that pattern of delay points at details like these.
Valleys carry far more water than their area suggests
A valley collects the drainage of two roof planes into a single channel, so the volume passing through it during a downpour is out of all proportion to how much of the roof it represents. That makes it unforgiving of any defect in the lining or the laps.
Debris is the local aggravator. Fine grit and material blown in through the dry months settles in valleys, and the National Weather Service office in Phoenix names dust storms among the monsoon hazards alongside damaging winds, torrential rainfall and lightning (NWS Phoenix).
A partially blocked valley does not leak in light rain because the reduced flow still gets away. In a real storm the water backs up above the designed waterline and goes in over the top of the lining, which is why this is such a reliable heavy-rain-only symptom. When a tile roof leaks in heavy rain covers that pattern in full.
Heat and ultraviolet light take the sealant first
Every one of these details relies to some degree on a sealed joint somewhere, and in this climate that is the component with the shortest life. At the Scottsdale Municipal Airport station, NOAA’s 1991 to 2020 normals average about 93 days a year at or above 100 degrees Fahrenheit and about 161 days at or above 90.
Metal flashing expands and contracts measurably through each of those days, and the sealant at its edges has to absorb that movement while being broken down by ultraviolet light at the same time (NOAA NCEI). National Weather Service figures for the Phoenix area add an average of 21 days a year at or above 110 degrees over the same period (NWS Phoenix).
The City of Scottsdale’s own homeowner guide describes this cycle working fasteners loose over time, and the same mechanism opens sealed joints. It is also why a flashing detail that performs for twenty years somewhere milder may need attention far sooner here.
Previous repairs are often the actual cause
A detail that has been sealed several times is usually a detail that was formed wrongly once. Each repair buys a year or two, and each one makes the underlying geometry harder to see, until nobody can tell what the original detail was supposed to be without stripping it back.
This matters when you are getting quotes. A contractor proposing to clean the area back and re-form the detail properly is describing a different and more expensive job from one proposing to seal it again, and the second will usually be cheaper and will usually be back.
Thick beads of mastic, mismatched metal, roofing cement smeared across a junction and tape over a joint are all signs that the detail is being held together rather than working. They are worth pointing out when you ask for an assessment.
Diagnosis means running water, not reasoning
Flashing leaks are found the same way as any other: by reproducing them. A hose test runs water deliberately in sections, starting below the suspected detail and working upward, with someone watching inside, so the moment water appears is tied to one specific part of one junction.
That sequence matters at a flashing more than anywhere else, because these details sit in a stack. Wetting a chimney head and a base flashing at once tells you only that water got in somewhere around the chimney, which you already knew.
Water also travels before it drops, running along battens, rafters and the underside of the deck, so the stain indoors can be a long way from the junction that is failing. A diagnosis offered without anyone going up is not a diagnosis.
Repair means re-forming rather than re-sealing
A lasting flashing repair restores the geometry: the counter-flashing goes back into the masonry, the laps are correct, the upstand is the right height, and the metal is continuous where it needs to be. Sealant then does its proper job as a secondary defence rather than as the whole defence.
Expect that to cost more than another bead of mastic and to take longer. It is the difference between a repair measured in years and one measured in seasons, and on a detail like a chimney it is usually the only approach that actually ends the problem.
There is a safety point throughout. These details are at the awkward parts of a roof, often at height and often on tile, which is brittle and breaks underfoot. The city’s own guide says roof work can be dangerous and lethal for the inexperienced, and flashing details are the worst place to learn that.
Where to start
If your leak appears near a chimney, skylight, wall junction or parapet, say so when you book an inspection, because it changes what the inspector brings and how long they allow. Flashing diagnosis takes longer than looking at a field of tiles.
If a detail has been repaired before, say that too, and say how many times. A junction on its third repair is telling you something about the first two, and it changes the right answer from sealing to re-forming.
Our guide to roof problems in Scottsdale sets out how the various failures present if you want to place your symptoms first. If you would rather have someone look at it, Residential Roofing Scottsdale works across Scottsdale and the surrounding Valley. Call (480) 919-1173, or read our roof repair in Scottsdale page for what an inspection and repair involves.
Frequently asked questions
These are the questions homeowners in this area ask most often about leaks at chimneys, skylights and wall junctions, which are the details that produce most of the leaks on most houses. Each answer is kept short deliberately, with a link to the detailed article where one exists.
What is roof flashing and what does it do?
It is shaped metal at a junction, such as where a roof meets a chimney or a wall, arranged so that its geometry directs water back onto the roof surface. Sealant at its edges is a secondary defence rather than the thing actually holding water out.
Why does my chimney leak only in heavy rain?
Usually because water is getting behind a counter-flashing that was sealed to the brick face rather than let into a mortar joint, or because a wide chimney has no cricket and water ponds against its uphill side. Light rain never generates enough volume to reach either fault.
Is my skylight leaking or is it condensation?
Water appearing on a cold morning with no rain is almost always condensation on the glass, and the answer to that is ventilation rather than roofing. A genuine skylight leak is tied to rain and usually originates at the curb or the head flashing rather than the glass.
Can flashing be repaired with sealant?
As a temporary measure before a proper repair, yes. As the repair itself it rarely lasts, because sealant on a surface that bakes and moves daily has a short life here, and because re-sealing does not address the geometry that let water reach the joint.
How long does flashing last in Arizona?
We publish no figure, because the variables are the quality of the original detail, the metal used and how much sun it takes, and no verified local source for a service life exists. The sealed joints age considerably faster than the metal itself.
Does a flashing leak mean the whole roof needs replacing?
Normally not. A flashing failure is a localised defect at a junction, and the surrounding roof is often sound. It becomes part of a larger conversation only when the underlayment has also reached the end of its life, which shows as leaks in several unrelated places.