Flashing: Where South San Francisco Roofs Actually Leak, and Why
When a roof leaks, the field of shingles is rarely the culprit. Far more often it is the flashing, the metal that seals the roof's transitions and penetrations. Here is what flashing does, why it fails first, and why reusing old flashing is a false economy.
The part of the roof that does the hardest job
Most people picture a roof as a field of shingles, and that field does the bulk of the visible work of shedding water. But the field is also the easy part. A flat or sloped plane of overlapping shingles sheds rain reliably with no help, and it is rarely where a roof leaks. The hard part of keeping a roof watertight is everything that interrupts that clean plane, the chimney rising through it, the walls it runs into, the vents and pipes poking through it, the valleys where two slopes meet. Those transitions and penetrations are where water wants to get in, and the metal that seals them is called flashing.
Flashing does the hardest job on the roof, sealing the very points where the simple overlap of shingles is no longer enough. When a roof leaks, the flashing is the first place an experienced roofer looks, because that is where leaks overwhelmingly originate. Understanding flashing, what it is, why it fails, and why it has to be done right, explains most of what a homeowner needs to know about why roofs leak and how to keep them from doing so.
Why flashing fails before the shingles do
Flashing tends to fail before the field of shingles for a few related reasons. It is metal, so over a long life it can corrode, and corroded flashing is no longer a reliable seal. It sits at the points of greatest stress on the roof, the transitions and penetrations where water concentrates and where the roof flexes and moves, so it takes more punishment than the open field. And it is often the part of the roof that was done least carefully in the first place, especially on older homes where past repairs caulked over a flashing problem rather than replacing the flashing properly.
That last point deserves emphasis, because caulk-over-corrosion is one of the most common shortcuts we find on older South San Francisco roofs. Instead of removing failed flashing and installing new, a previous crew smeared sealant over the problem. Caulk is not a permanent fix for flashing. It dries, it cracks, and once it does, the underlying flashing problem it was hiding lets water straight through. A roof can look fine, with an intact field of shingles and a tidy line of old caulk at the wall, while that wall flashing is actively failing underneath. This is exactly the kind of thing a careful inspection is built to find.
- Leaks overwhelmingly start at flashing, not the field of shingles
- Flashing seals the chimney, walls, valleys, vents, and penetrations
- It is metal at the points of greatest stress, so it fails first
- Caulking over failed flashing is a temporary fix that cracks and leaks
- A roof can look fine while its flashing is actively failing
Why a leak shows up far from where it started
One of the most confusing things about a flashing leak is that the water rarely shows up where it got in. When water enters at a flashing detail, it does not drop straight down onto the ceiling below. It runs along the underside of the deck and the framing, following the path of least resistance, sometimes a considerable distance, before it finally finds a spot to drip. The stain on the ceiling can be far from the actual failure, which is why chasing a leak by looking only at the spot where the water appears is a recipe for missing the real cause.
This is why diagnosis is most of the value of a leak repair. A crew that simply seals around the stain is treating a symptom in the wrong place and will be back at the next rain. Tracing the water back to its true origin, which on most roofs means working back to a flashing detail, a perished boot, or a leaking valley, is the part that takes experience and the part that determines whether the repair actually holds. Find the real entry point, fix that, and the leak is gone. Guess at it, and you have bought yourself a return visit.
Why reused flashing is a false economy
All of this is why we fit new flashing rather than reusing the old when we replace a roof, and why an estimate that quietly plans to reuse existing flashing is cheaper for a reason. Putting a new roof over old, corroded flashing is building a brand new roof around its own most likely future leak. The flashing is at the transitions that fail first, it is already aged, and reusing it means the new roof inherits a weak point on the very first day. The savings are real and so is the risk, and the risk is precisely the one you were trying to avoid by replacing the roof.
When we replace or work on a South San Francisco roof, the flashing gets the attention it deserves, new metal at every wall, chimney, and penetration, installed properly rather than caulked over. It is not the glamorous part of a roof and it does not show from the curb, but it is where roofs leak, so it is where careful work pays off most. If your roof is leaking, the flashing is the first thing we will look at, and if we are building you a new roof, the flashing is one of the things we will not cut corners on, because doing so would undermine the whole job.
If your roof is leaking, the flashing is the most likely culprit, and finding the true source is most of the fix. Call 650-477-1036 for a free inspection and a repair built on a correct diagnosis, not a guess.
Call 650-477-1036 and we will tell you honestly what the roof needs.