Leaks and salt corrosion
On the coast the failure is often not wear. It is corrosion, on parts that are cheap to replace and invisible until they fail.
Roofs rarely leak through the middle of a slope. They leak where the field stops and has to meet something else, and on the coast those junctions are mostly made of metal.
What salt actually attacks
Not the shingles. Asphalt is largely indifferent to salt. What corrodes is everything holding the system together.
Fasteners. Nails and screws in flashing and vents. When a fastener corrodes it loses grip and it opens the hole it was sealing.
Drip edge and flashing. Thin metal at the most exposed parts of the roof. Once it perforates, water gets behind rather than over.
Vent housings and pipe collars. Metal housings corrode and their seals fail.
Gutters and fascia. Not roofing exactly, and the commonest cause of what people report as roof leaks.
None of this is visible from the ground and almost none of it is visible from a photograph of the roof field. It is why a coastal inspection should spend as much time on the metal as on the shingle.
Where water gets in
Around plumbing vents, where heat and salt work on the collar together. At wall and chimney flashing. In valleys, which move enormous volumes in a coastal downpour. At the eaves, where wind driven rain pushes uphill under the bottom courses. And through gutters that are no longer doing anything.
Why the stain is not under the entry point
Water comes through, lands on the deck or a rafter, and travels along the timber until something stops it. It drips there, and that is where the ceiling marks. Ten feet of travel is ordinary.
Which means the useful search starts uphill of the stain and works the details, not directly above it.
Questions people ask
- Only in a driving rain. Is that still the roof?
Usually, and it narrows it. Leaks that need wind behind them point at flashing, at edges, or at water being driven sideways under courses rather than falling on them. That pattern is far more common here than inland.
- Can it be found without waiting for weather?
Often, with a methodical hose test working from low to high. It is slow. A company unwilling to do it is guessing, and guessing on a roof is expensive.
- The gutters are rotten. Does that matter to the roof?
Yes. A gutter that has corroded through or detached lets water run behind the fascia and under the bottom course of shingles, which presents exactly like a roof leak and cannot be fixed on the roof.