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Where the water shows up is not where it gets in

Most roof leaks are not holes in the roof. They are a detail — a boot, a flashing, a valley — and where the stain appears is a poor guide to which one.

How a leak is actually traced

1. Start in the attic

   Daylight, staining on the underside of the deck, rusted nail tips and wet insulation all point uphill towards where water is getting in.

2. Follow the water up

   Water runs along rafters and decking before it drops. The entry is uphill of the stain, and on a low-slope run it can be a long way uphill.

3. Inspect the suspect detail from above

   Once the attic narrows it to a penetration, a wall line or a valley, that detail gets looked at directly and photographed.

4. Fix the defect, not the symptom

   Replacing a boot, rebuilding a flashing or reworking a valley fixes the cause. Sealant over the top of a defect buys a season.

The usual suspects, in order of how often they turn up

1. Pipe boots. The rubber collar splits years before the shingles wear out.
2. Step flashing at a wall or chimney, especially where it has been replaced with a caulked strip.
3. Valleys, where nails too close to the center line or debris damming the trough both leak.
4. Counterflashing surface-mounted on masonry instead of let into a cut groove.
5. Nail pops, which lift the shingle above and leave a hole underneath.
6. Ice dams, which are a heat-loss problem the roof gets blamed for.