Why is my roof leaking in one spot?
Most single-spot leaks come from a cracked pipe boot, loose flashing at a wall or chimney, a worn valley, a failed vent or skylight seal, or wind-lifted shingles. The stain inside is usually downhill from the real entry point, because water runs along the decking and framing before it drips. The fix has to be made where the water enters.
Water travels before it drips
Rain that gets under a shingle does not fall straight through. It runs down the underlayment and the top of the decking, finds a nail or a seam, follows a rafter, and lands on a ceiling joist before soaking through the drywall. The spot on your ceiling can be several feet from where the roof is actually open.
That is why patching the shingles directly above a stain so often fails. The attic is the better place to start: dark streaks on the underside of the decking, rusty nail tips and damp insulation lead uphill to the source.
The five usual suspects
Pipe boots are first on the list. The rubber collar around a plumbing vent cracks after years of sun, and water runs straight down the pipe into the attic. Flashing is next: the metal that seals the roof where it meets a wall or a chimney can pull loose or be covered in caulk that has shrunk away.
Valleys carry more water than any other part of the roof and wear faster. Exhaust vents and skylights depend on seals that dry out. And shingles lifted by wind, often along ridges and rakes, let rain blow underneath when a storm comes in sideways.
Leaks that only happen in some storms
If the ceiling only drips when a storm comes from a particular direction, the leak is likely somewhere wind-driven rain can reach and ordinary rain cannot: under a lifted shingle, behind loose flashing on the windward side, or through a vent facing into the weather.
Keeping a note of which way the storm came from and how hard it rained is one of the most useful things you can bring to a leak inspection.
After the source is fixed
Let the next rain prove the repair before painting over the stain. Replace insulation that stayed wet, and have any soft decking at the entry replaced, since wood that has been wet for months will not hold a nail properly.
Related questions
Can a roof leak without a missing shingle?
Yes. Most leaks come from boots, flashing, valleys and vents rather than missing shingles.
Is it safe to wait on a small leak?
A small leak soaks insulation, decking and framing long before it shows much inside. It is cheaper to fix early.
