How a Roof Works
Lesson 2. How Water Actually Gets In
At a Glance
- Shingles are not waterproof. The arrangement is
- Water moves sideways and uphill under the right conditions
- Most leaks start at a joint, not at a shingle
- The stain inside is rarely below the entry point
The Roof Is a Waterfall, Not a Bucket
Every course of shingles laps over the one below it. Water lands, runs down the face of a shingle, lands on the next one, and keeps going until it reaches the gutter. No single piece is waterproof and none of them needs to be.
This is why slope matters so much. Take the slope away and water slows down, sits, and starts working sideways into the laps. It is also why nothing that interrupts the flow can be sealed with caulk alone and trusted.
The Three Ways In
- A hole. Rare, obvious and the least common cause
- A failed joint. Flashing, a pipe boot, a valley. This is most leaks
- Water that stopped moving. Ice dams, ponding and clogged valleys, which let water work uphill under the laps
Wind matters too, because wind driven rain travels sideways and gets into gaps gravity would never reach. That is why a roof can leak only in a storm from one direction.
Why the Stain Moves
Water that gets past the shingles hits the underside of the deck and runs downhill along the wood. It follows a rafter, reaches a ceiling joist, travels along that, and drops at the first seam it finds in the drywall.
A stain in the middle of a bedroom ceiling can come from an entry point ten or fifteen feet away, higher up the roof and often on a different plane entirely. Anybody who patches directly above a stain without going into the attic first is guessing.
What This Means for You
When you call about a leak, the useful information is not where the stain is. It is when it leaks. Only in heavy rain, only in wind from one direction, only in spring, or all the time.
Those answers narrow it down fast. Only in wind driven rain points at flashing or siding. Only in spring points at ice damming. All the time points at a real opening.