How do gutters work?
Short answer. Rain runs down the shingles, drips off the drip edge into the gutter, flows along the gutter's slight slope to an outlet, falls through the downspout, and is discharged away from the foundation. Each step depends on the one before it, so a missing drip edge, a clog, a sag or a short downspout breaks the whole chain.
The path of the water
- Roof surface. Shingles overlap like fish scales, so water moves downhill on top of them toward the eave.
- Drip edge. The metal strip at the edge of the roof carries water past the fascia and drops it into the gutter. Without it, surface tension pulls some water back along the underside of the shingles and down behind the gutter.
- Gutter trough. The gutter collects water along the whole eave. It is hung with a slight fall toward each outlet, so gravity moves water sideways.
- Outlet. A hole in the gutter bottom feeds the downspout. This is usually the bottleneck: if the outlet is small or clogged, the gutter fills and overflows no matter how big the gutter is.
- Downspout. The vertical pipe carries water to the ground through elbows that hug the wall.
- Discharge. An extension, splash block or buried pipe carries the water away from the foundation so it does not soak back toward the house.
Where it usually breaks
- Leaves and needles packed at the outlet.
- A sag that holds standing water mid-run.
- Water running behind the gutter because drip edge is missing.
- Undersized downspouts on a big, steep roof plane.
- Downspouts that end right at the foundation.
Knowing the path makes problems easy to trace: find the first step where water goes the wrong way. More in roof anatomy explained, drip edge explained and gutter sizing, seamless gutters and guards.