| In the rain you see | Likely cause | Fix |
|---|---|---|
| Spills along the run in every rain | Clogged trough | Cleaning |
| Spills near the top of a downspout | Packed downspout | Clear the downspout |
| Spills at one low spot mid-run | Sag between hangers | Rehang |
| Water pools and stays | Pitched away from the outlet | Rehang at the right slope |
| One corner spills in hard rain | Valley corner, outlet too far | Add a downspout or a splash guard |
| Whole run spills only in downpours | Too little capacity | More or larger downspouts, then a larger gutter |
| Water runs down the wall behind | No drip edge or apron | Add flashing at the roof edge |
Capacity is about the hardest minutes
A gutter that only fails in the worst storms is usually not dirty but short of capacity, most often at the downspouts. The calculator takes your town's five-minute, ten-year downpour from NOAA Atlas 14 and shows how many downspouts a roof section needs.
Behind the gutter is a different problem
Water running behind the gutter does not come over the top; it slips between the roof edge and the gutter. It rots the fascia first, so fix it early: see fascia and soffit.