Gutters Overflowing in Heavy Rain? How to Tell if It's a Clogged Downspout, an Underground Drain, or the Gutter Itself

Read the overflow before you climb

Stand outside during the next real rain and look at where the water is coming over. Water sheeting over the lip in one place, near a downspout, with the rest of the run behaving, is a blocked downspout. Water coming over the whole length of a run is a gutter full of debris, or a run that no longer drains toward its outlet. Water pouring over an inside corner where two roof planes meet is a valley delivering more water than the gutter can take at that point, which may be a pitch or sizing issue. Water leaking from a downspout's own seams, or bubbling back out of the top elbow, is a downspout packed below that point.

Then look at the ground. A downspout that drains onto a splash block will show you whether water is coming out. A downspout that disappears into the ground next to the foundation drains to an underground line, and if that line is blocked, the downspout backs up from the bottom. The tell is a downspout that gurgles, keeps dripping for hours after the rain stops, or has a soggy, heaving patch of soil or a wet basement corner at its base.

The four causes, in order of likelihood

Packed downspout elbows. Downspouts clog at the top elbow, where debris funnels in, and at the bottom elbow, where it settles. The elbows are the narrowest point, and fir needles felt up in them. This is the cause most of the time, and it is why a gutter cleaning that does not flush every downspout does not fix overflow.

A full gutter. Needles, maple seeds, shingle grit and moss form a mat that holds water and slows the flow to a trickle. Under firs this happens two or three times a season; in the open, once a year in fall.

An underground drain line that backs up. Many Portland homes route downspouts into buried lines that run to the street, a dry well or a rain garden. Those lines fill with roots, silt and debris, and when they do the downspout has nowhere to send the water. The gutter looks clean and still overflows.

Pitch and sizing. Gutters should fall toward the downspout about a quarter inch per ten feet. Hangers pull loose, fascia sags, and a run can end up draining backward or pooling in the middle. Some older homes also have five-inch gutters under big roof planes that need six-inch, or a single downspout serving a run that needs two.

What fixes each one

Elbows and full gutters are a cleaning. The gutters are cleared, the outlet is opened, and every downspout is flushed with water and watched at the bottom until it runs clear. A top elbow is cleared from above; a bottom elbow that is packed is opened and cleared, not forced down the pipe.

An underground line is a different trade. We flush the downspout and confirm whether the line takes water. If it backs up, we document it with photos and tell you what we saw, and the fix is a drain specialist with a camera and a jetter. What we will not do is blindly jet a line we cannot see, because a collapsed clay line or a root ball can turn a clog into a repair.

Pitch and sizing are repairs. Loose hangers and a sagging run can be re-pitched; an undersized gutter or a missing downspout is a gutter contractor's job. We photograph and flag them in the visit report so you can get them fixed before the winter.

The Portland calendar for this

The first atmospheric river of the fall is when overflow shows up, usually late October into November, right after the firs and maples have dropped. A cleaning in late October or early November, with every downspout flushed, prevents most of it. Homes under heavy tree cover need a second visit after the winter storms or in spring, when the last of the needles and the maple seeds come down. Homes in the open usually get through on one fall visit.

Questions homeowners ask