Why are my gutters overflowing?

Most overflowing gutters are blocked. Leaves, seed pods, roof grit and the odd tennis ball pack into the gutter or, more often, into the outlet and downspout, and the water has nowhere to go but over the front. If the gutters are clean and still spill in an ordinary rain, the gutter system itself is the problem: too small for the roof, too few downspouts, lost slope, or sagging hangers. In winter, ice and snow can do the same.

It is worth tracking down. A few inches of rain on a roof can produce several thousand gallons of runoff,^[4] and an overflowing gutter drops that water along the foundation. Fairfax County's soil and water conservation district lists gutters that need to be cleaned or are undersized among the usual sources of wetness and erosion near a foundation.^[1]

Find where it spills

Go out in a steady rain and watch. Where the water leaves the gutter tells you most of what you need to know:

• Over the front along the whole run: the gutter is full of debris, the downspout is blocked, or the gutter is too small for the roof.
• At one spot: a low point, a sag between hangers, or debris caught at a seam or corner.
• At the downspout end: a plug in the outlet or downspout, or a downspout too small for the water reaching it.
• Behind the gutter, down the fascia: the gutter is not catching the roof edge properly.

When it is dry, the marks are still there. The district notes that streaks or watermarks on the outside of the gutters and downspouts, and erosion along the drip line, are signs the gutters overflow when it rains and need cleaning.^[1]

Cause 1: debris in the gutter or downspout

This is the usual one, and the cheapest. A gutter can look clear from the ground and still have a packed outlet. Clear the gutter, lift any plug out of the outlet, then run a hose in the far end and watch the bottom of the downspout. Water should come out steadily. If it backs up, the plug is in the downspout or an elbow. Screens and strainers that keep leaves out of the outlet help, but they clog too and need rinsing when you clean.

For doing this safely from a ladder, see how to clean gutters safely from a ladder.

Cause 2: gutters or downspouts too small

Fairfax County's soil and water district treats spillover from clean gutters in a normal rainstorm as an indication of an undersized gutter system, and suggests considering larger gutters.^[1] Before you buy bigger gutters, rule out the other causes below: too few or too small downspouts, lost slope and sagging can all make a clean gutter spill.

Gutter channels typically come in 4-, 5- and 6-inch sizes, and a larger one carries more water faster, provided there are enough downspouts to drain it.^[2]

Downspouts are often the real bottleneck. For sizing, allow 1 square inch of downspout cross-section for every 100 square feet of roof area, and place downspouts at least 20 feet apart but no more than 50 feet apart.^[2] Size makes a large difference: a 2-by-3-inch downspout drains 186 gallons per minute and a 3-by-4-inch one drains 434.^[3] Larger or more downspouts are recommended for wide roof areas, heavy downpour areas and gutter systems that overflow.^[3] A long run with one small downspout at the far end, or a big roof valley dumping into a short gutter, is a pattern to look for.

Cause 3: lost slope or sagging

A gutter has to fall toward its outlet. Gutters should slope at least 1/16 inch per foot of run toward the downspouts.^[2] A gutter that is level, tilts the wrong way, or dips in the middle holds water and spills at the low point.

Sagging usually comes from loose or missing hangers, a gutter weighed down by wet debris, or ladders leaned on it. Typical spacing is one hanger every 24 inches, or every 18 inches in areas with heavy ice and snow loads.^[3] Hangers pulling out of soft wood can mean the fascia behind them is rotting, which is a repair, not a cleaning.

Cause 4: water getting behind the gutter

If water runs down the fascia behind the gutter, the gutter may sit too low or too far out to catch the roof edge. If the house has a drip edge, the gutter slips under it.^[3] That sits at the roof edge, so it is a job for a gutter or roofing pro, not a ladder fix.

Cause 5: ice and snow

Where winters freeze, make sure the gutters are clean as winter approaches so they can drain and do not fill with water that could freeze.^[2] Gutters do not cause ice dams, but where snow is heavy they can fill with snow that freezes and becomes part of an ice dam already forming on the roof.^[2] Do not chip ice out of a gutter from a ladder; let it melt, and deal with the cause before next winter.

While you wait for a repair

If the gutters are hard to reach or cost too much to replace right away, the district suggests protecting the soil instead: a band of bluestone or pea gravel about 10 to 12 inches wide along the drip line, or ground covers and thick leafy shrubs, protects the soil.^[1]

Stop and call a pro if

• The gutters are on a second storey, or the only way to reach the problem is from the roof.
• A section sags, has pulled away from the fascia, or the wood behind the hangers is soft or rotten.
• Water runs behind the gutter, which involves the roof edge and drip edge.
• The gutters are clean and still overflow, and the slope, hangers and downspouts look right, so the fix may be larger gutters or more or larger downspouts. In Virginia, a contractor's Class C licence covers construction, removal, repair or improvement jobs with a single contract over $1,000 and under $30,000,^[6] so check the licence of anyone quoting a repair or replacement in that range.
• Ice has built up in the gutters or along the roof edge.
• A ladder position would bring you or the ladder near a power line or the service wire to the house. Keep clear of every wire; metal ladders must not be used on or near electrical wires.^[5]

Browse gutter repair on Loclis and the other exterior answers, look up exterior pros around Blacksburg, or read how often gutters should be cleaned. When it is time to fix it, describe the gutter repair once and licensed local businesses quote it.