Why Clogged Gutters Become a Fall Pest Magnet

In Southern California you can go seven or eight months without meaningful rain, which is long enough to forget you own gutters at all. Meanwhile the sycamore next door, the neighbor's jacaranda and the pine across the street have been quietly filling them. By fall, a lot of gutters around here are less a drainage system than a long, narrow planter box. That matters to pests twice: once now, while the gutter holds a little standing water in the shade, and again when the first real storm hits and all that water goes over the edge instead of down the spout.

A clean gutter drains in minutes. A clogged one doesn't drain at all. Leaves, pine needles, seed pods and roof grit settle into a dense mat that dams the water, and in the shade of an eave that water can sit for a long time. Even without rain, sprinkler overspray, a dripping AC condensate line or a neighbor washing a car can top it up.

For mosquitoes, that is a breeding site. The invasive day-biting species now common around Los Angeles, the ones people call ankle-biters, are especially good at using small, overlooked pockets of water: plant saucers, buckets, tarps, and yes, gutters. Southern California's warm falls keep them active well after people assume mosquito season is over, so a gutter holding water in October is still doing them a favor.

Our West Nile virus article covers what county health officials have been tracking this year, and we won't repeat it here. The point for this piece is simpler: standing water near your house is the part of the mosquito problem you actually control, and the gutter is one of the places people forget to look.

A gutter can be free of leaves and still hold water if it has lost its slope. Sagging sections, loose hangers and spots where the gutter has pulled away from the fascia all create low points. If you see a line of water sitting in a clean gutter a day after watering or rain, the gutter needs adjusting, not just emptying.

The second problem starts when it rains. Water hits the clog and spills over the front edge of the gutter in a sheet, landing in the same strip of soil along the foundation every storm. That strip gets saturated, the soil erodes, water splashes up onto the bottom of the stucco, and in some cases it finds its way under the slab edge or into a crawl space. If this winter turns out as wet as the El Niño forecast suggests, expect it to happen more often than usual.

Saturated soil against the house is about the most pest-friendly condition a property can have. Several of the pests that matter most follow moisture directly.

Subterranean termites live in the soil and need moisture to survive. Wet soil right against the foundation lets foraging workers work close to the house, and wood that stays damp is easier for them to attack. Our article on El Niño and termite swarms goes into how rain sets off their swarming season; this is the everyday version of the same problem.

Argentine ants nest in shallow soil, mulch and leaf litter. They like moist ground, but when a nest floods they pick up their brood and move, often indoors. A soaked strip along the foundation is close to the house in both senses. Ant trails along the baseboard during a storm are a familiar rainy-season sight in this region.

Earwigs, pillbugs, sowbugs, millipedes, crickets and silverfish are not structural threats, but they all gather where it is damp. When you find them in numbers at a door threshold or in the garage, the overflow spot outside is often the reason.

Gutters hang on fascia boards, and fascia boards hold up badly to being soaked over and over. Overflow runs down the back of the gutter, stains and softens the wood, and eventually rots the fascia and rafter tails behind it. Damp, decaying wood is attractive to wood-destroying pests, and gaps where rotted fascia has pulled away from the roofline are exactly the kind of opening roof rats use to get into an attic.

Rain barrels and other roof-water catchment have become popular around Los Angeles, and they are a good idea. Left unscreened, they are also a very large mosquito nursery fed directly by your downspout. Any barrel connected to a gutter should have a tight lid and fine mesh over every opening, including the overflow, and the water should get used rather than left to sit all winter.

The same downspout often feeds other water traps nobody intended: a plant saucer under the spout, a tarp folded over the woodpile, a toy in the side yard, the low corner of a patio cover. After the first storm, walk the drip line and the downspout outlets and tip out anything holding water. It takes five minutes and it is one of the more useful things a homeowner can do about mosquitoes.

One more gutter detail, for anyone with roof rats in the neighborhood, which covers a good part of Los Angeles. Roof rats travel along rooflines, climb downspouts and overhanging branches to get up there, and look for gaps at the fascia and eaves to get in. A gutter pulling away from a rotted fascia board leaves exactly that kind of gap. While you are up on the ladder, notice whether anything along the roof edge has been chewed, and whether there are droppings mixed in with the gutter debris.

Clean gutters solve the gutter part. They don't solve a yard that slopes toward the house, a patio poured with a slight tilt in the wrong direction, a planter built up against the stucco, or a sprinkler that has been soaking the same wall every morning for a decade. Often the gutters are just the most visible part of a broader water problem.

Plenty of Southern California houses also have no gutters at all. That is not automatically a problem, but it means the roof sheds water straight down along the drip line. Walk the house and look at what sits under that line. Wood fencing, mulch beds piled against the wall, firewood and stored lumber in the drip line all stay wetter than they should once the rain starts.

This piece is about water. The gaps pests use to get in, like door sweeps, garage seals and weatherstripping, are a separate job that our Labor Day exclusion checklist covers, and the two work best together: drainage makes the outside of the house less inviting, and exclusion makes the inside harder to reach.

Gutter cleaning is a homeowner job, or a job for a gutter or roofing company. Where pest control comes in is when the moisture has already drawn something in, or when you aren't sure whether it has.

When we inspect a property, the exterior perimeter and the places where water collects are part of what we look at, since that's where a lot of pest activity starts. We can't regrade your yard or rehang your gutters, but we can tell you what the moisture is attracting and what it would take to deal with it.