Standing Water Under a Gutter Guard Can Breed Mosquitoes

A gutter guard is supposed to keep leaves out of the trough and let water through. In a St. Louis summer, it can also leave a shallow pool of water sitting in the gutter where nobody looks. Mosquitoes need only a small amount of still water to lay eggs, and a clogged or badly pitched gutter under a guard often holds exactly that. If you have guards on your gutters and you have noticed buzzing near the eaves in July, this page covers what to check and what fixing it usually involves.

Gutters and guards

Most gutter guards are a mesh, a perforated metal, a reverse curve surface, or a brush or foam insert. Water is supposed to pass over or through the guard into the trough and out the downspout. The trough is the part that holds water when something blocks the flow. The guard hides the debris, which means you cannot see whether the trough is still full after a rain.

Three things usually create standing water in a guarded run. The first is a low spot where the trough sags between hangers, so water settles instead of running toward the outlet. The second is a downspout outlet that is partly blocked by granules, seed casings or a crimp in the elbow. The third is fine debris that works its way under the guard edge and forms a wet mat on the bottom of the trough. That mat holds water even after the rain stops.

How quickly it becomes a problem

Mosquito life cycles move fast in warm weather. Eggs laid on a damp surface can hatch within days in summer heat, and the larvae develop in water that stays still long enough for them to finish. A trough that drains completely after each storm is unlikely to produce a crop. A trough that holds water for a week or more is the kind that gets noticed. The practical test is simple: after a rain, look into the gutter at the lowest point between two hangers, then check again two or three days later. If there is still water, or a dark wet mat with a sour smell, the run is holding water.

The guard does not cause the mosquitoes. The water does. A guard only matters because it hides the water from view, so the trough can stay wet for weeks without anyone noticing. Do not pour a chemical into the gutter to deal with this without reading the label for use near plants, pets and any rain barrel on the property. In most cases the fix is mechanical: get the water moving and keep the trough from sagging.

What to check at each outlet

Start at the downspout outlet, where the trough meets the elbow. Look for granules, seed casings and shingle grit packed into the opening, and for a crimped or dented elbow. A flashlight helps. If the outlet is clear and the elbow is round, look at the trough slope. Set a level on the trough at the far end from the outlet. Gutter pitch is meant to carry water toward the outlet, and a run that has settled or was installed with too little fall will hold water at the far end no matter how clean it is.

Then check the hangers. A hanger that has pulled out of the fascia or the mortar lets a section sag, and sag is what keeps the water in place. On brick, the hanger often lets go where the mortar was thin. On older wood fascia, the screw may have stripped the wood. Both are structural problems, and both are easier to fix when they are found before a freeze arrives and the trough is carrying ice along with the water.

What fixing it usually involves

When the outlet is blocked, the repair is clearing it and checking that the elbow and the first several feet of downspout are open. When the trough sags, the repair is re-hanging or adding hangers and re-pitching the section so water runs to the outlet. When the wet mat is under the guard, the guard usually has to come up enough to clean the trough. A guard that snaps in can be lifted. A guard that is fastened may need its fasteners backed out, and that makes the job more than a quick clean.

If the trough is cracked, a joint has separated, or the seal at an end cap has failed, that section needs its own repair. A seam leak shows as a drip line or a stain on the fascia below the joint. Water that leaves through a seam never reaches the downspout, so the trough stays partly full and the mosquitoes keep their water.

What it costs

Cost depends on what is actually wrong and on how much of the guard has to come off to reach it. Clearing an outlet and checking an elbow is usually a short visit. Re-hanging a sagging section, re-pitching a run, or replacing a cracked section adds labor, and both the number of hangers and the length of the section matter. Work on a second story or over a walkway takes more setup. A number is only useful once someone has walked the run and found the cause, because a figure given from a phone description tends to miss the part that drives the labor.

Ask for the cost to be split by item: outlet clearing, hangers, re-pitching, sections replaced, and guard removal and reinstallation. A single lump figure hides whether the guard has to come off, and on a guarded run that is often the largest single part of the job.

What to ask

• Where the standing water was found, and how many days it had been sitting when you looked.
• Whether the trough was checked for slope with a level at the far end from the outlet.
• Whether the downspout elbow and the first several feet of downspout were checked for blockage.
• Which hangers were loose, and what each one was fastened into.
• Whether the guard has to come off to clean the trough, and how it goes back on.
• Whether the repair is written as a separate line from the guard itself.

Put these questions to any contractor in writing before you agree to a visit. If you want to talk it through by phone first, the Keys office is at (314) 220-2333.