Standing water in a gutter means water cannot reach the downspout: a clog is damming it, the run has sagged into a low spot, or the slope is wrong. Beyond the rot and ice risks, still water at roof height is mosquito habitat. Toronto Public Health lists clogged gutters as a place mosquitoes breed.
How much water in a gutter is normal?
Very little. Aluminum eavestroughs are never perfectly smooth, so after rain you will often see a thin film or a few shallow puddles that evaporate within a day. What is not normal is a pool you could measure with a ruler, more than about a centimetre deep, still sitting there two days after the rain stopped. That water has nowhere to go.
Why does standing water matter?
- Mosquitoes. Toronto Public Health's West Nile virus guidance specifically notes that clogged gutters accumulate water and create a place for mosquitoes to breed.
- Rot and corrosion. Water held against seams breaks down sealant, and on steel gutters it rusts through. Wet debris sitting in the trough keeps the fascia behind it damp.
- Weight. A metre of 5-inch gutter holding water or ice carries roughly ten kilograms, and that load works hangers out of the fascia.
- Ice. Water that never drains is the first thing to freeze in November, and a frozen gutter is where ice dams start.
How do you find the cause?
Look at where the water sits. You can see most gutters from an upstairs window, or check with a short ladder on a dry, calm day.
| Where the water sits | Most likely cause | The fix |
|---|---|---|
| Next to the downspout outlet | A clog in the outlet or an elbow backing water up | Clear and flush the spout (how-to) |
| In one stretch mid-run | A sag from loose hangers or a bent section | Re-fasten hangers and lift the section back to line |
| Along the whole run | The gutter was hung flat, or the fascia has settled | Re-pitch the run toward the outlet |
| At the far end, away from the outlet | Slope running the wrong way | Re-hang so the run falls toward the downspout |
To confirm, lay a short level in the bottom of a clean section: the bubble should sit slightly off-centre toward the high end, showing a fall toward the outlet of roughly 1 to 2 millimetres per metre. Or run a hose at the far end and watch: water should travel steadily to the outlet without collecting anywhere. Checking gutter slope has the full method.
How do you fix a gutter that holds water?
- 1Clean the run and flush the downspout. A surprising share of 'slope problems' disappear once the outlet actually drains.
- 2If water still pools in one stretch, check the hangers there. Spikes that have backed out and hidden hangers with stripped screws let the gutter drop.
- 3Re-fasten the hangers so the run follows a straight line falling toward the outlet, adding hangers where the spacing is too wide.
- 4Test with a hose before calling it done. The water should run to the outlet and leave nothing behind.
What else holds water around a GTA house?
If you are tackling standing water for mosquitoes, the gutter is one of several spots. Toronto Public Health's list includes pool covers, buckets, planters, toys and waste containers, and its guidance adds making sure water does not pool around downspouts. A downspout that discharges into a hollow in the lawn creates its own puddle; an extension onto ground that slopes away solves both problems. Where downspouts should drain covers the setup, and a rain barrel needs a sealed, screened lid.
What does this look like in Ajax?
Ajax grew along the lake and Duffins Creek, and the mature maples and willows along the creek, plus big street trees in the older south end, keep gutters loaded well into summer. Willow leaves are small and slender, and they pack into outlets in a dense wad that holds water behind it. Newer blocks up toward Audley shed less, but their builder downspouts clog quickly when they do.
A cleaning clears the dam and proves the flow: every visit ends with each downspout flushed and flow-tested, and the before and after photos show you the trough. In 2026 gutter cleaning in Ajax runs $299 to $499 before HST depending on home type and storeys.
Sources
- City of Toronto: Reducing the risk of West Nile virus
- City of Toronto: first West Nile-positive mosquitoes of 2026
Frequently asked questions
Can mosquitoes breed in gutters?
Yes. Toronto Public Health notes that clogged gutters accumulate water and create a place for mosquitoes to breed. A gutter that drains properly after rain does not hold water long enough to be a problem.
How long does water have to sit before mosquitoes breed in it?
In warm summer weather mosquitoes can develop from egg to adult in about a week, which is why public health units advise emptying standing water at least weekly. A gutter that never drains is a permanent breeding site.
Will gutter guards stop standing water in my gutters?
No. Guards keep leaves out, but water still pools under them if the gutter has a low spot or the wrong slope. A guarded gutter that holds water needs the same re-pitching as any other.
Is standing water in gutters a sign they need replacing?
Rarely. It usually means a clog or a few loose hangers, both of which are routine fixes. Replacement only comes up if the run is badly bent or rusted through.
Keep reading
- How much slope should gutters have? Checking the pitch
- Why your gutters overflow even when they're clean
- Signs your gutters are clogged: the five-minute check
- Gutter & eavestrough cleaning: what is included and how it works
- Gutter & eavestrough cleaning in Ajax: local prices and booking
- Everything we do in Ajax: neighbourhoods, trees and services
