The Slope
A trough that holds water after the rain stops is sloped wrong, and that one fault causes most of the others.
Short answer
An eavestrough needs a consistent fall toward the outlet — a small amount over a long run, but consistent. Standing water means the slope is flat, reversed, or sagging in the middle. It breeds mosquitoes, freezes first in winter, accelerates corrosion and overflows in a storm.
Why standing water matters
It freezes first and most, which in Toronto is the beginning of the ice problem in that run.
It holds debris in place rather than letting it move to the outlet.
It accelerates corrosion, particularly where the sediment sits.
It adds weight, which pulls on the hangers and makes the sag worse — a feedback loop.
It overflows in a storm, because the trough is already part full before the rain starts.
And it breeds mosquitoes in summer, which is the complaint that frequently brings it to attention.
How to check it
After rain, look along the run from the ground on a sunny day — standing water reflects and is easy to see. Or run a hose into the far end and watch where it goes. If it pools anywhere, or runs away from the downspout, the slope is wrong.
Why it goes wrong
Installed level, which looks right from the ground and does not drain.
Installed with the fall toward the wrong end.
Sagging in the middle from hangers too widely spaced or working loose.
Ice load over several winters, which permanently deforms the run and changes the slope.
Fascia rot, which lets one section drop.
And a long run with the outlet at one end only, where the required total fall over the distance becomes visually noticeable and the installer compromised.
The repair
Reslope by repositioning the hangers rather than bending the trough.
Replace hangers that have pulled, and screw into solid fascia — which may mean fascia repair first.
Tighten the hanger spacing, particularly on a long run or where ice load is heavy.
Consider adding a downspout mid-run, which halves the required fall and is frequently the better answer than trying to slope a very long run to one end.
Replace a trough that has been permanently deformed by ice, because resloping a bent trough does not produce a clean fall.
And check it with water afterwards rather than by eye.
Common questions
How much slope does an eavestrough need?
A small but consistent fall toward the outlet. The test is that it drains completely.
Why does water sit in mine?
Level or reversed slope, sagging hangers, or ice deformation.
Is adding a downspout an option?
Frequently the better answer on a long run, because it halves the required fall.