Overflowing
A trough overflowing in a storm is not always blocked. There are five other reasons and three of them are design rather than maintenance.
Short answer
Blockage is the first thing to check and the easiest to fix. After that: too few downspouts for the roof area, trough or downspout undersized, slope wrong or flat, the trough mounted too low relative to the roof edge so water overshoots, and valley discharge concentrating flow at one point.
The six causes
Blockage in the trough or, more often, at the downspout outlet and the elbow.
Too few downspouts. A long run with one outlet cannot move the volume a Toronto summer storm delivers, however clean it is.
Trough or downspout undersized for the roof area, which on a large or steep roof means the standard size is not enough.
Slope wrong or flat, so water sits in the middle of the run and overflows there rather than reaching the outlet.
Trough mounted too low, so fast water coming off a steep roof overshoots it entirely — which looks like overflow and is not.
And valley discharge, where a valley dumps a concentrated stream into one point of the trough and overwhelms it locally.
Overshoot versus overflow
Watch it in a storm. Water arcing over the front of the trough and missing it is overshoot — the trough is too low or too far out, or the roof is steep enough that the water leaves fast. Water filling the trough and spilling over the edge is overflow. They look the same from the window and need completely different fixes.
Working out which
Watch it in a real downpour, which is the only diagnosis that works.
Note whether it overflows along the whole run or at one point. One point is a slope problem, a blockage or a valley discharge.
Note whether it overflows at the downspout end, which means the downspout cannot take it.
Note whether it spills over the back toward the fascia, which means the trough is too far out or the drip edge is missing.
Check the trough for standing water after rain, which shows the slope.
And count the downspouts against the roof area, because that is frequently the answer.
The fixes
Clear it, and clear the downspouts and the elbows, which is where most blockages are.
Add a downspout, which on a long run is the single most effective change.
Reslope the run, which means moving the hangers rather than bending the trough.
Upsize the trough and the downspout on a large or steep roof.
Add a splash guard at any point where a valley empties into the run.
And adjust the trough position relative to the drip edge, which is a small adjustment that solves overshoot.
Common questions
My eavestrough is clean and still overflows. Why?
Usually too few downspouts, wrong slope, or the trough is undersized.
What is overshoot?
Water arcing over the front and missing the trough entirely. Different problem, different fix.
What is the most effective single change?
Adding a downspout on a long run.