Size And Capacity
Standard size suits a standard roof. On a large, steep or valley-fed Toronto roof it overflows regardless of how clean it is.
Short answer
Capacity depends on the roof area draining to the run, the roof pitch, the local rainfall intensity, the trough size and the number and size of downspouts. A steep roof delivers water faster than a shallow one, which effectively increases the area. Where the standard is marginal, the answer is usually more downspouts rather than a bigger trough.
What determines capacity
The roof area draining into that run, which is the main term.
The pitch, because a steep roof delivers the same volume in less time — which for sizing purposes behaves like a larger area.
Rainfall intensity, which for Toronto means sizing for a short intense summer storm rather than for average rainfall.
Trough cross-section, which is the headline size.
Downspout number and size, which is frequently the actual bottleneck rather than the trough.
And valley discharge, which concentrates a large area into one point and overwhelms the trough locally even when the overall sizing is fine.
Downspouts before trough size
A bigger trough with the same single downspout fills up and still overflows at the outlet. Adding a second downspout halves the distance water has to travel and doubles the discharge capacity, and it is usually both cheaper and more effective than upsizing the whole run.
When to go larger
A large roof area draining to one run, which on a bungalow with a long uninterrupted eave is common.
A steep roof, which is most of the Toronto housing stock with a gable.
Valley-fed runs, where a valley discharges into the trough.
A metal roof, which sheds water faster than asphalt.
A location where overflow causes a real problem — over a door, over a walkway, against a foundation with a known water issue.
And a commercial or institutional building, where the areas involved are larger and the consequence of overflow is greater.
The practical options
Add a downspout, which is the first and usually the best answer.
Upsize the downspouts, keeping the trough, which handles a bottleneck at the outlet.
Upsize the trough and the downspouts together, which is the full answer on a genuinely undersized run.
Fit a valley splash guard where a valley discharges into the trough.
Oversize the outlet opening in the trough, which is a detail frequently done badly — a large downspout fed by a small hole is still a small downspout.
And check it in a real storm afterwards rather than assuming the calculation was right.
Common questions
Is a standard eavestrough big enough?
For a standard roof. On a large, steep or valley-fed roof it frequently is not.
Bigger trough or more downspouts?
More downspouts, usually. It is cheaper and more effective.
Does roof pitch matter?
Yes. A steep roof delivers the same volume in less time, which behaves like a larger area.