Sizing Eavestrough: 5-Inch vs 6-Inch and What Actually Drives the Choice
Eavestrough size is not a style preference. Roof area, pitch, valley placement, downspout count and rainfall intensity all feed the decision — and metal roofs change the maths.
Five-inch eavestrough is the default on Ontario houses because it is what was on the last one. That is not a sizing method. Trough size is a hydraulic decision, and it is made together with downspout size and downspout count — change one and you change the others. Here is what actually drives it.
Start With Drainage Area, Not Wall Length
The number that matters is not how long the fascia is. It is how much roof drains into that particular run. A 30-foot run under a wide, low roof plane collects far more water than a 30-foot run under a narrow one. So the first step is to break the roof into planes, work out which plane feeds which trough, and add up the area. If a contractor sizes your eavestrough by walking the perimeter without ever looking at the roof planes, they are guessing.
Pitch Multiplies the Load
A steep roof does not just have more surface area than a flat one covering the same footprint — it also throws water at the eave faster and with more momentum. Drainage calculations account for this by adjusting the effective area upward as pitch increases. In practical terms, two houses with identical footprints can need different eavestrough if one has a shallow hip roof and the other has a steep gable. Steep roofs push the case for 6-inch trough and larger outlets, both for capacity and to reduce water shooting straight over the front lip.
Valleys Are the Real Trouble Spot
A valley funnels two roof planes into one narrow channel and dumps the combined flow at a single point on the eave. That point sees far more water per second than the rest of the run. Undersized trough below a valley overflows in exactly one place, over and over, and the homeowner assumes the whole system is failing. Sizing up, adding an outlet close to the discharge point, or fitting a valley splash guard are all legitimate answers — ignoring it is not.
Rainfall Intensity, Not Annual Rainfall
Eavestrough is not sized for how much rain falls in a year. It is sized for the worst few minutes of a summer storm. Southern Ontario's convective storm season delivers short, extremely intense downpours — the kind where the street floods for ten minutes and then the sun comes out. That short-duration intensity is the design condition. A system that copes fine with a day of steady drizzle can be comprehensively overwhelmed by a July cell.
Metal Roofs Change the Calculation
Water leaves a standing seam or metal panel roof faster than it leaves asphalt. There is no granular surface slowing it down and no absorbency at all. That means a higher peak flow rate arriving at the eave in a shorter window, and it means water arrives with enough velocity to overshoot a trough that is hung too low or too far out. On metal roofs the sizing conversation should always include trough position and eave trim detail, not just the number of inches.
- ✓Large or steep roof planes draining into a single run — points to 6-inch.
- ✓Valleys discharging onto the run — points to 6-inch, or an outlet placed close to the discharge.
- ✓Long runs with only one practical downspout location — points to 6-inch.
- ✓Metal roofing with fast runoff — points to 6-inch and to careful trough positioning.
- ✓Heavy tree cover, where partial blockage is normal — points to 6-inch and larger downspouts.
- ✓Small, simple, moderately pitched roof with two or more good downspout locations — 5-inch is usually genuinely sufficient.
Trough Size and Downspout Size Are One Decision
A 6-inch trough draining through a small 2x3 downspout is a bottleneck with extra capacity upstream. The outlet and the downspout are the restriction in nearly every overflowing system. Six-inch eavestrough is normally paired with 3x4 downspouts, and the outlet punched into the trough should be sized to match. If a quote lists 6-inch trough but keeps the existing 2x3 downspouts, ask why — sometimes there is a good reason involving an existing underground drain, but it should be a stated reason, not an oversight.
The most common cause of an overflowing eavestrough is not undersized trough. It is too few downspouts, undersized outlets, or an outlet placed at the wrong end of the run. Before you pay to upsize everything, have someone confirm where the water is actually backing up.
The Aesthetic Objection, Honestly
Six-inch eavestrough is visibly larger, and on a small house with delicate trim it can look heavy. That is a real consideration, not a fake one. The usual compromise is to run 6-inch only where the drainage demands it — the long back elevation under the main roof, or the run below a valley — and 5-inch elsewhere, with the changeover at a corner so the difference is not read as a mistake. On most homes with a substantial roof, the size difference is far less noticeable than homeowners expect once it is installed and colour-matched.
Signs Your Current System Is Undersized
Sheeting over the front lip during heavy rain, water running behind the trough at the fascia, staining on the soffit, erosion trenches directly below the eave, and an ice ridge that always forms in the same spot are all consistent with an undersized or badly fed system. So is a basement that only gets damp during intense storms rather than during long wet spells. Note where it happens and when, and bring that to the contractor — it narrows the diagnosis quickly.
Frequently Asked Questions
Is 6-inch eavestrough always better?
No. On a modest roof with good downspout placement, 5-inch handles Ontario storms fine and looks lighter on the trim. Six-inch earns its place when the drainage area per run is large, when valleys concentrate flow, when downspout locations are limited, or when a metal roof is shedding water quickly.
Can I upgrade to 6-inch without changing my downspouts?
You can physically, but you usually should not. The outlet and downspout are the throttle on the whole system. Upsizing the trough while keeping small downspouts moves the bottleneck rather than removing it.
Does a bigger eavestrough stop ice problems?
No. Ice in an eavestrough is a symptom of meltwater refreezing at a cold eave, which is driven by heat loss through the roof, not by trough size. A larger trough holds more ice. Fixing ice means addressing attic air sealing, insulation and ventilation, and keeping the drainage path clear.
How do I know whether my contractor sized it properly?
Ask them to walk you through it: which roof planes drain into which run, roughly how much area each run carries, where the outlets go and why, and how they handled the valleys. You do not need to check their arithmetic. You need to hear that arithmetic exists.
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