Eavestrough Sizing and Downspouts: Roof Area Per Downspout, 5 in. vs 6 in., One Drop Every 40 ft, the Slope, and Why the Water Has to Land 6 ft From the Wall

The three rules

1. Roof area per downspout. A 5 in. K-style trough with a standard 2×3 in. downspout handles about 600 sq ft of roof per drop in an Ontario downpour. A 6 in. trough with a 3×4 in. downspout handles about 1,200. Count the roof area draining to each stretch of trough, not the floor area of the house.
2. One downspout per 40 ft of run. Past 40 ft the water has too far to travel at the slope a trough can carry, and the far end is dry while the outlet end is over the lip. Long runs get a drop at each end, sloped from the middle.
3. Discharge 6 ft from the foundation, or into a drain. The roof collects the rain from the whole house footprint and puts it in four places. If those four places are the backfill against the basement wall, you have built a watering system for the footing.

The slope is the fourth rule nobody argues about: about 1/16 in. per foot toward the outlet, which is 2.5 in. over a 40 ft run, enough to move the water and not enough for the eye to catch from the street. More slope looks crooked; less holds water, and held water is ice in December.

Roof area is not floor area

The trough sees the water that lands on the roof surface, and a pitched roof is bigger than the footprint under it. Multiply the footprint by a pitch factor: about 1.05 for 4/12, 1.12 for 6/12, 1.20 for 8/12, 1.30 for 10/12, 1.41 for 12/12. A 30 x 40 ft house with an 8/12 gable roof has a 1,200 sq ft footprint and about 1,440 sq ft of roof, and each eave takes half, 720 sq ft. That is more than one 5 in. downspout is meant to carry.

Then there are valleys. Where two roof planes meet in a valley, the valley collects water from both and delivers it to the trough in one spot, at speed. A trough that is fine on average can overflow at the valley in every storm. The fix is a downspout close to the valley outlet, or a 6 in. trough along that run, or a splash guard riveted to the front lip at the valley so the water does not shoot straight over it. On a hip roof with a bump-out at the back, the valley corner is where I would put the fifth downspout, and it is $85 for the outlet and elbows plus $12 a foot of pipe.

Use whichever rule gives more downspouts. A steep roof sheds water faster than a shallow one at the same area, so on a 10/12 or steeper roof I lean to 6 in. before the arithmetic says I have to.

Two roofs worked through

The 40 x 38 ft hip bungalow, 6/12. Footprint 1,520 sq ft, roof about 1,700 sq ft with the pitch factor. A hip roof drains to all four sides, so each side of the 156 lf trough gets roughly a quarter, 425 sq ft, under the 600 sq ft limit for 5 in. The run rule says 156 divided by 40 is 3.9, so four downspouts, one at each corner, each side sloped toward its corner. 5 in. does it. The cost is on the eavestrough cost page: $2,820 before HST with removal.

The 30 x 40 ft gable two-storey, 8/12. Each 40 ft eave takes 720 sq ft. In 5 in. that is two downspouts per side, four on the main roof, each 22 ft of pipe on a two-storey wall: 88 lf at $12 is $1,056 of downspout. In 6 in. it is one per side, 44 lf, $528, and the trough costs $3 a foot more, 104 at $3 is $312. The 6 in. job is $216 cheaper and has two fewer pipes on the front of the house. On a two-storey the pipe is expensive and the trough is not, so go bigger on the trough.

Where the water lands

Six feet from the wall is the rule, and it is the rule because the backfill against a basement wall is loose soil that drains straight down to the footing. Put 400 sq ft of roof into that soil at every storm and the weeping tile gets more water than it was built for, the sump runs constantly, and in a house with no sump the water finds the crack. It is the commonest cause of a wet basement I see, and the cheapest to fix.

The options, from cheapest up: a 6 ft extension on the downspout, hinged so the mower can get by, which is in the $12 a foot pipe rate; a buried 4 in. pipe from the downspout to daylight at a low spot on the lot, which is a trenching job priced per job; or a connection to a storm drain where the municipality has one and allows it, which many Ontario towns no longer do and some require you to disconnect. Never into the weeping tile or the sanitary sewer. Never onto a walkway that freezes. Never onto the neighbour.

On a lot that slopes toward the house, which is common on the Georgian Bay side of Simcoe County where the houses sit below the road, the downspout on the uphill side wants a buried line, because 6 ft of extension just puts the water 6 ft closer to coming back. The foundation types page and the basement pages on this site cover what the wall is dealing with from the other side.

Snow and ice

Simcoe County roofs carry real snow, and a trough on the eave takes the load when it slides. Hidden hangers screwed into sound fascia every 16 to 24 in. hold it; spikes do not. A 6 in. trough is stiffer than a 5 in. and tears off less. Snow guards on a metal roof above a trough keep the whole winter from arriving at once, and the metal roof page covers those. Ice in the trough itself is an attic problem wearing an eavestrough costume: the ice dam page explains why, and sizing the trough bigger does not warm the attic any less.

The one sizing choice that does help with ice is the downspout outlet: a 3×4 in. outlet on a 6 in. trough is harder to plug with slush than a 2×3 on a 5 in., so the trough drains during the January thaw instead of filling and freezing.

Questions people ask

Sources and further reading

• Alu-Rex: seamless eavestrough and gutter-guard systems made for Canadian snow and ice; sizing and installation notes.
• Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.
• Ontario’s Building Code: the official page with the current regulation; cladding is Part 9 Section 9.27, masonry veneer 9.20, attic ventilation 9.19.