Gutter calculator: hangers, downspouts, elbows and straps
One row per gutter run: its length, its outlets, the drop of its downspouts and, optionally, the roof area draining to it. Enter your spacings: the calculation counts the rest.
Spacing and lengths: the makers' figures (reference)
What the makers' installation guides say, cited as a guide: your spacing stays yours.
| Gentek | Amerimax | |
|---|---|---|
| Hangers | At most 800 mm (32 in) apart | One every 24 in, every 18 in where ice and snow are heavy |
| Downspout straps | One next to the upper elbow, then at least every 3 m (10 ft) | 2 bands per 10 ft section |
| Slope | At least 1/4 in per 10 ft (6 mm per 3 m) toward the nearest outlet | About 1/4 in per 10 ft toward the downspout |
| Lengths sold | 3 m and 6 m gutters, 3 m downspouts (French edition) | Gutters and downspouts in 10 ft lengths |
Sources: Gentek installation guide; Amerimax metal gutter guide
What one downspout serves (reference)
Two guides for placing the outlets. To size by rainfall, the plumbing codes start from the roof area in plan (Canada adds half of the largest wall that sheds water onto it).
| One downspout | |
|---|---|
| Gentek | Usually enough for 475 to 600 sq ft (45 to 55 m²) of roof |
| SMACNA | At most 50 ft (15 m) of gutter, in practice, because of thermal expansion |
Sources: Gentek installation guide; SMACNA calculator; IPC 2021, 1106
Where the water goes: check your city's rule (reference)
Downspout discharge is up to each city. A few rules, as read: yours may differ.
| Place | Rule as read |
|---|---|
| US (IRC 2021) | On expansive or collapsible soils: roof water discharges at least 5 ft (1524 mm) from the foundation walls, or into an approved drainage system |
| Portland | Voluntary disconnection: the end of the extension at least 6 ft from a basement and 5 ft from the neighbor's property line, among others |
| Toronto | No downspout connected, directly or not, to a storm or combined sewer (an exemption can be requested) |
| Montreal | Onto a permeable surface (flower bed, lawn) at least 1.5 m from the foundation; never into the foundation drain, toward a hard surface or the street |
Sources: IRC 2021, R801.3; Portland, downspout disconnection; Toronto Municipal Code 681-11S; Ville de Montréal, gouttières
How it is calculated
For each run: hangers = length / spacing, rounded up, + 1. Each outlet has its downspout, of the drop height entered: downspout length = outlets x drop, then rounded up at the total to the lengths sold (10 ft, or 3 m in meters). Straps = downspouts x (drop / strap spacing, rounded up, + 1). Elbows = downspouts x elbows per downspout. Roof per downspout = the run's area / its outlets, and gutter per downspout = length / outlets: the calculation shows the most loaded run and the longest. No spacing comes filled in: they are yours.
The whole job in MesuPro
Gutter survey
Runs by size, hangers, end caps, corners and outlets, downspouts with elbows and straps, old gutters to remove.
Feet and inches by default
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