Gutters are usually sloped about ¼ in for every 10 ft of run toward the downspout. A 30 ft gutter drops ¾ in from end to end and a 40 ft gutter 1 in. Hung on hangers every 2 ft, the 30 ft run takes 16 hangers, and the table under the calculator gives the mark for each one below a level line.
How gutter slope is calculated
Slope is a drop per length. Divide the run by 10 ft and multiply by the drop per 10 ft; in metric, multiply the run in meters by the drop in millimeters per meter.
Total drop = run length ÷ 10 ft × drop per 10 ft · Hangers = run ÷ spacing, rounded up, + 1
Worked example with the defaults: 30 ft ÷ 10 ft × ¼ in = ¾ in. At 2 ft spacing, 30 ft ÷ 2 ft = 15 spaces, so 16 hangers. The drop at each hanger is its distance from the high end times the slope: 1/16 in at 2 ft, ⅛ in at 4 ft, and so on to ¾ in at the downspout. In metric, 2 mm per meter is close to ¼ in per 10 ft (¼ in over 3.05 m is 2.1 mm per m).
How much slope does a gutter need?
Family Handyman's gutter installation guide slopes each run down toward the downspout about ¼ in for every 10 ft. More slope drains faster and less standing water is left behind, but on a long run it lowers the downspout end noticeably below the fascia line. If you want a steeper gutter on a short run, enter ½ in per 10 ft; the marks follow. The IPC sizes gutters at much steeper slopes (⅛ to ½ in per foot) because it is sizing drains, not hanging house gutters.
Long runs: slope both ways
For runs of 40 ft or more, Family Handyman slopes the gutter down both ways from the middle with a downspout at each end. Switch on "Slope both ways" and the calculator halves the run: a 40 ft gutter drops ½ in from the middle to each end instead of 1 in over the whole length, and it counts two downspouts.
Marking the slope on the fascia
- Mark the high end about ½ in below the roof edge, so the shingles or drip edge overhang the gutter.
- Measure the run and read the total drop from the calculator. Mark the low end that far below a level line from the high mark.
- Snap a chalk line or stretch a string between the two marks, then fasten hangers along it at the spacing you entered.
Family Handyman puts hangers every 2 ft. Where snow and ice load the gutters, follow the hanger maker's spacing for your snow load if it is closer.
How many downspouts do I need?
The International Plumbing Code sizes roof drainage from the 100-year, 1-hour rainfall for your location (IPC 1106.1, with maps in Figure 1106.1) and the horizontally projected roof area, meaning the roof seen from above, not its sloped area. One inch of rain per hour on one square foot is about 0.0104 gallons per minute. IPC Table 1106.3 lists what each downspout (vertical leader) can carry:
| Downspout | Capacity |
|---|---|
| 2½ × 3 in rectangular or 3 in round | 92 gpm |
| 3 × 4¼ in rectangular or 4 in round | 192 gpm |
| 4 × 5 in rectangular | 360 gpm |
Worked example: 750 sq ft of roof draining to one gutter, in an area with 3 in per hour of design rainfall, sheds 750 × 3 × 0.0104 = 23.4 gpm: one 2½ × 3 in downspout carries it. A 3,000 sq ft roof at 4 in per hour produces 125 gpm and needs two of them, or one 4 in round. The calculator also never gives fewer downspouts than the runs have low ends.
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Replacing the roof at the same time? The roofing calculator gives the eave lengths and the roof area, the roof pitch calculator turns plan area into sloped area when you need it, and the shingle calculator and metal roof calculator order the roofing itself. The roof replacement cost calculator prices the whole job.