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Downspout & Gutter Sizing

Use the tables below to size your gutters and downspouts according to your catchment area and potential maximum rainfall per hour.
Note:
I will err on the side oversizing gutters as I want all the rain I can get when it falls.

You can get data on rainfall intensity for your site here [link to come], courtesy of NOAA, or additional site data (water-harvesting potential; evapotranspiration rate; selected temperature data; and information to help with tank-, pipe-, drain-, and gutter-sizing) via the free Rainwater Site Analysis Tool created by Doug Pushard of HarvestH2O.com.

SIZING GUTTERS (English measurements) Based on maximum allowable catchment surface area (in square feet) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-3.)

Maximum Rainfall Rate
Diameter of Gutter
sloping 1/16 inch per foot
2 inch/hr3 inch/hr4 inch/hr5 inch/hr6 inch/hr
4 inch720 ft2480 ft2360 ft2288 ft2240 ft2
5 inch1,250 ft2834 ft2625 ft2500 ft2416 ft2
6 inch1,920 ft21,280 ft2960 ft2768 ft2640 ft2

SIZING GUTTERS (Metric measurements) Based on maximum allowable catchment surface area (in square meters) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-3 (Metric).)

Maximum Rainfall Rate
Diameter of gutter
sloping 5.2 mm/m
50.8 mm/hr76.2 mm/hr101.6 mm/hr127 mm/hr152.4 mm/hr
101.6 mm66.9 m244.6 m233.4 m226.8 m222.3 m2
127 mm116.1 m277.5 m258.1 m246.5 m238.7 m2
152.4 mm178.4 m2119.1 m289.2 m271.4 m259.5 m2

Note: For figures associated with larger-diameter gutters see the Uniform Plumbing Code Table 11-3.

SIZING VERTICAL DOWNSPOUT PIPES (English measurements) Based on Maximum Allowable Catchment Surface Area (in square feet) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-1.)

Maximum Potential Rainfall Rate
Diameter of downspout pipe1 inch/hr2 inch/hr3 inch/hr4 inch/hr5 inch/hr
3 inch6,440 ft23,220 ft22,147 ft21,610 ft21,288 ft2
4 inch13,840 ft26,920 ft24,613 ft23,460 ft22,768 ft2
5 inch25,120 ft212,560 ft28,373 ft26,280 ft25,024 ft2

SIZING VERTICAL DOWNSPOUT PIPES (Metric measurements) Based on Maximum Allowable Catchment Surface Area (in square meters) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-1 (Metric).)

Maximum Potential Rainfall Rate
Diameter of downspout pipe25 mm/hr50 mm/hr75 mm/hr100 mm/hr125 mm/hr
75mm600 m2300 m2200 m2150 m2120 m2
100mm1,286 m2643 m2429 m2321 m2257 m2
125mm2,334 m21,117 m2778 m2583 m2467 m2

Notes:

1. For rainfall rates other than those listed, establish the allowable roof area by dividing the area given in the 1 inch/hour (25 mm/hour) column by the desired rainfall rate.

2. For figures associated with larger-diameter pipe see the Uniform Plumbing Code, Table 11-1

SIZING HORIZONTAL DOWNSPOUT PIPES (English measurements) Based on Maximum Allowable Catchment Surface Area (in square feet) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-2.)

Maximum Potential Rainfall Rate
Diameter of pipe sloping 1/8-inch per foot1 inch/hour2 inch/hour3 inch/hour4 inch/hour5 inch/hour
3 inch3,288 ft21,644 ft21,096 ft2822 ft2657 ft2
4 inch7,520 ft23,760 ft22,506 ft21,880 ft21,504 ft2
5 inch13,360 ft26,680 ft24,453 ft23,340 ft22,672 ft2
Diameter of pipe sloping 1/2-inch per foot1 inch/hour2 inch/hour3 inch/hour4 inch/hour5 inch/hour
3 inch6,576 ft23,288 ft22,192 ft21,644 ft21,310 ft2
4 inch15,040 ft27,520 ft25,010 ft23,760 ft23,101 ft2
5 inch26,720 ft213,360 ft28,900 ft26,680 ft25,320 ft2

SIZING HORIZONTAL DOWNSPOUT PIPES (Metric measurements) Based on Maximum Allowable Catchment Surface Area (in square meters) at various rainfall rates (Adapted from Uniform Plumbing Code, 1997 Edition, International Association of Plumbing and Mechanical Officials, Table 11-2 (Metric).)

Maximum Potential Rainfall Rates
Diameter of pipe sloping 10 mm/m 25 mm/hr50 mm/hr75 mm/hr100 mm/hr125 mm/hr
75 mm 305 m2153 m2102 m276 m261 m2
100 mm 700 m2350 m2233 m2175 m2140 m2
125 mm 1,241 m2621 m2414 m2310 m2248 m2
Diameter of pipe sloping 40 mm/m25 mm/hr50 mm/hr75 mm/hr100 mm/hr125 mm/hr
75 mm611 m2305 m2204 m2153 m2122 m2
100 mm1,400 m2700 m2465 m2350 m2280 m2
125 mm2,482 m21,241 m2827 m2621 m2494 m2

Note: Install cleanouts in any horizontal pipe run exceeding 100 feet (30 meters).

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