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/hr | 3 inch/hr | 4 inch/hr | 5 inch/hr | 6 inch/hr |
| 4 inch | 720 ft2 | 480 ft2 | 360 ft2 | 288 ft2 | 240 ft2 |
| 5 inch | 1,250 ft2 | 834 ft2 | 625 ft2 | 500 ft2 | 416 ft2 |
| 6 inch | 1,920 ft2 | 1,280 ft2 | 960 ft2 | 768 ft2 | 640 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/hr | 76.2 mm/hr | 101.6 mm/hr | 127 mm/hr | 152.4 mm/hr |
| 101.6 mm | 66.9 m2 | 44.6 m2 | 33.4 m2 | 26.8 m2 | 22.3 m2 |
| 127 mm | 116.1 m2 | 77.5 m2 | 58.1 m2 | 46.5 m2 | 38.7 m2 |
| 152.4 mm | 178.4 m2 | 119.1 m2 | 89.2 m2 | 71.4 m2 | 59.5 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 pipe | 1 inch/hr | 2 inch/hr | 3 inch/hr | 4 inch/hr | 5 inch/hr |
| 3 inch | 6,440 ft2 | 3,220 ft2 | 2,147 ft2 | 1,610 ft2 | 1,288 ft2 |
| 4 inch | 13,840 ft2 | 6,920 ft2 | 4,613 ft2 | 3,460 ft2 | 2,768 ft2 |
| 5 inch | 25,120 ft2 | 12,560 ft2 | 8,373 ft2 | 6,280 ft2 | 5,024 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 pipe | 25 mm/hr | 50 mm/hr | 75 mm/hr | 100 mm/hr | 125 mm/hr |
| 75mm | 600 m2 | 300 m2 | 200 m2 | 150 m2 | 120 m2 |
| 100mm | 1,286 m2 | 643 m2 | 429 m2 | 321 m2 | 257 m2 |
| 125mm | 2,334 m2 | 1,117 m2 | 778 m2 | 583 m2 | 467 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 foot | 1 inch/hour | 2 inch/hour | 3 inch/hour | 4 inch/hour | 5 inch/hour |
| 3 inch | 3,288 ft2 | 1,644 ft2 | 1,096 ft2 | 822 ft2 | 657 ft2 |
| 4 inch | 7,520 ft2 | 3,760 ft2 | 2,506 ft2 | 1,880 ft2 | 1,504 ft2 |
| 5 inch | 13,360 ft2 | 6,680 ft2 | 4,453 ft2 | 3,340 ft2 | 2,672 ft2 |
| Diameter of pipe sloping 1/2-inch per foot | 1 inch/hour | 2 inch/hour | 3 inch/hour | 4 inch/hour | 5 inch/hour |
| 3 inch | 6,576 ft2 | 3,288 ft2 | 2,192 ft2 | 1,644 ft2 | 1,310 ft2 |
| 4 inch | 15,040 ft2 | 7,520 ft2 | 5,010 ft2 | 3,760 ft2 | 3,101 ft2 |
| 5 inch | 26,720 ft2 | 13,360 ft2 | 8,900 ft2 | 6,680 ft2 | 5,320 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/hr | 50 mm/hr | 75 mm/hr | 100 mm/hr | 125 mm/hr |
| 75 mm | 305 m2 | 153 m2 | 102 m2 | 76 m2 | 61 m2 |
| 100 mm | 700 m2 | 350 m2 | 233 m2 | 175 m2 | 140 m2 |
| 125 mm | 1,241 m2 | 621 m2 | 414 m2 | 310 m2 | 248 m2 |
| Diameter of pipe sloping 40 mm/m | 25 mm/hr | 50 mm/hr | 75 mm/hr | 100 mm/hr | 125 mm/hr |
| 75 mm | 611 m2 | 305 m2 | 204 m2 | 153 m2 | 122 m2 |
| 100 mm | 1,400 m2 | 700 m2 | 465 m2 | 350 m2 | 280 m2 |
| 125 mm | 2,482 m2 | 1,241 m2 | 827 m2 | 621 m2 | 494 m2 |
Note: Install cleanouts in any horizontal pipe run exceeding 100 feet (30 meters).
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