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Engineering ToolBox > Draining Roof Areas by Vertical Downspouts
The capacity of vertical downspouts draining roof areas can be found in the table below.
| Downspouts Size | Roof Area | ||
| mm | inches | m2 | ft2 |
| 50 | 2 | 65 | 700 |
| 65 | 2 1/2 | 120 | 1300 |
| 80 | 3 | 205 | 2200 |
| 100 | 4 | 430 | 4600 |
| 150 | 6 | 1255 | 13500 |
The table is based on a rainfall intensity of 100 mm per hour (4 inches per hour). The anticipated rainfall intensity should always be checked with the local authorities.