Getting the water off the roof and away from the house
Gutters collect rain from the roof, but it is the downspouts that carry it to the ground. Too few or too small downspouts make gutters overflow during heavy storms, and the water then runs down the walls and soaks the soil next to the foundation, a leading cause of wet basements and cracked foundations.
Two simple sizing rules
- Area rule: allow 1 square inch of downspout opening for every 100 square feet of roof, measured in plan view (the footprint of the roof, not its sloping surface).
- Spacing rule: place a downspout at least every 40 feet of gutter, and at the ends of long runs.
The calculator applies both and keeps the larger number.
| Downspout | Opening | Roof area served |
|---|---|---|
| 2 × 3 in rectangular | 6 sq in | about 600 sq ft |
| 3 in round | 7 sq in | about 700 sq ft |
| 3 × 4 in rectangular | 12 sq in | about 1,200 sq ft |
| 4 in round | 12.6 sq in | about 1,250 sq ft |
Worked example
A house has 1,500 sq ft of roof and 60 ft of gutter, with standard 2 × 3 inch downspouts:
- Area needed: 1,500 ÷ 100 = 15 sq in, so 3 downspouts.
- Spacing: 60 ÷ 40 = 1.5, so 2 downspouts.
- Result: 3 downspouts.
- During a storm of 5 inches per hour, the roof sheds about 77.9 gallons per minute.
With 3 × 4 inch downspouts, two would cover the area.
Choosing the rainfall intensity
Gutter design usually uses the heaviest 5-minute rainfall expected about once every 10 years at your location, which NOAA’s Atlas 14 publishes. It ranges from about 2 inches per hour in the Pacific Northwest to 7 or more on the Gulf Coast and in Florida. The intensity matters most for very large or flat roofs and for sizing gutters.
Installation tips
- Slope gutters about 1/4 inch per 10 feet toward the downspouts.
- Use 3 × 4 inch downspouts with 6 inch gutters, under trees, and on steep roofs, where water arrives faster.
- Extend outlets at least 4 to 6 feet from the foundation with extensions or splash blocks, or connect them to an underground drain.
- Consider rain barrels for garden watering. Check local rules first, as some places limit rainwater collection.
- Clean gutters and downspout strainers in late fall and spring.