Size gutters and downspouts correctly based on your roof's drainage area and local rainfall intensity to prevent overflows and water damage.
Gutter size depends on two factors: how much roof area drains into the gutter, and how intense your local rainfall gets during storms. Bigger drainage area or heavier rainfall requires larger gutters.
The drainage area is the roof surface that sheds water into a specific gutter run. For a gable roof, each side drains separately.
Use the maximum 5-minute rainfall intensity for your area (inches per hour). This data is available from NOAA or your local building department.
| Region | Typical Max (in/hr) |
|---|---|
| Pacific Northwest | 2-3 |
| Northern Plains | 3-5 |
| Midwest | 4-6 |
| Mid-Atlantic | 5-7 |
| Southeast | 6-8 |
| Gulf Coast / Florida | 8-11 |
Match your adjusted area × rainfall intensity against gutter capacity charts. The capacity is rated at 1 inch/hour — divide by your actual rainfall intensity.
| Gutter Size/Style | Capacity @ 1 in/hr | @ 6 in/hr |
|---|---|---|
| 5" K-Style | 5,520 sq ft | 920 sq ft |
| 6" K-Style | 7,960 sq ft | 1,327 sq ft |
| 5" Half-Round | 3,840 sq ft | 640 sq ft |
| 6" Half-Round | 5,520 sq ft | 920 sq ft |
If your drainage area (adjusted for pitch) exceeds the capacity at your rainfall rate, upgrade to the next gutter size or add a downspout to split the run.
Each downspout handles a limited drainage area. Place downspouts to limit gutter run length to 40 feet maximum per downspout.