Max roof drainage area per gutter size by rainfall
This is our own cross-size reference and the moat behind the site. For every common gutter size and profile it gives the labeled SMACNA-style max roof area at a 6 in/hr design rain and then derives — unique to this site — the max roof area that same gutter drains across the whole US rainfall-intensity range, from the identity max area(intensity) = max area at 6 in/hr × 6 ÷ intensity. It is a dated snapshot of stable typicals, not a live feed. See how it’s derived in the methodology, and use it with the gutter size calculator and the capacity / flow reference.
| Gutter size | 2 in/hr | 4 in/hr | 6 in/hr | 8 in/hr |
|---|---|---|---|---|
| 5" K-style | 7,500 ft² | 3,750 ft² | 2,500 ft² | 1,875 ft² |
| 6" K-style | 11,520 ft² | 5,760 ft² | 3,840 ft² | 2,880 ft² |
| 7" K-style | 16,560 ft² | 8,280 ft² | 5,520 ft² | 4,140 ft² |
| 5" half-round | 5,760 ft² | 2,880 ft² | 1,920 ft² | 1,440 ft² |
| 6" half-round | 7,500 ft² | 3,750 ft² | 2,500 ft² | 1,875 ft² |
Labeled planning typicals (max roof area in ft²) — the figures scale inversely with rainfall intensity, so at 6 in/hr a 5" K-style drains ~2,500 ft² and at 3 in/hr the same gutter drains ~5,000 ft². Confirm your local rainfall and follow local code and the manufacturer’s data for complex roofs, valleys and long runs. Snapshot: 2026-07-11.