Gutter and Downspout Sizing
The gutter size and number of downspouts a roof needs.
Example
You enter
- Plan roof area (sq ft) 1200
- Roof-pitch factor 1.10
- Rainfall (in/hr) 5
- Downspout (sq in) 12
You get
- Adjusted roof area 1320 sq ft
- Downspouts 2
Details, formula, and sources
The adjusted (design) roof area from the plan area, the roof pitch, and the local rainfall intensity, then the gutter and downspout count.
Adjusted area = plan area x pitch factor x (rainfall / 5 in/hr); downspouts = ceil((adjusted / 100) / downspout sq in); gutter 5 in K-style up to ~5,520 sq ft else 6 in.
SMACNA / standard residential gutter method (by name).
SMACNA licensed; rainfall intensity free from NOAA Atlas 14 at hdsc.nws.noaa.gov.
Estimate. AHJ and licensed professional govern.
Field names used by the API: roof_area_sqft, pitch_factor, rainfall_in_hr, downspout_sqin, adjusted_area, downspouts
- Reference intensity 5 in/hr reference; 1 sq in of downspout per ~100 sq ft of roof (editable)SMACNA practice