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 1,320 sq ft
- Downspouts 2
Checked against published worked examples from: Berger Building Products (reproducing SMACNA ASMM Table 1-1).
Details, formula, and sources
ScopeThe adjusted (design) roof area from the plan area, the roof pitch, and the local rainfall intensity, then the gutter and downspout count.
Formula or tableAdjusted area = plan area x pitch factor x (rainfall / 5 in/hr); downspouts = ceil((adjusted / 100) / downspout sq in); gutter design area = plan area x pitch factor x rainfall (in/hr): 5 in K-style up to 5,520, 6 in K-style up to 7,960, else split the run or go larger.
Source and editionSMACNA / standard residential gutter method (by name).
Free accessSMACNA licensed; rainfall intensity free from NOAA Atlas 14 at hdsc.nws.noaa.gov.
What governsEstimate. AHJ and licensed professional govern.
API field namesroof_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