Roof & Gutter Details
Plan area draining to gutters
Default: 75 mm/hr
Enter the actual largest pitch-adjusted tributary area. It cannot be below the even share; leave blank only when outlets divide the effective area equally.
5% typical
Results
Enter details and click Calculate to see results
UKUK Building Regulations
References & Sources
Standards Cited
- Approved Document H — Drainage and waste disposal — H3 rainwater drainage
H3 Table 1 effective-area pitch factors and Table 2 gutter/outlet sizing - BS EN 12056-3 — Gravity drainage systems inside buildings — Roof drainage, layout and calculation
Gutter sizing, flow capacity, and downpipe design
Formulas Used
- UK effective roof area:
A_effective = A_plan × (1 + 0.5 × tan(roofPitch))
Source: Approved Document H H3, Table 1 pitch-factor anchors — Reproduces the published factors 1.29 at 30°, 1.50 at 45°, and 1.87 at 60°. Over 70°, Table 1 requires an elevational-area method, so this plan-area calculator fails closed. - Required flow rate:
Q = effectiveRoofArea × rainfallIntensity / 3600
Source: Approved Document H / BS EN 12056-3 — Q in litres/second, area in m², rainfall in mm/hr - Number of brackets:
brackets = ceil(gutterRun / bracketSpacing) + 1
Source: BS EN 12056-3
Key Assumptions
- Design rainfall: 75 mm/hr for UK (BS EN 12056-3) — BS EN 12056-3
- Half-round and ogee profiles available in 75-150 mm widths — Industry standard
- Bracket spacing: 900 mm maximum — BS EN 12056-3