Guttering Calculator
Calculate gutter lengths, downpipe quantities, and fascia board requirements from roof dimensions. Includes runoff capacity checks. Free, no signup.
Angle Finder
Calculate roof angles for flashing, bends, and junctions
Use when two roof planes meet around an internal or external corner (usually a 90° building corner).
Usually 90° for square building corners
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Guttering — did you know?
Gutter sizing from roof area
Effective roof area for gutter sizing = plan area × pitch factor (bigger roof = more water). Half-round 112mm gutter handles ~36 m² effective area per downpipe. Square 125mm: ~50 m². Always check manufacturer flow rates.
Downpipe spacing
UK rule of thumb: 1 downpipe per 6m of gutter run, or per 36 m² effective roof area (whichever is less). At 35° pitch with 100 m² plan (122 m² effective): 122 ÷ 36 = 3.4 → 4 downpipes minimum.
Fascia and soffit calculation
Fascia length = total eaves length (both sides for gable, all 4 sides for hip). Soffit area = eaves overhang × perimeter. Standard overhang: 150-300mm. Fascia board: 150-200mm deep typically.
Rainwater capacity check
UK design rainfall: 75mm/hour intensity. Each downpipe should handle roof_area × 75mm/h. A 68mm circular downpipe handles ~12 m² at 75mm/h. A 100mm square: ~50 m². Size up in high-rainfall regions.
Formulas used
Effective roof area
effective = plan_area × (1 ÷ cos(pitch°))
Downpipes needed
downpipes = ceil(effective_area ÷ m²_per_downpipe)
Gutter length
gutter = eaves_length (gable: 2 sides; hip: 4 sides)
Fascia length
fascia = eaves_perimeter
Frequently asked questions
How do I calculate gutter length?
Total eaves length. Gable roof: 2 × building_length. Hip roof: perimeter of building. Add 10% for cuts and joints.
How many downpipes do I need?
1 per 36 m² effective roof area (half-round 112mm). At 35°, 100 m² plan = 122 m² effective → 4 downpipes.
What size gutters for my roof?
112mm half-round: up to 36 m²/downpipe. 125mm square: up to 50 m². 150mm industrial: up to 100 m². Check manufacturer flow rates.
Best free guttering calculator?
One that handles gutter length, downpipe count, and runoff capacity. This tool does all, free.