Home & Construction⏱ 4 min read
How to Calculate Gutter Size and Downpipe Capacity
Undersized gutters overflow in heavy rain and cause damp. Here is the formula for calculating the roof catchment area, the rainfall intensity to design for, and the correct gutter profile size.
Most UK houses have 112mm half-round gutters, which are adequate for average roofs in average rainfall areas. But larger roofs, steep pitches, or high-rainfall locations can require bigger systems -- and the calculation is straightforward.
Roof Catchment Area
The effective catchment area is the plan (horizontal) area of roof draining to one gutter.
For a pitched roof: use the horizontal plan area, not the slope area.
Roof pitch does affect effective area slightly:
Effective area = Plan area x pitch factor
For pitches below 45 degrees: factor = 1.0 (use plan area)
For pitches 45-60 degrees: factor = 1.05-1.10
Above 60 degrees (near vertical): factor up to 1.40
Example: Semi-detached house, one roof slope:
Roof slope: 5m wide x 8m long
Plan area: 5 x 8 = 40 m2
Pitch 30 degrees: factor 1.0
Effective catchment area: 40 m2
Design Rainfall Intensity
UK Building Regulations Approved Document H uses:
Design rainfall intensity: 75 mm/hour (for most of UK)
For higher-risk areas (some Scottish uplands): 100 mm/hour
Flow rate calculation:
Q = Catchment area x Rainfall intensity / 3,600
Q in litres/second, area in m2, rainfall in mm/hour
Example: 40 m2 catchment, 75 mm/hour:
Q = 40 x 75 / 3,600 = 3,000 / 3,600 = 0.833 litres/second
Gutter Capacity vs Flow Rate
Half-round gutter flow capacities (level gutter):
75mm gutter: 0.38 L/s
100mm gutter: 0.78 L/s
112mm gutter: 1.22 L/s
125mm gutter: 1.67 L/s
150mm gutter: 2.71 L/s
Our example: 0.833 L/s needed
112mm gutter (1.22 L/s): ADEQUATE (capacity exceeds demand)
100mm gutter (0.78 L/s): INADEQUATE
Gutter fall helps: a 1:500 fall (2mm drop per metre run)
increases capacity by approximately 40% for a half-round gutter.
For long gutter runs or large roofs:
Add a downpipe at each end instead of just one.
Each downpipe splits the catchment area and reduces flow per gutter section.
Downpipe Sizing
Downpipe capacity for vertical pipes:
68mm downpipe: 1.6 L/s
100mm downpipe: 5.2 L/s
UK standard 68mm round downpipe is adequate for:
Most domestic situations where gutter is correctly sized
(The gutter is always the bottleneck, not the downpipe)
Number of downpipes needed:
One downpipe per 50m2 catchment area (guideline)
or: Downpipes = Catchment area / 50
For 40 m2: 40/50 = 0.8 -- one downpipe is sufficient
For 80 m2 (larger house): 1.6 -- two downpipes recommended