Rainwater
Effective roof area draining to a gutter (Approved Document H)
- where
- plan area draining to the gutter (m²)
- length of roof draining to the gutter (m)
- plan depth from gutter to ridge (m)
- pitch factor
- roof pitch (°)
- effective roof area (m²)
- area of wall draining onto the roof (m²)
Explain this equation
Works out the effective area of roof that sends rain to a gutter, following the Approved Document H method. It is the starting point for sizing gutters and downpipes.
A_p is the area of roof seen on plan that falls to the gutter. Wind-driven rain hits a sloping roof at an angle, so a steeper roof catches more than its plan area suggests; the factor 1 + tan θ / 2 allows for that. A wall above the roof that runs onto it catches driving rain too, and half its face area is added.
The pitch factor approximates rain falling at an angle onto the slope; at 0° it is exactly 1.
Sense check: a 10 × 4 m plan at 30° gives a factor of about 1.29 and an effective area of about 51.5 m². At 45° the factor is 1.5.
Watch out: for very steep roofs (70° and over) Approved Document H treats them like a wall, using half the elevation area instead.
Converts the plan area of a roof into the effective area used to size gutters and downpipes, allowing for wind-driven rain on pitched roofs and walls.
Enter your values to see the result.
Good to know
Approved Document H, Table 1, uses the plan area for flat roofs, multiplies it by 1.29 at 30°, 1.50 at 45° and 1.87 at 60°, and takes half the elevation area for walls and roofs over 70°. These factors equal plan area plus half the roof’s elevation area, which is what 1 + tan θ / 2 gives. Add half the area of any wall above that sheds water onto the roof.
Common mistake
Using the sloping area of the roof. The method uses plan area with a pitch factor, which is smaller than the slope area on steep roofs.
Limits
Pitches up to 70°. Steeper roofs and walls use half the elevation area only. For larger buildings and internal gutters use BS EN 12056-3 in full.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.