Guttering calculator: gutter size, flow and fall
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Roof area draining to this gutter, on plan | Roof: rectangle 10 m × 6 m |
| Roof pitch | 0 ° |
| Pitch factor from Approved Document H | 1 × |
| Rainfall intensity | 0.021 L/s/m² |
| Outlets | 1 |
| Outlet position | At the end of the run |
| Gutter run | 10 m |
Rainwater flow: 1.26 L/s from 60.0 m² of effective roof
- Limit not yet confirmed Flow along the gutter against its capacity: 1.26 L/s Enter the gutter maker's flow capacity for your gutter and outlet to check it. England: Gutter maker's flow capacity (Approved Document H, Section H3). Check with the source.
| Result | Value |
|---|---|
| Rainwater flow | 1.26 L/s |
| Effective roof area | 60 m² |
| Pitch factor | 1 × |
| Flow to each outlet | 1.26 L/s |
| Flow along the gutter to an outlet | 1.26 L/s |
| Downpipes | 1 |
Show the working
- Effective area: 60 m² × 1 (your factor) = 60 m²
- Flow: 60 m² × 0.021 L/s/m² = 1.26 L/s
- Per outlet: 1.26 ÷ 1 = 1.26 L/s
How it works
- Effective area (m²) = plan area × pitch factor (Approved Document H)
- Flow (L/s) = effective area × rainfall intensity (L/s/m²)
- Flow to each outlet = flow ÷ outlets; with the outlet in the middle, each side of the gutter carries half of that
- Fall (mm) = run ÷ n for a fall of 1 in n; brackets = run ÷ bracket spacing, rounded up, + 1
Gutter capacity depends on the gutter profile, the outlet and where it sits: use the gutter maker's flow figures.
Downpipes discharge to a drain or soakaway; check with building control where new drainage is needed.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Rainfall intensity | 0.021 L/s/m2 | Regulation or HSE figure Approved Document H (Researcher verifies) Not yet checked against the source | Source to be added by the Researcher brief v2 R6: default 0.021 L/s/m² (about 75 mm/h) from Approved Document H. Researcher verifies the clause and value. |
Regulations and standards
- Approved Document H (2015), Section H3: rainfall intensity, effective roof area and gutter sizes
Explained: Approved Document H (drainage).
Approved Document H covers England. Wales, Scotland and Northern Ireland have their own drainage guidance.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Effective roof area: design factor by roof pitch | Approved Document H, Section H3 (Researcher verifies the table and row pitches) | England | 0/5 values entered Not yet checked against the source |
| Gutter and outlet sizes by flow capacity | Approved Document H, Section H3 (Researcher verifies) | England | 0/0 values entered |