Gutter and outlet capacity checker
See whether a roof’s gutters and outlets can take the rain that falls on it. Enter the roof, the rainfall for the site, the gutter and the outlets, and compare what runs off with what the drainage can carry.
The method follows BS EN 12056-3, the standard for roof drainage, as HR Wallingford’s manual for it sets it out, and Approved Document H. It checks capacity, not condition. Blocked outlets, flat gutters and failed joints need a roof condition survey to find.
The roof and its drainage
One gutter run and the outlets that drain it. Repeat for each run.
0.022 l/s per m² is the top of the range on Approved Document H’s map of rainfall for eaves gutters, and the figure MGMA uses for England. A valley or internal gutter, where an overflow goes into the building, needs a higher rate for the site, which BS EN 12056-3 takes from its maps of rarer storms. Enter that rate.
Result
Run-off from the roof against what the gutter and the outlets can carry.
- Run-off in a design storm
- Effective catchment
- Rainfall rate used
- Gutter capacity level box gutter
Run-off is 29% of what the gutter can carry.
- Outlet capacity 3 × 100 mm
Run-off is 164% of what the outlets can pass. The outlets are the limit.
Undersized outlets. In a design storm the gutter fills faster than three 100 mm outlets can empty it. Two more outlets, or outlets of at least 129 mm, would bring the run-off within capacity. An internal gutter would also need overflows, so that water goes outside rather than into the building.
Capacities follow BS EN 12056-3, by the method HR Wallingford’s manual for the standard sets out, for a level gutter and open outlets with no grating. A gutter laid to a fall, or a sump at each outlet, carries more, and a grating or leaf guard halves what an outlet passes. Leaves, silt and birds’ nests are not in the standard.
These results are a guide only and don’t replace a survey or professional advice.
Sources
- Approved Document H, Drainage and waste disposal, 2015 edition, Section 1 of H3, Gutters and rainwater pipes
- HR Wallingford, R W P May, Manual for the Design of Roof Drainage Systems, a guide to the use of European Standard BS EN 12056-3:2000, Report SR 620, 2003, for the standard’s equations and factors
- MGMA, Guidance Document GD03, Rainwater drainage design BS EN12056:3-2000, June 2026
- MGMA, Guidance Document GD18, Eaves gutter design flow, June 2026
- MGMA, Guidance Document GD21, Guidance on selecting the correct rainfall intensity, June 2026
- ACO, Rainwater Outlets, its design considerations
- Wavin, Osma Rainwater Systems, its design procedures
- Wade, Roof outlets technical handbook, its flow rates
- Stormguard Rainwater Systems, Technical Information, its notes on flow capacities
How to use it
Describe one gutter run
Enter the roof area that drains to it, the pitch, and any wall above that sheds rain onto it. Use the roof measurement tool if you don’t have the figure.
Pick the rainfall
Use 0.022 l/s per m² for an eaves gutter, or enter the site’s own rate for an internal gutter, where an overflow would go into the building.
Enter the gutter and outlets
The gutter’s shape and size, its length, and the number, size and place of its outlets. The result shows which is the limit, and by how much.
What it tells you, and what it cannot
Capacity is arithmetic. Whether the gutter is actually draining is a question for the roof.
It tells you
- How much water runs off the roof in a design storm, in litres per second
- How much the gutter can carry, and how much the outlets can pass
- Which of the two is the limit, and by how much
- What would bring the run-off within capacity: more outlets, bigger outlets or a bigger gutter
It cannot tell you
- Whether the outlets are blocked, which is the usual reason a gutter overflows
- Whether the gutter falls to its outlets or holds water
- Whether the joints, sole and outlets are sound, or the downpipes below them
- What the downpipes and the drains can take, which the standard covers separately
Questions about gutter capacity
Why do my gutters overflow in heavy rain?
Usually one of three things: the outlets are blocked, the gutter was never big enough for the roof, or the gutter has lost its fall and holds water that a storm tops up. The tool answers the second. A condition survey answers the other two.
What rainfall rate should I use?
For an eaves gutter, where an overflow spills over the edge, Approved Document H takes the rate from its map, which runs from 0.012 l/s per m² in mid Wales to 0.022 around London and in East Anglia. MGMA uses 0.022 for England and 0.018 for Scotland, Wales and Northern Ireland. For a valley or internal gutter, where an overflow comes into the building, BS EN 12056-3 uses a rate for the site that depends on its location and on how serious an overflow would be, from its maps of rarer storms. HR Wallingford’s manual for the standard says the old fixed rate of 75 mm/h should no longer be used.
Does roof pitch matter?
Yes. A steeper roof catches more rain per square metre of plan, so the standard increases the effective area with pitch, by half the roof’s area in elevation. In a valley between two roofs of the same height it adds nothing, since the wind that drives more rain onto one roof shelters the other, so enter a pitch of 0. A wall above the roof adds half its face as well, up to 10 m above the gutter, because wind drives rain against it and the water runs down onto the roof.
Why are the outlets the limit and not the gutter?
A gutter holds a lot of water, but an outlet can only pass so much through a hole of its size, and only once the water over it is deep enough. On many commercial roofs the outlets, not the gutter, decide when it overflows. Sumps at the outlets and larger outlets both help.
What should an internal gutter have that an eaves gutter needn’t?
Overflows: weirs or pipes set below the top of the gutter that send water outside when the outlets can’t cope, rather than over the inside edge. The standard also designs internal gutters to a higher rainfall rate for that reason.
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Roof condition surveys
Gutters, outlets and drainage recorded with every other defect, located on a plan of the roof.
About condition surveysAll measurements, calculations and estimates provided by Roofcheck’s online tools are for guidance purposes only and should not be relied upon as definitive or technically accurate. Results should be independently verified through a professional roof survey or assessment before undertaking any works or making decisions. Roofcheck accepts no liability for any loss, damage or costs arising from reliance on these results, to the fullest extent permitted by UK law.