Roof Gutter Calculator
Design flow, what the gutter can carry, and how many downpipes.
Quick answer
With the default inputs: Catchment area 500 m² · Storm intensity q 200 L/(s·100m²) · Runoff coefficient 0.9 etc.Design rainwater flow 9.00 L/s
Edit the inputs below to recalculate.Inputs
Result
Design rainwater flow
9.00L/s
| Gutter flow area | 0.0700m² |
| Hydraulic radius | 0.0933m |
| Gutter capacity | 60.7L/s |
| Capacity ratio | 6.74× |
| Full-flow velocity | 0.87m/s |
| Downpipe capacity | 11.78L/s |
| Downpipes required | 1bars |
| Catchment per downpipe | 500m² |
Design flow Q = runoff coefficient × storm intensity × catchment ÷ 10000, since q is per 100 m². Gutter capacity uses Manning: Q = (1/n)·A·R^(2/3)·√S, with A = b·h and wetted perimeter b + 2h for a rectangular channel. Check two things: capacity above design flow, preferably 1.2×, and full-flow velocity not below 0.6 m/s or silt will settle. Gutter slope must not be under 1‰.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Design flow | Q = ψ × q × F ÷ 10000 (L/s) |
| Gutter area | A = width × depth |
| Hydraulic radius | R = A ÷ (width + 2 × depth) |
| Gutter capacity | Q = (1 ÷ n) × A × R^(2/3) × √S × 1000 (L/s) |
| Downpipe capacity | q = pipe area × velocity × 1000 |
| Downpipe count | n = ⌈design flow ÷ pipe capacity⌉ |
500 m² of roof at 200 L/(s·100m²) and 0.9 runoff: 9 L/s design flow; a 350×200 gutter at 3‰ with n = 0.013 carries 60.7 L/s, 6.7 times over; a DN100 pipe at 1.5 m/s handles 11.78 L/s, so one suffices.
How to work it out
- Design rainfall: Q = q × F × ψ ÷ 100, with q in L/(s·100m²), F the catchment on plan and ψ the runoff coefficient (0.9-1.0 for roofs).
- Gutter capacity by Manning: Q = A × (1/n) × R^(2/3) × I^(1/2), with A the flow section, R = A/wetted perimeter and I the slope as a decimal (3‰ = 0.003).
- Compare: capacity must exceed design flow, or widen the gutter or add outlets.
- Downpipes: capacity per pipe = pipe area × velocity; number of pipes = Q ÷ capacity, rounded up.
- Check velocities: gutter flow should be 0.6-1.2 m/s, below which silt settles and above which it splashes; downpipes run 1-2 m/s.
Where it goes wrong
- Using the wrong unit for q — L/(s·ha) and L/(s·100m²) differ by a hundred, and the whole flow calculation collapses.
- Setting the gutter slope to zero — Manning cannot be evaluated at I = 0, and a flat gutter does not drain; the minimum is 3‰.
- Sizing the gutter but not the downpipes — a generous gutter with too few stacks overflows anyway.
- Ignoring blockage — blocked outlets from leaves cause most wet-season leaks, so allow spare outlets with strainers and an overflow.
Material and cost
Stainless gutter 200-400 CNY/m, coated steel 80-150, PVC 60-120. uPVC DN100 downpipe 25-45, roof outlets 80-200 each.
On site
- Fit strainers or leaf guards in every outlet; blocked outlets are the leading cause of roof overflow, especially near trees.
- Gutter movement joints every 20-30 m with a 20-30 mm gap and flexible sealant, or thermal movement cracks it.
- Provide an overflow on important buildings, set 30-50 mm above the outlet, so storms beyond the design return period shed water instead of overloading the roof.
Basis
- GB 50015-2019 Standard for design of building water supply and drainage, Chapter 5
- GB 50345-2012 Technical code for roof engineering, Chapter 4
- GB 50400-2016 Technical code for rainwater control and use in buildings and communities
FAQ
Why is the calculated capacity so much larger than the flow?
Because storm intensity on a small roof yields little flow, while gutters are sized by construction and cleaning needs rather than hydraulics. A 500 m² roof produces 9 L/s and a single DN100 pipe carries 11.78. The real constraints are downpipe spacing (usually no more than 20 m apart, at least two per catchment) and minimum gutter size for leaf blockage and cleaning. A large margin simply means the section is not the bottleneck.
How many downpipes per catchment?
Code requires at least two outlets or downpipes per catchment so a blockage or failure still leaves a path instead of overflowing. Downpipe spacing should also not exceed 20 m to limit gutter run and flow distance. So even when one is hydraulically sufficient, install two — and add outlets mid-run on gutters longer than 20 m.
What happens with insufficient gutter slope?
Standing water, silt, leaks and eventually damp ceilings and cracked parapet flashings. Below 1‰, velocity is too low to move grit, leaves and droppings, so the section silts up and the gutter overflows in the very storm it was designed for, while the trapped water ages the waterproofing. Code requires at least 1‰, commonly 2-5‰, with the outlet at the low point and no backfalls. Check levels section by section on site.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Roof Gutter Calculator. 2026-10-25. https://jianzhujisuan.com/en/roof-gutter
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.