How to Calculate Gutter Size for Any UK Roof
The gutter size you need depends on your roof’s effective catchment area and local rainfall intensity. For most UK homes, a standard 112 mm half-round or 114 mm square-line gutter is sufficient, handling roofs up to roughly 50–60 m² of effective area. Larger roofs, steeper pitches, or high-rainfall regions require deep-flow or high-capacity profiles rated above 1.5 litres per second. The definitive method is set out in BS EN 12056-3, which ties gutter capacity to effective roof area and a rainfall intensity of 75 mm/h for standard UK design.
Getting this calculation wrong causes real problems. Undersized gutters overflow in heavy downpours, saturating fascia boards, staining render, and sending water straight to your foundations. Oversized gutters cost more and can look out of proportion on smaller properties. Whether you are a roofer pricing a job, a builder specifying materials, or a homeowner replacing tired plastic guttering, the maths is straightforward once you understand the three key variables: roof area, rainfall intensity, and gutter flow capacity.
Why Gutter Sizing Matters
Gutters do one job — carry rainwater from the roof to the downpipe safely. When a gutter is too small, water spills over the front edge during storms. Over time that leads to:
- Damp and rot in fascia boards and soffit behind the gutter
- Staining and algae on brickwork and render
- Foundation damage as water pools around the base of the building
- Ice dams in winter when overflow water freezes at the eaves
Conversely, fitting a 150 mm industrial system on a single-storey porch is overkill. The right size balances performance, aesthetics, and cost.
Step 1 — Measure Your Effective Roof Area
The effective roof area is not simply the footprint of your house. Roof pitch, wind exposure, and the number of elevations draining into each gutter run all affect the figure.
The BS EN 12056-3 Formula
The standard formula used across the UK is:
Effective Roof Area (m²) = (W + H ÷ 2) × L
Where:
| Symbol | Meaning | How to Measure |
|---|---|---|
| W | Horizontal span of roof draining to the gutter (m) | Half the total ridge-to-ridge span for a symmetrical pitched roof |
| H | Height from gutter level to ridge (m) | Measure at the gable end or use pitch tables |
| L | Length of the gutter run (m) | Measure along the fascia board |
Worked Example — Semi-Detached House
| Parameter | Value |
|---|---|
| Horizontal span (W) | 5.0 m |
| Height to ridge (H) | 3.2 m |
| Gutter run length (L) | 8.5 m |
Effective area = (5.0 + 3.2 ÷ 2) × 8.5 = (5.0 + 1.6) × 8.5 = 56.1 m²
For hip roofs, calculate each elevation separately because different lengths drain into different gutter runs.
Step 2 — Establish Rainfall Intensity
BS EN 12056-3 uses a design rainfall intensity measured in millimetres per hour (mm/h). For the majority of England, Wales, and lowland Scotland, the standard figure is:
| Location | Design Intensity |
|---|---|
| Most of England and Wales | 75 mm/h |
| Western Scotland, Lake District, Snowdonia | 100–150 mm/h |
| Exposed coastal and highland sites | Up to 150 mm/h |
If you are specifying for a high-rainfall area — western Scotland, the Pennines, parts of Devon and Cornwall — you should use the higher figure. The Met Office publishes regional data, and most local authorities accept 75 mm/h unless the site is in a recognised high-exposure zone.
Tip: When in doubt, use 75 mm/h for standard domestic work. If you are working to a Building Control specification, they will state the design intensity in the drainage calculation.
Step 3 — Calculate Required Flow Rate
Once you have the effective roof area and the rainfall intensity, the required flow rate in litres per second is:
Flow rate (l/s) = Effective Area (m²) × Rainfall Intensity (m/s)
Converting 75 mm/h to m/s: 75 ÷ 3,600,000 = 0.0000208 m/s
For our example:
56.1 × 0.0000208 = 0.00117 m³/s = 1.17 l/s
A simpler shortcut used on-site:
Flow rate (l/s) = Effective Area (m²) × 0.0208 ÷ 1,000 × Intensity (mm/h)
Or even simpler — use the table below.
Gutter Flow Capacity Table
Every gutter profile has a tested flow capacity. The table below shows the most common PVC-U profiles available from Kalsi Plastics’ rainwater systems range:
| Gutter Profile | Nominal Size | Flow Capacity (l/s) | Max Effective Roof Area at 75 mm/h |
|---|---|---|---|
| Mini gutter | 76 mm | 0.5 | ~24 m² |
| Half round | 112 mm | 0.9 | ~43 m² |
| Square line | 114 mm | 1.1 | ~53 m² |
| Ogee | 116 mm | 1.1 | ~53 m² |
| Deep flow | 115-170 mm | 1.7–2.5 | 82–120 m² |
| Hi-Cap | 150 mm | 2.2 | ~106 m² |
| Industrial | 150+ mm | 2.5+ | 120+ m² |
These figures assume:
- Gutter is installed with the correct fall (typically 1:350 to 1:600)
- Adequate downpipe outlets (one per 50 m² as a rule of thumb)
- Clean, unobstructed flow
Matching Our Example
Our 56.1 m² effective roof area at 75 mm/h requires approximately 1.17 l/s. Looking at the table:
- Half-round (0.9 l/s) — too small
- Square-line (1.1 l/s) — marginal, risky
- Deep-flow (1.7 l/s) — safe choice with headroom
- Ogee with two downpipe outlets — could work if the run is split
The safe specification for this property would be a deep-flow gutter system or a square-line system with two downpipe outlets splitting the run length.
Step 4 — Consider Downpipe Size and Position
Gutter capacity is only half the equation. The downpipe must be large enough to take the water away. Common downpipe sizes in the UK:
| Downpipe Size | Approximate Capacity (l/s) | Typical Use |
|---|---|---|
| 65 mm round | 0.7 | Mini gutter, small shed roofs |
| 68 mm round | 1.0 | Standard domestic — half-round, ogee |
| 80 mm round | 1.5 | Deep-flow and high-capacity |
| 65 × 65 mm square | 1.0 | Square-line systems |
| 80 × 80 mm square | 1.8 | Industrial and commercial |
Key Rules
- Never have fewer outlets than the gutter needs. A single 68 mm downpipe handles roughly 1.0 l/s. If your gutter requires 1.5 l/s, you need two outlets or a larger downpipe.
- Maximum gutter run before an outlet — BS EN 12056-3 recommends no more than 12 m of gutter before a downpipe outlet for domestic installations.
- Position downpipes at low points. If the gutter has a central fall, put the downpipe in the middle. If it falls in one direction, the downpipe goes at the lower end.
Step 5 — Factor in Gutter Fall
PVC-U gutters in the UK are typically installed with a gentle fall towards the nearest downpipe outlet. The recommended fall is:
- Minimum: 1:600 (roughly 1 mm per 600 mm of run)
- Standard domestic: 1:350 (about 3 mm per metre)
- Maximum before aesthetic issues: 1:100
A steeper fall increases flow capacity marginally but becomes visually noticeable on long runs. For most domestic projects, 3 mm per metre is the sweet spot — enough gradient to keep water moving without looking crooked against the roofline.
Some installers fit gutters level and rely on the natural flow to the downpipe outlet. This works on short runs (under 4 m) but is not recommended for longer lengths because standing water encourages debris build-up, algae growth, and potential overflow during heavy rain.
Quick-Reference Sizing Guide
For installers who want a fast answer without running calculations, here is a simplified guide based on typical UK house types:
| Property Type | Typical Effective Area | Recommended Gutter |
|---|---|---|
| Shed, porch, lean-to | 5–15 m² | Mini gutter (76 mm) |
| Small terraced house | 20–40 m² | Half-round (112 mm) or square-line (114 mm) |
| Semi-detached house | 40–70 m² | Square-line, ogee, or deep-flow |
| Large detached house | 70–120 m² | Deep-flow or hi-cap |
| Commercial / industrial | 120+ m² | Industrial gutter or bespoke |
Common Sizing Mistakes
1. Ignoring the Pitch
A steep roof (45°+) has a much larger effective area than a shallow one. If you just measure the footprint and ignore the height, you will undersize the gutter.
2. Forgetting Extensions
A rear extension with its own roof often drains into the same gutter run as the main house. Add both effective areas together.
3. One Downpipe for Everything
Long gutter runs with a single outlet at one end overwhelm the downpipe. Split the run or add a second outlet.
4. Not Accounting for Trees
Gutters near trees catch leaves, moss, and debris. While a leaf guard helps, you should also upsize by one profile as a precaution — blocked gutters have zero capacity.
5. Using the Wrong Rainfall Intensity
If you are in a high-rainfall area (western Scotland, Lake District, parts of Wales), using 75 mm/h will undersize the system. Check with your local authority or use 100 mm/h for safety.
BS EN 12056-3 — What You Need to Know
BS EN 12056-3: Gravity Drainage Systems Inside Buildings — Roof Drainage, Layout and Calculation, is the European standard adopted in the UK for sizing rainwater systems. The key points:
- It specifies how to calculate effective roof area, including wind-driven rain adjustments
- It provides test methods for gutter flow capacity
- It defines rainfall intensity values by region
- It applies to both eaves gutters and valley gutters
- Building Control in England and Wales reference it via Approved Document H
For straightforward domestic projects, the calculations in this guide are fully aligned with BS EN 12056-3. For complex or commercial projects with multiple roof levels, valley gutters, or unusual geometry, a drainage engineer should carry out a full calculation.
Putting It All Together — Step-by-Step Checklist
- ☐ Measure the horizontal span (W), ridge height (H), and gutter run length (L)
- ☐ Calculate effective roof area: (W + H/2) × L
- ☐ Determine design rainfall intensity (75 mm/h for most UK sites)
- ☐ Calculate required flow rate or look up the area in the capacity table
- ☐ Select a gutter profile that exceeds the required flow rate
- ☐ Choose downpipes large enough to handle the flow
- ☐ Plan downpipe positions — one per 50 m² or as calculation requires
- ☐ Allow for fall — 3 mm per metre towards each outlet
- ☐ Add 10–20% headroom if the property is near trees or in an exposed location
- ☐ Document the specification for Building Control if required
Frequently Asked Questions
What size guttering do I need for a standard 3-bed semi?
A typical 3-bed semi-detached house has an effective roof area of around 50–65 m². A 114 mm square-line gutter with two downpipe outlets, or a deep-flow profile with one outlet, will handle this comfortably. For properties in western or highland regions with higher rainfall, step up to deep-flow or hi-cap.
Can I use mini gutters on a house?
Mini gutters (76 mm) are designed for small structures — sheds, porches, canopies, and lean-to extensions — with effective roof areas below 25 m². They are not suitable as the primary gutter on a house because their flow capacity of around 0.5 l/s is too low for anything beyond the smallest single-storey structures.
How many downpipes do I need?
As a rule of thumb, plan one downpipe per 50 m² of effective roof area. For a gutter run longer than 10–12 m, it is usually better to have two downpipes with the gutter falling from the centre towards each end, rather than a single outlet at one end trying to handle the entire run.
Does gutter colour affect performance?
No. Colour has no effect on flow capacity. Black, white, brown, grey, and anthracite all perform identically. Choose the colour that complements your roofline and property style. Dark colours (black, anthracite grey) tend to show less staining over time.
What is the difference between nominal gutter size and flow capacity?
Nominal size (e.g., 112 mm) refers to the width across the top of the gutter. Flow capacity (measured in litres per second) is the actual water-carrying ability, which depends on the profile shape, depth, and outlet position. Two gutters of the same nominal width can have different capacities if one is deeper or more efficiently shaped. Always specify by flow capacity, not just width.
