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Gutter Size Calculator

Calculate the right gutter size for a UK roof from area, pitch and design rainfall using BS EN 12056-3 NA hydraulic capacities. Half-round, ogee, deepflow and square profiles.

Gutter Size Calculator

Size guttering from roof area, pitch and design rainfall using BS EN 12056-3 NA hydraulic capacities.

Recommended gutter size
DN 125 halbrund
Peak flow: 2.59 L/s · Flow per downpipe: 1.29 L/s
Effective drainage area: 122 m² (Pitch factor: ×1.2)
Minimum acceptable
DN 125 halbrund
Downpipe cross-section
75 × 100 mm
Suggested downpipe count
1
Reference standard
BS EN 12056-3:2000 NA / BBA Agrément

What this calculator does

This calculator sizes UK guttering from three inputs: the projected roof area that drains into the gutter, the roof pitch, and the design rainfall intensity for your region. It applies the BS EN 12056-3:2000 NA rational method, accounts for wind-driven rain on steeper pitches via a pitch factor, then matches the resulting peak flow against the BBA-published hydraulic capacities for half-round, ogee, square and deepflow profiles to recommend a nominal size.

It also sizes the downpipes and tells you the minimum number needed to handle the total peak flow — separating the gutter sizing question from the downpipe sizing question, which trip up about a third of DIY installs.

How to use it

  1. Enter the projected roof area in m². Plan-view footprint, not on-slope area. For a simple gable, length × width. For a hip or L-shape, sum each slope’s projected area.
  2. Pick the pitch. This sets the wind-correction factor that converts projected area to effective drainage area. UK roofs typically sit between 30° and 45°.
  3. Set the design rainfall. The default is 75 mm/hr (BS EN 12056-3 NA category 1 — most of England below 200 m elevation). Use 100 mm/hr for west Scotland, Cumbria, Snowdonia, and west Wales (NA category 2).
  4. Choose the profile. Half-round (or ogee) for traditional and most modern semis; square or deepflow for higher capacity at the same face width; box for commercial.
  5. Set the number of downpipes. Two is typical for terraced and semi-detached. Three for detached over 200 m² or for split-pitch front gutters with corner downpipes.
  6. Read the result. The big number is the recommended nominal size. The minimum-acceptable line is the smallest size that just handles the load.

The BS EN 12056-3 rational method

Peak flow into a gutter is calculated by:

Q (L/s) = effective drainage area (m²) × rainfall intensity (mm/hr) ÷ 3,600

Effective drainage area is the projected (plan-view) area multiplied by a pitch factor that accounts for wind-driven rain:

PitchPitch factor
Flat (≤ 4°)1.00
14° (3/12 equivalent)1.05
22° to 26°1.10
30°1.20
35°1.25
40° to 45°1.30

The pitch factor is empirical — calibrated against UK Met Office driving-rain index data and BRE Digest 127 corrections for windward elevations. East-coast and exposed coastal sites can add another 10% if the gutter is on the windward side of the roof during prevailing storms.

Per-downpipe capacity tables

For half-round at 1:600 standard fall:

Nominal sizeCapacity per downpipe
100 mm half-round1.4 L/s
112 mm half-round1.85 L/s
125 mm half-round2.5 L/s
150 mm half-round3.4 L/s

For square profile at 1:600 fall:

Nominal sizeCapacity per downpipe
112 mm square2.2 L/s
125 mm square3.0 L/s
150 mm square4.5 L/s

The recommended size is the smallest profile that handles the per-downpipe flow with at least 15% reserve. The reserve covers partial leaf-clogging, gradients flatter than nominal, and rainfall events above the 1-in-1-year design storm. For 1-in-30-year design (commercial and critical infrastructure), step up one nominal size from the calculator’s recommendation.

When to step up a size

Step up from the recommended size if any of these apply:

  • Concentrated valleys. A roof valley dumps the flow from two slopes into a short section of gutter. The capacity tables assume even distribution; concentrated valley loads can swamp a borderline-sized gutter.
  • Long single-fall runs. Beyond 8 metres of single-fall gutter, the high end sees stagnant water during heavy rain because the downpipe is too far away. Step up one nominal size or add a midpoint downpipe.
  • Steep pitches above 45°. The pitch factor undercounts wind-driven rain on slopes above 45°; designers in the Highlands typically add 15% to the calculated flow.
  • High-rainfall regions. West Scotland, Cumbria, Snowdonia, west Wales, and Cornwall can experience 100+ mm/hr in convective storms; use the BS EN 12056-3 NA category 2 design value of 100 mm/hr rather than the default 75 mm/hr.
  • Smooth roof coverings. Standing-seam metal, glazed clay tile and slate shed water faster than concrete tile, concentrating flow at the eave. Add 10–15% to calculated flow for these coverings.

Downpipe sizing — the BBA rule of thumb

The traditional UK rule is 1 cm² of downpipe cross-section per 10 m² of effective drainage area. Standard pairings:

DownpipeCross-sectionDrains up to
65 mm round33 cm²80 m²
76 mm round45 cm²130 m²
110 mm rectangular110 cm²250 m²

Underspouting is the most common UK gutter failure. A correctly sized 125 mm deepflow on undersized 65 mm round downpipes will overflow at the high end during a 75 mm/hr storm because the downpipe chokes the flow before the trough fills. Pair sizes properly: 100 mm gutter to 65 mm downpipe; 112 mm half-round to 65 mm round; 125 mm deepflow to 76 mm round; 150 mm to 110 mm rectangular.

Common UK edge cases

Listed building, cast-iron replacement. Listed building consent typically requires you to match the existing profile and downpipe diameter exactly. Use a like-for-like cast-iron or composite cast-iron-look profile from Hargreaves or Dales. The calculator’s hydraulic recommendations still apply for capacity-checking, but you may not be able to step up size without LBC variance.

Terraced row, party-wall downpipe shared. When the downpipe is shared at a party wall, the contributing area is both halves of both roofs. Calculate accordingly — a 5 m × 7 m terraced house with shared party-wall downpipe sees 70 m² of contributing area, not 35 m².

Conservation area, modern composite replacement. Conservation area amenity societies often allow Marley Alutec or Lindab aluminium half-round to replace cast-iron, with similar visual profile but ~30% better hydraulic capacity for the same nominal size. Use the half-round table for sizing.

Combined sewer connection. If your downpipes feed into a combined sewer, Thames Water or your local authority may require flow-restricting devices on new builds (rainwater attenuation). This doesn’t change gutter sizing but can affect downpipe sizing and the number of underground rainwater connections.

Reference standards (UK)

  • BS EN 12056-3:2000 + UK National Annex — Gravity drainage systems inside buildings — Roof drainage, layout and calculation.
  • BS EN 612 — Eaves gutters and rainwater downpipes of metal sheet (specification).
  • BBA Agrément Certificates — Per-manufacturer published flow capacities (Marley, Hunter, FloPlast, Marshall-Tufflex).
  • Approved Document H (2015 + 2018 amendments) — Drainage and Waste Disposal, references BS EN 12056-3.
  • NHBC Standards Chapter 7.2 — Roof drainage for new-build warranty.
  • BRE Digest 127 — Driving rain in the UK, for wind-correction guidance.

Sources: BS EN 12056-3:2000 + UK NA; BBA Agrément Certificates for Marley, Hunter, FloPlast guttering; Approved Document H 2015 + 2018; NHBC Standards Chapter 7.2; BRE Digest 127 driving rain index; Met Office UK climate data.

Frequently asked questions

What size gutter do I need for a UK semi-detached house?
A standard UK semi-detached with a 100 m² projected roof area, a 30° pitch and the BS EN 12056-3 NA design rainfall of 75 mm/hr produces a peak flow of about 2.5 L/s. Split between two downpipes (one at each end of the front gutter), that's 1.25 L/s per downpipe — well within the capacity of a standard 112 mm half-round (about 1.85 L/s). Most UK new-build semis use 112 mm half-round (sometimes called 4½-inch ogee in older trade catalogues) as the default, with 76 mm round downpipes. Step up to 125 mm deepflow or square if the projected area exceeds 130 m², if the roof pitch is below 25°, or if you have only a single downpipe serving a long run.
Is 112 mm or 125 mm gutter better for a UK home?
112 mm half-round (4½-inch in old-money) is the default for terraced and semi-detached homes built since the 1960s. It handles up to about 130 m² of projected roof area at standard rainfall. 125 mm deepflow or 125 mm square is recommended when projected area exceeds 130 m², when the roof has long single-fall runs over 8 metres, when you're in a high-rainfall region (Wales, Cumbria, west Scotland) where the BBA-recommended design rainfall rises to 100 mm/hr, or when you're feeding rainwater into a combined sewer that's already at capacity (London, Manchester, Glasgow combined-sewer-overflow zones). The 25% material cost step-up to 125 mm is small compared to the call-out cost of overflow remedial work.
How does BS EN 12056-3 size gutters?
BS EN 12056-3:2000 with the UK National Annex specifies gutter sizing as a function of effective drainage area (m²), design rainfall intensity (L/s/m² or equivalently mm/hr), gutter geometry (half-round, ogee, square, deepflow) and gradient (typically 1:600 minimum to 1:300 typical). The flow capacity Q for a given gutter section is published in the BBA Agrément certificates for major manufacturers (Marley, Hunter, FloPlast). The calculator uses the rational method peak flow Q = effective area × intensity, then matches against the BBA-published capacity tables to recommend the smallest profile that handles the per-downpipe flow with a 15% reserve.
What's the difference between half-round, ogee, square and deepflow guttering?
Half-round (also sold as 'round' or, in 4½-inch nominal, 'ogee') is the original Victorian profile and the modern UK default — semicircular cross-section, low capacity per face width but cheap and well-supported by every UK supplier. Ogee strictly refers to the S-curve cast-iron profile common on heritage homes and modern composite replicas; the back is shaped to the fascia. Square is a flat-bottomed rectangular profile with higher capacity than half-round at the same face width; common on modern semis. Deepflow is a square-bottomed half-round-front profile with the highest residential capacity per face width, suitable for high-rainfall regions and large projected areas. Cast-iron heritage replacements (Hargreaves, Dales) use half-round or ogee almost exclusively.
How many downpipes does my UK gutter run need?
BS EN 12056-3 sizes downpipes from total peak flow: divide Q by per-downpipe capacity. A 65 mm round downpipe handles about 1.5 L/s, a 76 mm round handles about 2.2 L/s, a 110 mm rectangular handles about 4.5 L/s. The companion rule of thumb for residential is one downpipe per 8 metres of single-fall gutter or per 10 metres of centre-high (split-pitch) gutter. A typical 14 metre front-of-house gutter run on a UK semi wants two downpipes, one at each end. Add a third midpoint downpipe if the projected area exceeds 200 m² or if you're in a high-rainfall region.
Do I size from the projected roof area or the actual sloped area?
Projected area — the plan-view footprint from above, not the slope-length area. BS EN 12056-3 then applies a wind correction (clause 6.2) for steeper roofs that catches additional driving rain. Most UK pitched roofs sit between 30° and 45° where the wind correction is modest (≤ 20%); the calculator handles this automatically with the pitch factor. For a 10 m × 7 m semi with a 30° pitch, projected area is 70 m² and effective area is 70 × 1.20 = 84 m². Don't double-count by using the slope-length area (~80 m²) and then also applying the pitch factor.
What downpipe size pairs with 112 mm and 125 mm guttering?
Standard pairings: 112 mm half-round with 65 mm round downpipe; 125 mm deepflow or square with 76 mm round; 150 mm commercial deepflow with 110 mm square or rectangular. The 1 cm² of downpipe cross-section per 10 m² of effective drainage area rule of thumb gives a conservative starting point — a 100 m² roof wants ~10 cm² of downpipe cross-section, met by a single 65 mm round (33 cm²) but the rule says use two 65 mm rounds for redundancy and to reduce overflow risk if one chokes with leaves. UK downpipe standards are the same diameter regardless of gutter profile, so you can mix half-round gutter with rectangular downpipe if it suits the elevation.
Are gutter sizes a Building Regulations requirement in the UK?
Approved Document H (Drainage and Waste Disposal) requires that surface water from roofs be drained to a soakaway, watercourse or sewer, but does not specify numerical gutter sizes. BS EN 12056-3 with the UK National Annex is the cited British Standard for sizing — referenced in Approved Document H for guidance but not strictly mandated. NHBC Standards Chapter 7.2 (cited for new-build warranty work) requires BS EN 12056-3 sizing as part of warranty compliance. Local authority Building Control accepts SMACNA-equivalent sizing where the property is being converted from a non-residential use; for new residential, BS EN 12056-3 is expected. Cast-iron heritage replacements on listed buildings may need conservation officer approval before changing profile or downpipe size.

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