⌂ RoofingCalculatorHQ

Downpipe Calculator

Size rainwater downpipes from roof area, pitch and rainfall using BS EN 12056-3 and BS 6367 methods — round and square profiles in cast iron, uPVC and aluminium.

Downpipe Calculator

Downpipe shape:
Effective drainage area
154 m²
Cross-section
80 cm²
Drains per downpipe
80 m²
Min downpipes
2
Peak flow
3.21 L/s
BS EN 12056-3 simplified rule: 1 cm² of downpipe cross-section per 1 m² of effective catchment.

What this calculator does

This calculator sizes the vertical downpipes that carry water from the gutter to ground level. It takes the roof catchment area, pitch, rainfall intensity, and your chosen downpipe profile and dimensions, then tells you how many downpipes you need and what the peak flow into the system will be.

Downpipe sizing is a separate calculation from gutter sizing: a 150 mm deep-flow gutter sized for a large detached’s catchment still needs the right downpipes under it or you’ll get overflow at the gutter’s high point during a 75 mm/hr design storm.

How to use it

  1. Enter the projected roof area in m². Plan-view footprint, not sloped surface — for a simple gable, length × width. For a hipped or complex roof, sum the projected area of each slope.
  2. Set the pitch factor. Default is 1.10 (around 35° pitch, the British residential default). Use 1.00 for flat, 1.05 for very shallow pitches (15–20°), 1.20 for 40–50° pitches, 1.30 for steep alpine-style 60° pitches.
  3. Enter the rainfall intensity in mm/hr. Default is 75 mm/hr — the BS EN 12056-3 design baseline for the UK. The Met Office UK 5-minute rainfall atlas shows 60–80 mm/hr for most of England and Wales, 50–70 for Scotland, and locally 100+ mm/hr for the M5 corridor and parts of Cornwall.
  4. Pick the downpipe shape and dimensions. Round 68 mm uPVC for the residential default with 112 mm half-round gutter, round 110 mm uPVC for deep-flow systems, square 65 × 65 mm or 75 × 75 mm uPVC for square-line gutter systems. Cast iron sizes match.
  5. Read the result. The big number is the minimum number of downpipes needed to handle the effective catchment area.

The BS EN 12056-3 simplified rule

The rule of thumb is: 1 cm² of downpipe cross-section drains 1 m² of effective catchment. So a 68 mm round uPVC downpipe (cross-section π × 34² = 3,632 mm² = 36.3 cm²) handles 36 m² of catchment, and a 110 mm round (cross-section π × 55² = 9,503 mm² = 95 cm²) handles 95 m². This rule produces the same answer as the full BS EN 12056-3 rational method for the British 75 mm/hr design intensity and is what most installers use day-to-day.

For higher-intensity regions (M5 corridor, Cornwall, parts of West Wales), step up one nominal size or add a downpipe.

Common UK downpipe sizes and capacities

SizeCross-sectionDrains up to (75 mm/hr)
65 × 65 mm square uPVC42 cm²42 m²
68 mm round uPVC36 cm²36 m²
75 × 75 mm square uPVC56 cm²56 m²
80 mm round cast iron50 cm²50 m²
100 mm round uPVC78 cm²78 m²
110 mm round uPVC95 cm²95 m²
100 mm round cast iron78 cm²78 m²
150 × 100 mm rect. uPVC150 cm²150 m²

Pairing downpipes to gutter sizes

Standard pairings, never undersize:

  • 112 mm half-round gutter (the British residential default) with 68 mm round (or 65 × 65 mm square) uPVC downpipe — drains up to ~40 m² per downpipe.
  • 125 mm half-round gutter (mid-size detached) with 80 mm round cast iron or 75 × 75 mm square uPVC — drains up to ~55 m² per downpipe.
  • 150 mm deep-flow gutter (larger detached) with 110 mm round uPVC — drains up to ~95 m² per downpipe.
  • Box / OG profile gutter (heritage / commercial) with 100 × 75 mm rect. cast iron — drains up to ~75 m² per downpipe.

A correctly sized 150 mm deep-flow gutter on a 68 mm round downpipe will overflow at the high end during a 75 mm/hr storm because the downpipe chokes the flow before the gutter trough fills.

When to step up a size

Add cross-section or add downpipes if any of these apply:

  • Concentrated valleys. A roof valley dumps the flow from two slopes into a short stretch of gutter. Place a downpipe directly under the valley termination if possible.
  • Long single-fall runs. Beyond 12 m of single-fall gutter, the high-end gutter sees standing water during heavy rain because the nearest downpipe is too far away.
  • Steep pitches above 45°. Wind-driven rain factor increases nonlinearly above 45° — apply 1.20 to 1.30 pitch factor.
  • High-rainfall regions. M5 corridor (Worcester to Bristol), Cornwall, parts of West Wales — use 90–100 mm/hr in the calculator.
  • Smooth metal or glazed clay tile roofs. Standing-seam metal and glazed clay shed rain faster than concrete or slate, concentrating flow at the eave with less detention. Add 10–15% to the calculated peak flow.
  • Box gutters and parapet gutters. Lined parapet gutters carry concentrated flow over a long horizontal distance; sizing is governed by BS 6367, not the rule of thumb. Always specify with overflow weirs.

Discharge to drains, soakaways and SuDS

Building Regulations Approved Document H Part 3 governs discharge:

  • Combined sewer. Permitted on existing properties where the connection already exists, but most water companies (Thames Water, Anglian Water, Severn Trent, Yorkshire Water, etc.) charge a fee for new connections and the trend is to refuse new combined-sewer rainwater connections.
  • Separate surface-water sewer. Preferred where available — newer estates and post-2010 development.
  • Soakaway. Mandatory in most new-build outside the public-sewer zone under Sustainable Drainage Systems (SuDS) guidance. Size at 3 m³ per 100 m² of impermeable area; locate at least 5 m from any building and 2.5 m from any boundary; perform a BRE Digest 365 percolation test before specifying.
  • Watercourse discharge. Requires Environment Agency permission and is increasingly difficult to obtain.
  • Rainwater harvesting. Increasingly common — Building Regulations now allow non-potable reuse for toilet flushing and garden irrigation. Tank sizing typically 1,500–4,000 L for a 4-bed detached.

Common edge cases

Two-storey house, no upper-storey gutter. Combine the upper and lower roof catchments for the lower-storey downpipe count. Better fix: add a small upper-storey gutter routed via a kickout into the lower-storey gutter.

Porch roof tied to the main eave. Dedicate a corner downpipe where the porch eave meets the main gutter, or step up the main gutter one size.

Edwardian / Victorian house with original cast-iron downpipes. The 75 mm round cast-iron downpipes specified in 1900–1920 are typically undersized for modern rainfall intensity — overflow at the gutter is the symptom. Either replace with 100 mm cast iron (Hargreaves Foundry / Saint-Gobain Cast Iron) preserving the heritage profile, or add a third downpipe at mid-run.

Conservation area, listed building. uPVC downpipes are usually refused planning consent. Cast-iron round or moulded box only — Hargreaves Foundry, Saint-Gobain Cast Iron Drainage Systems, or Heritage Fabrications. Aluminium with a cast-iron-look powder coat (Marley Alutec, Lindab) is often accepted as an alternative if the LPA approves it.

Reference standards (UK)

  • BS EN 12056-3:2000 — Gravity drainage systems inside buildings — Roof drainage layout and calculation.
  • BS 6367:1983 — Code of practice for drainage of roofs and paved areas (legacy reference, withdrawn but still cited).
  • Approved Document H Part 3 — Sanitary pipework and drainage (Building Regulations 2010, 2015 edition).
  • NHBC Standards Chapter 7.2 — Pitched roofs.
  • NHBC Standards Chapter 5.3 — Drainage below ground.
  • BRE Digest 365 — Soakaway design.
  • BBA Agrément certificates — for FloPlast, Hunter Plastics, Marshall Tufflex, Polypipe, Marley Alutec, Lindab, Hargreaves Foundry.
  • BS EN 607:2004 — Eaves gutters and fittings made of PVC-U.
  • BS EN 1462:2004 — Brackets for eaves gutters — Requirements and testing.

Sources: BS EN 12056-3:2000 Gravity drainage systems inside buildings; BS 6367 Drainage of roofs and paved areas; Approved Document H Part 3; NHBC Standards Chapters 5.3 and 7.2; BRE Digest 365 Soakaway Design; BBA Agrément certificates; Met Office UK 5-minute rainfall climatology; Hargreaves Foundry technical manual; FloPlast / Hunter Plastics / Marshall Tufflex / Polypipe installation guides.

Frequently asked questions

How many downpipes do I need for a 140 m² roof?
Apply the BS EN 12056-3 simplified rule of thumb — 1 cm² of downpipe cross-section per 1 m² of effective catchment. A 35° pitched roof has a pitch factor of about 1.10, so 140 m² of plan-view area becomes 154 m² of effective catchment. A 100 mm round uPVC downpipe (78.5 cm² cross-section) drains 78 m² per pipe, so you need ceiling(154 / 78.5) = 2 downpipes. A 110 mm Hunter Plastics uPVC downpipe (95 cm²) would just about handle the same area on a single run, but the standard British semi works as two — one at each rear corner of the rear elevation, or one at each end of the front gutter.
What size downpipe pairs with a 112 mm half-round gutter?
A 68 mm round (or 65 × 65 mm square) uPVC downpipe pairs with the standard 112 mm half-round gutter — that's the FloPlast / Marshall Tufflex / Polypipe / Hunter system default. Cross-section is 36 cm², drains 36 m² per pipe. For larger semi-detached or detached homes step up to a 110 mm round pipe with a 150 mm deep-flow gutter (drains 95 m² per pipe). For Grade II listed and conservation-area homes you'll usually be specifying cast-iron round or moulded box (Hargreaves, Saint-Gobain, Heritage Fabrications) — sizes match the uPVC equivalents but profiles vary.
Round vs square downpipes — which is better?
Hydraulically identical for the same cross-section. Round 68 mm uPVC has 36 cm² and a 65 × 65 mm square has 42 cm² — close enough that they're treated as interchangeable in BS EN 12056. Round is the heritage default and the only profile permitted in many conservation areas (square is read as 'modern' by planning officers). Square is cheaper at the merchant and easier to fit through a 90° bend at a soil pipe boxing without losing flow. uPVC is the volume default; cast iron is for Grade II listed buildings and increasingly for high-end new builds where embodied carbon and longevity (60+ years vs 25 years for uPVC) justify the premium.
What's the BS EN 12056-3 method for downpipe sizing?
BS EN 12056-3:2000 specifies the rational method for gravity drainage from buildings. The full calculation is Q = (C × i × A) / 3600 in litres per second, where C is the runoff coefficient (1.0 for non-permeable roofs), i is the rainfall intensity in mm/min, and A is the catchment area in m². The simplified rule of thumb that drops out of the rational method for typical UK rainfall (75 mm/hr design intensity, 4-minute peak duration) is 1 cm² of downpipe cross-section per 1 m² of catchment. The full BS EN 12056-3 calculation is mandatory under Approved Document H for new-build commercial work; residential typically uses the rule of thumb.
Where should downpipes be placed?
Two principles: never longer than 12 metres of single-fall gutter without a downpipe, and place at the ends not the middle on hipped or split-fall front gutters. For a typical 12 m semi-detached front gutter run with a single fall, one downpipe at the low corner is sufficient. For a 15 m+ front gutter on a detached, two downpipes — one at each end — works better than one in the middle (the middle position assumes water flows uphill on the high half). Place downpipes adjacent to the soil stack on the rear elevation when possible; that simplifies discharge to the combined sewer.
Where can downpipes discharge to in the UK?
Three options: combined sewer, separate surface-water sewer, or soakaway. Combined sewer discharge requires Building Regulations Approved Document H Part 3 compliance plus water company approval (Thames Water / Anglian Water / etc. — most charge a fee for new connections). Separate surface-water sewer is the preferred option in newer estates where it's available. Soakaway is mandatory in most newbuild now under Sustainable Drainage Systems (SuDS) guidance — sized at 3 m³ per 100 m² of impermeable area, located at least 5 m from any building and 2.5 m from any boundary, with a permeability test (BRE Digest 365). Discharge to a watercourse requires Environment Agency permission. Direct surface discharge to ground via a soakaway is the cheapest residential option for a self-build outside the SuDS-mandated zones.
How do I handle a two-storey house with no upper-storey gutter?
The upper roof discharges directly onto the lower roof, so the lower-storey downpipes must be sized for the combined catchment (upper + lower roof). For an 80 m² upper roof discharging to a 100 m² lower roof at 35° pitch, the combined effective catchment is (80 + 100) × 1.10 = 198 m². Two 68 mm round downpipes (36 cm² each, drains 36 m² each) would only handle 72 m² — you'd need six. The better fix is to add a small upper-storey gutter that catches the upper roof and routes via a kickout into the lower-storey gutter, distributing the flow. Many older British houses have this gutter design retrofitted because the original Edwardian / Victorian configuration was undersized for modern rainfall intensity.
Are downpipe sizes required by UK building code?
Approved Document H Part 3 (Sanitary Pipework and Drainage) requires rainwater discharge from the building, with sizing referenced to BS EN 12056-3 in commercial work. NHBC Standards Chapter 7.2 (Pitched Roofs) and Chapter 5.3 (Drainage Below Ground) reference BS EN 12056 plus the simplified residential rule of thumb. Building Regulations don't mandate a numerical downpipe size — they require that the drainage system be 'adequate' under expected rainfall, which the BBA Agrément test certificates for FloPlast / Hunter / Marshall Tufflex / Polypipe demonstrate. For Grade II listed buildings and conservation areas, planning consent (not Building Regulations) typically dictates profile and material — cast iron round is the standard expectation.

Related calculators