Calculator Methodology & Sources

The effective area method

Rain does not fall vertically. BS EN 12056-3 assumes it is driven at 26.6° from vertical, which means a pitched roof or a wall intercepts rain across its vertical face as well as its footprint. The standard turns that into one rule:

A = A_H + 0.5 × A_V

A_H is the horizontal (plan) area of the surface. A_V is the area of its vertical projection. The 0.5 coefficient is tan(26.6°), rounded. For a pitched roof, the vertical projection is the plan area multiplied by the tangent of the pitch, so the whole rule collapses to a single multiplier on plan area:

effective area = plan area × (1 + ½ · tan(pitch))

Approved Document H Table 1 is the same rule, pre-computed

Approved Document H Table 1 ("Calculation of drained area") prints three factors for three pitches. They are not a separate method. They are the BS EN 12056-3 formula evaluated at 30°, 45° and 60° and rounded to two decimal places. The right-hand column below is computed live by this page:

Because the two agree exactly, our calculators use the continuous formula and accept any pitch from 0° to 70°. Real roofs are rarely 30°, 45° or 60°, and rounding a 35° roof up to 45° (or down to 30°) discards accuracy for no reason. Above 70° we switch to the elevational area × 0.5 rule, as Table 1 does. BS EN 12056-3 caps the contributing wall height at 10m above the gutter, so measure to that limit.

The design flow load then follows from BS EN 12056-3 as Q = A × r, where r is the design rainfall intensity in litres per second per square metre and A is the effective area. A run-off coefficient of 1.0 is used, which is standard for impermeable UK roof surfaces.

Design rainfall intensity

Approved Document H paragraph 1.1 states: "For eaves, gutters the rainfall intensity should be obtained from Diagram 1." Diagram 1 is titled "Rainfall intensities for design of gutter and rainfall pipes (litres per second per square metre)". It is a contour map covering England and Wales only. Scotland and Northern Ireland are not on it.

Diagram 1 carries six contour values in total, and these are the only intensities our calculators offer:

To convert between units, multiply l/s/m² by 3600 to get mm/hour. So the map's range of 0.012 to 0.022 l/s/m² is 43.2 to 79.2 mm/hour, a spread of nearly two to one across two countries.

The geography is counter-intuitive, and that trips people up

The highest design intensities are not in the wet west. They are in the Midlands, East Anglia and the South East. The lowest value on the entire map, 0.012 l/s/m², sits in a closed contour in mid-Wales, one of the wettest parts of the country by annual rainfall.

That is not an error in the map. It reflects what a gutter is actually sized for. A gutter has to carry a short, intense burst, and short intense bursts in Britain are summer convective thunderstorms, which are most severe in the warmer, drier east and south east. The west gets far more rain in total, but it arrives as prolonged frontal drizzle at a low rate that any competently sized gutter absorbs. Annual rainfall totals are the wrong intuition here, and designing from them will lead you to undersize gutters in Norwich and oversize them in Snowdonia.

Why we default to the worst case

This is the part of our method you should scrutinise most, so we will be plain about it.

We cannot read Diagram 1 accurately enough to give you your local figure automatically. Diagram 1 is a contour map published as artwork. It has no text layer and no usable coordinate reference. There is no lookup table behind it, no gridded dataset published alongside it, and no reliable way to take a postcode, place it on the map, and determine which contour band it falls in. We verified this against the source document directly rather than assuming it.

Some tools guess anyway, by eyeballing the map or by interpolating from a rough regional lookup. We are not willing to do that, because a wrong guess in the wrong direction produces an undersized gutter, and an undersized gutter overtops. So our calculators default to 0.022 l/s/m², the highest value on the map. That choice is safe by construction: 0.022 is the worst case anywhere in England and Wales, so it can never undersize, wherever your building is.

The cost of this is real and we will name it. If your site is genuinely in a 0.014 band, defaulting to 0.022 sizes your gutter for roughly 57% more flow than you need, and may push you up a profile size. That is why every calculator lets you select your own intensity from the six Diagram 1 values. If you have read your figure off Diagram 1 for your location, select it, and you will get the benefit of it. We would rather be transparently conservative by default and precise on request than fake a precision the source document does not support.

It is worth noting that Approved Document H's own Table 2 does exactly the same thing. Table 2's roof areas are built on roughly 0.021 to 0.022 l/s/m²: divide the 150mm row's 2.16 l/s capacity by its 103.0 m² area and you get 0.021. In other words, if you use Table 2 straight from the page without touching Diagram 1 at all, you are already applying a near worst-case intensity. Our default is not more conservative than the Approved Document. It is the same posture, stated out loud.

One consequence of this is that our calculators select on flow rather than on Table 2's area column. Selecting on area would silently lock in the worst-case intensity even for a user who told us their local figure is lower. Selecting on flow means a lower intensity actually earns you a smaller gutter, which is the point of reading the map in the first place.

Numbers you will see elsewhere that are wrong

Two figures circulate widely in UK gutter sizing content, including on merchant and manufacturer sites. Both are wrong for eaves gutters, and both err in the unsafe direction.

75 mm/hour, from a withdrawn standard

The 75 mm/hour design figure comes from BS 6367, which has been withdrawn. HR Wallingford's Report SR 620, the free official UK companion guide to BS EN 12056-3, states that it should no longer be used in the UK. Quoting a withdrawn standard is not a technicality here: 75 mm/hour is 0.0208 l/s/m², which is below the 0.022 that applies across the Midlands, East Anglia and the South East. It is the wrong number and it is low.

0.014 l/s/m², taken from the wrong section of the right document

This one is more insidious, because it is a genuine figure from Approved Document H, quoted accurately, and applied to the wrong thing.

Approved Document H paragraph 3.8 says: "Design rainfall intensities of 0.014 litres/second/m² may be assumed for normal situations. Alternatively the rainfall intensity may be obtained from Diagram 2." Paragraph 3.8 sits in Section 3, Surface Water Drainage. It is about paved areas and underground rainwater drainage. It is not a gutter figure, and it does not point at Diagram 1.

Diagram 2 is a different map, titled "Rainfall intensities for design of drainage from paved areas and underground rainwater drainage (litres per second per square metre)", with different contours: 0.010, 0.012, 0.014 and 0.016 l/s/m². Its range tops out where Diagram 1's range is only two-thirds of the way up. The two maps are not interchangeable and the Approved Document never suggests they are.

The consequence is specific. Across the Midlands, East Anglia and the South East, the correct gutter intensity from Diagram 1 is 0.022. Using 0.014 instead sizes for 36% less flow than required, an undersizing of roughly a third, which is easily enough to drop a roof from a 125mm gutter to a 100mm one. The gutter will look fine for years and then overtop in the first serious summer storm.

For the avoidance of doubt: Approved Document H paragraph 1.1 gives no permitted default for gutters at all. It directs you to Diagram 1 and nowhere else. Any "default" you see quoted for eaves gutters has been imported from somewhere it does not belong.

Two claims we deliberately do not make

You will sometimes see Diagram 1 described as being based on a "1 year return period" or a "2 minute storm duration". Approved Document H is silent on return period and storm duration for Diagram 1. Those figures may well have informed the underlying research, but they are not in the document, so we do not attribute them to it. The only return period stated anywhere in Approved Document H is at paragraph 3.27: "Soakaways should be designed to a return period of once in ten years." That is about soakaways, not gutters.

Approved Document H Table 2, as published

This is Table 2 ("Gutter sizes and outlet sizes") reproduced as printed. It applies to nominal half-round eaves gutters laid level, with a sharp-edged outlet at one end.

The 6.0 m² row prints dashes in the source for gutter, outlet and flow. That means Approved Document H mandates no particular size at or below that area, not that no gutter is needed. We report it as "Not specified" rather than inventing a size for it.

Table 2 carries two caveats that our calculators cannot check for you, because they depend on the depth of flow in your specific gutter:

• Table 2 is valid where the distance from the stop end to the outlet is no more than 50 times the water depth. On long runs, capacity falls below the tabulated figure.
• Where there are two end outlets, they may be up to 100 times the depth of flow apart. A centre outlet roughly doubles capacity compared with a single end outlet, because each half of the run drains a shorter distance.

What these tools do not do

The limits below are not disclaimers bolted on at the end. They are the boundary of what the source documents actually support.

• Eaves gutters only. Approved Document H paragraph 1.2 requires valley gutters, parapet gutters, siphonic systems and flat roof drainage to be designed to BS EN 12056 in full, not by Table 2. We do not cover any of them. The reason is physical, not procedural: when an eaves gutter overtops, water goes over the edge of the building. When a valley or parapet gutter overtops, water goes inside it.
• England and Wales only. Diagram 1 stops at the Scottish border and does not include Northern Ireland. Scotland works to the Scottish Technical Handbooks and Northern Ireland to its Technical Booklets. The effective area method is identical everywhere, but you need a local design intensity, which the BS EN 12056-3 National Annex provides.
• Half-round profiles, laid level, outlet at one end. Those are the conditions Table 2 is published under. Deeper and squarer profiles carry more than a half-round of the same nominal width, and manufacturers publish their own tested capacities. If you are using a proprietary high-capacity profile, use the manufacturer's figure, not ours.
• No postcode lookup. As set out above, Diagram 1 cannot be resolved to a location programmatically. We default to the worst case instead of guessing.
• No check on gutter falls, run length, bracket spacing, thermal movement, overflow provision or downpipe routing below the outlet. These matter and they are outside the calculation.
• Not a substitute for design by a competent person. These calculators are a specification aid to help you order the right parts and sanity-check a design. Building control will not accept a printout from a merchant's website as a design.

Sources

Every figure on this page and in the calculators comes from one of the following. We have not interpolated, averaged or estimated any of them.

• Approved Document H (Drainage and Waste Disposal), Building Regulations, England. Section H3, Rainwater Drainage. Specifically: paragraph 1.1 (gutter intensity from Diagram 1), paragraph 1.2 (valley, parapet, siphonic and flat roof systems to BS EN 12056 in full), Diagram 1 ("Rainfall intensities for design of gutter and rainfall pipes (litres per second per square metre)", contours 0.012 to 0.022 l/s/m², England and Wales), Table 1 ("Calculation of drained area"), Table 2 ("Gutter sizes and outlet sizes"), and, for the numbers we explicitly do not apply to gutters, paragraph 3.8 with Diagram 2 ("Rainfall intensities for design of drainage from paved areas and underground rainwater drainage (litres per second per square metre)", contours 0.010 to 0.016 l/s/m²) and paragraph 3.27 (soakaway return period).
• BS EN 12056-3, Gravity drainage systems inside buildings, Part 3: Roof drainage, layout and calculation. Source of the effective area rule A = A_H + 0.5 × A_V and the 26.6° driven-rain assumption.
• HR Wallingford Report SR 620, the free official UK companion guide to BS EN 12056-3. Source for the position that the withdrawn BS 6367 figure of 75 mm/hour should no longer be used in the UK.
• BS 6367, withdrawn. Listed here only because its 75 mm/hour figure is still widely quoted. Do not design to it.

Diagram 1 and Diagram 2 are different maps

Because this is the single most common error in UK gutter sizing content, it is worth restating in isolation. Diagram 1 is for gutters and rainwater pipes, runs 0.012 to 0.022 l/s/m², and is referenced by paragraph 1.1. Diagram 2 is for paved areas and underground rainwater drainage, runs 0.010 to 0.016 l/s/m², and is referenced by paragraph 3.8. If you are sizing an eaves gutter, Diagram 2 and its 0.014 default have nothing to do with you.

Corrections

If you believe anything on this page misreads a source, we want to know. The figures here were checked against the primary documents rather than against secondary summaries, but that is not a guarantee of infallibility. Get in touch through help and support and cite the paragraph or table you think we have wrong.

Use the calculators

Gutter size calculator applies everything on this page end to end. Roof area calculator works out effective area on its own. How to install guttering covers the fitting side, and all tools lists the rest.