Rainwater: Effective Roof Area, Gutter Sizing and Outlet Positions

A customer wants the guttering replaced on a rear extension. You measure the roof: 8 m along the eaves, 3 m to the ridge on plan, pitched at 30°. That is 24 m² on the tape measure. It is not 24 m² of catchment.

The short answer

Rain does not fall straight down. Wind drives it at an angle, so a pitched roof presents more surface to the rain than its plan area suggests — and a wall standing above a roof sheds its own water onto that roof.

The figure a gutter is sized for is the effective roof area: the plan area corrected for pitch, with any contributing wall added. That is why an online calculator asks for the angle of the roof as well as its size.

And there are two accepted methods — Approved Document H Table 1, or BS EN 12056-3. Never a rafter-length method, and never both multiplied together. Pick one, state which one you used, and stay with it.

The two methods

Method 1 — Approved Document H Table 1. Multiply the plan area by a factor for the pitch.

Method 2 — BS EN 12056-3. The UK National Annex assumes rain is wind driven at 26° to the vertical, that is 2 in 1, so:

A = L_R × (B_R + H_R ÷ 2)

where L_R is the length of roof to be drained, B_R the plan width from gutter to ridge, and H_R the height from gutter to ridge. In plain terms: a sloping roof collects its plan area plus half its elevation area. Each gutter is designed for the wind direction that gives it the maximum flow.

Two worked examples

A sloping roof 10 m long, plan width 6 m, rise 4 m:
A = 10 × (6 + 4 ÷ 2) = 10 × 8 = 80 m²

The extension, with a wall above it. 8 m along the eaves, 3 m plan width, pitched at 30°, with the house wall rising 2.5 m for the same 8 m:

• Plan area of roof = 8 × 3 = 24 m²
• Pitch factor 1.29, so roof effective area = 31 m²
• Wall elevational area = 8 × 2.5 = 20 m²; contribution = 20 × 0.5 = 10 m²
• Total effective area = 41 m²

Nearly double the tape measure figure.

BS EN 12056-3 adds one limit to the wall rule: a single wall contributes 50 per cent of its area up to a maximum exposed height of 10 m. Where two walls form an angle or a bay, assume the wind direction presenting the greatest vertical area.

And not every roof needs a gutter: gutters and rainwater pipes may be omitted from a roof of not more than 6 m², at any height, provided no other area drains onto it. That is the reason for the blank row at the top of the gutter table. A complex roof is dealt with by calculating each portion separately and adding the results.

Sizing the gutter

BS EN 12056-3 puts the flow into one equation:

Q = r × A × C

where r is the rainfall intensity in l/s per m², A the effective roof area, and C a runoff coefficient taken as 1.0. Rainfall intensity is a local figure; a return period of one year is the minimum for eaves gutters and flat roofs, and for underground surface water drainage Approved Document H uses a design intensity of 0.014 l/s per m². Where no statistical data exists, minimum intensities run from 0.010 to 0.060 l/s per m², multiplied by a risk factor:

For ordinary domestic eaves gutters you do not have to work Q out by hand. Approved Document H3 Table 2 lists the largest effective roof area that may drain into each common gutter size:

Read down the first column to the first area equal to or greater than yours. The chief factor that decides the choice of guttering is the area of the roof — not the colour of the tiles, the age of the building or the material of the fascia.

So for the 41 m² extension: 41 is more than 37.0, so the 100 mm row is too small. The next row up is 53.0 — a 115 mm gutter with a 63 mm outlet, capacity 1.11 l/s. In practice you fit the standard domestic round system: a 112 mm half round gutter with a 68 mm downpipe. A 65 mm pipe is the square section equivalent, and 110 mm is soil pipe, not rainwater.

Remember what the table assumes: a gutter laid level, half round in section, with a sharp edged outlet at one end only, and the distance from stop end to outlet not more than 50 times the water depth. Where the outlet is not at the end, size the gutter for the larger of the two areas draining into it; with two end outlets they may be up to 100 times the depth of flow apart. A fall towards the outlet, a deeper section, or a round edged outlet can all allow a smaller size than the table gives.

The rainwater pipe should be at least the size of the outlet from the gutter. A downpipe serving more than one gutter must have an area at least as large as the largest contributing outlet. At a filling degree of 0.33, a 65 mm internal bore carries 3.4 l/s and a 70 mm bore 4.1 l/s — far more than the 1.11 l/s the gutter above can deliver. That is the usual outcome: the capacity of a rainwater system is normally set by the gutter or its outlet, not by the downpipe.

Long runs and outlet positions

Five terraced houses share 40 m of continuous gutter with a downpipe at each end. Number 3 overflows in any real storm. Nothing is blocked — the run is simply too long for two outlets.

A gutter is only as good as the head of water that can build behind its outlet. It is hydraulically short if its length is not greater than 50 times the design depth of water; beyond that, the design capacity has to be reduced by a capacity factor.

For the terrace, the answer is not a bigger gutter. It is more outlets.

• A stop end outlet takes everything from one direction only.
• A running outlet, part way along a run, receives water from both directions, so it can drain almost double the area of an end outlet.

So a centre outlet is more efficient than two end outlets, and a run split into quarters by three outlets leaves each one handling a quarter of the flow. On a terrace, one downpipe on each party wall line is the usual and neatest answer.

To decide how many outlets you need, divide the expected flow rate from the roof by the flow rate of one outlet, using the manufacturer's figure. The gutter is laid with any fall towards the nearest outlet, and outlets should not be located close to a change in direction.

Several details quietly take capacity away:

• A gutter laid at a nominal gradient of 3 mm/m or less is designed as level. In practice a fall of about 1 to 3 mm per metre, roughly 1:600, is good workmanship.
• Support brackets at maximum 1 m intervals, so the run holds its line when full.
• Where a gutter contains one or more angles greater than 10°, the design capacity is multiplied by 0.85. Angles should be avoided close to an outlet.
• Where an outlet from a non-flat soled gutter has a strainer or leaf guard, the capacity is multiplied by 0.5. A balloon grating halves the design.

Gutters must also be laid so that any overflow above the design capacity discharges clear of the building. Where a downpipe discharges onto a lower roof or a paved area, fit a pipe shoe; and where a roof of more than 25 m² effective area discharges through a single downpipe onto a lower roof, add a distributor pipe so the flow spreads and does not over-top the receiving gutter.

Keeping it working

A rainwater system needs regular checking and cleaning, because it can get blocked with leaves. A blocked gutter overflows down the wall and into the fabric of the building.

How much maintenance a system needs depends on trees close to the building, the material the gutter is made from, and the age of the installation. It does not depend on the height of the gutter — height changes how you reach it, not what it needs.

Profiles matter to the customer as well as the calculation: half round is the domestic standard; ogee the ornamental Victorian pattern often required on period terraces; square section pairs with square downpipes; high capacity profiles suit larger and steeper roofs.

Where rainwater is harvested, BS 8515 applies. Storage is the lesser of 5 per cent of the annual rainwater yield or 5 per cent of the annual non-potable demand, about 18 days of supply. Yield is the collection area times the rainfall depth, reduced by a yield coefficient of about 0.7 to 0.8 and a filter efficiency of about 0.9; demand is about 50 litres per person per day. Harvested rainwater pipework must be marked and labelled so it can never be confused with wholesome water.