Most people choose guttering by looking at what is already on the house. That works until you build an extension, re-roof a garage, or discover that the old half-round gutter overflows down the wall in every heavy shower. Then the question becomes a practical one: how much water does this roof deliver, and what size gutter and downpipe can carry it away? England's Approved Document H gives a simple method for ordinary domestic roofs, and it is worth understanding even if you end up buying the standard 112 mm gutter off the shelf, because it tells you when that is enough and when it is not.
What the rules actually require
Gutters and rainwater pipes fall under requirement H3 of the Building Regulations in England, which is explained in Approved Document H (Drainage and waste disposal). It says that the flow into a gutter depends on the area of surface being drained, and whether that surface is flat or pitched, and that capacity depends on the size and gradient of the gutters and pipes. Rainwater should not be discharged to a cesspool or septic tank. For eaves gutters, the design rainfall intensity is taken from a map in the document, and for valley gutters, parapet gutters, siphonic systems and drainage from flat roofs, where overflow would have particularly serious consequences, the design should follow BS EN 12056.
New buildings and extensions are checked by building control, but even if you are only replacing a gutter, using the same method is sensible. Wales publishes its own version of the approved documents, and Scotland and Northern Ireland have separate technical standards, so look at the documents for your nation if you are not in England.
Step one: find the effective roof area
Rain falls vertically, so what matters for a pitched roof is the plan area, the footprint seen from above, adjusted for the angle at which the rain is driven onto the slope. Approved Document H gives these multipliers in its Table 1 for calculating the effective design area:
| Type of surface | Effective design area |
|---|---|
| Flat roof | Plan area of the relevant portion |
| Pitched roof at 30 degrees | Plan area x 1.29 |
| Pitched roof at 45 degrees | Plan area x 1.50 |
| Pitched roof at 60 degrees | Plan area x 1.87 |
| Pitched roof over 70 degrees, or any wall | Elevational area x 0.5 |
The plan area is the horizontal projection: the length of the gutter run multiplied by the horizontal distance from the eaves to the ridge (or to the point where water stops draining to that gutter). Do not measure along the slope. If your pitch lies between the figures, you can interpolate, or use the higher figure to be safe.
Worked example (illustrative numbers): a house with a 30 degree pitched roof, where the front slope drains to a gutter 8 m long, and the horizontal distance from eaves to ridge is 3.5 m.
- Plan area = 8 x 3.5 = 28 square metres.
- Effective area = 28 x 1.29 = 36.1 square metres.
Step two: choose the gutter size
Approved Document H's Table 2 lists the largest effective roof area that should drain into each of the commonly used gutter sizes, along with the smallest outlet to use. The values apply to nominal half-round eaves gutters laid level, with a sharp-edged outlet at one end only.
| Maximum effective roof area (square metres) | Gutter size (mm) | Outlet size (mm) | Flow capacity (litres per second) |
|---|---|---|---|
| 18 | 75 | 50 | 0.38 |
| 37 | 100 | 63 | 0.78 |
| 53 | 115 | 63 | 1.11 |
| 65 | 125 | 75 | 1.37 |
| 103 | 150 | 89 | 2.16 |
The flow column works out at about 0.021 litres per second for each square metre of roof, the figure that corresponds to a heavy rainstorm of roughly 75 millimetres an hour, which is a figure that many manufacturers also quote. The exact design figure for your location comes from the rainfall map in the document, so a very wet or exposed site, or one where the consequences of overflow would be severe, may justify going a size up.
Applying the example: 36.1 square metres of effective area is under 37, so a 100 mm half-round gutter with an outlet of at least 63 mm would just manage it. That is too close to the limit to be comfortable, because any extra roof area, such as a dormer or a valley feeding into the run, would push it over. A 115 mm gutter, or splitting the run with a second downpipe at the other end, gives better margin. Many UK houses use a 112 mm half-round PVC-U gutter, which sits between those sizes, so check the manufacturer's capacity table for your actual product.
Where the outlet sits matters
The table figures assume a single outlet at one end of the gutter, with the distance from the stop end to the outlet no more than 50 times the depth of the water. For greater distances, the capacity should be reduced. Where the outlet is not at the end, the gutter should be sized for the larger of the two areas draining towards it. If there are two end outlets, they may be up to 100 times the depth of flow apart. In practice, this means that placing the downpipe in the middle of a long run lets each half carry only its share, and often permits a smaller gutter than an end outlet does. Many manufacturers publish tables for centre outlets, with higher permitted areas.
Gutters can be sized down if they are laid with a fall towards the outlet, have a section with greater capacity than a half-round profile, or use round-edged outlets, but only where the manufacturer's data or BS EN 12056 supports it. Do not assume a smaller gutter is acceptable just because it has a slight fall.
Step three: downpipes
The rainwater pipe must be at least as large as the outlet from the gutter. A downpipe serving more than one gutter should have an area at least as large as the largest of the contributing outlets, and be large enough for the flow from the whole contributing area. Domestic downpipes are commonly 68 mm round or 65 mm square, which cover the 63 mm outlet in the table. Larger roofs, valleys and flat roof outlets can need bigger pipes.
- Number of downpipes. Divide the effective area by the maximum area the chosen pipe can serve, and round up. Then place them to share the roof evenly, which also shortens each gutter run.
- Shoes and distributors. When a downpipe discharges onto a lower roof or paved area, a pipe shoe should divert the water away from the building. Where rainwater from a roof with an effective area greater than 25 square metres drains through a single downpipe onto a lower roof, a distributor pipe should be fitted to the shoe so that the flow does not over-top the receiving gutter.
- Combined drains. A rainwater pipe that discharges into a combined foul and surface water system should do so through a trap.
Falls and fixings
The Approved Document asks for gutters to be laid with any fall towards the nearest outlet. A gentle gradient is usual, and manufacturers' instructions normally specify around 1 in 600 (3 mm in 1.8 metres, for example) as a standard for PVC-U systems. Do not exaggerate the fall; a gutter that appears to slope visibly looks poor, and steep falls cut the effective depth at the stop end. Support brackets at spacing set by the manufacturer, typically around 1 metre or less, prevent sagging, and extra brackets near joints and outlets avoid leaks. Use the proper union brackets and seals, and set the gutter so the front edge is slightly lower than the back so that overflow in a storm spills forward and away from the wall, not behind it. The Approved Document also says gutters should be laid so that any overflow beyond design capacity is discharged clear of the building.
Where does the water go?
The sequence the Building Regulations expect for disposal of rainwater is: first an adequate soakaway or other infiltration system; where that is not reasonably practicable, a watercourse; and where neither is practicable, a sewer. In practice, existing houses often connect to a combined or surface water drain. For a new extension, check with your water company before connecting anywhere new, particularly if the route crosses a public sewer, and see local authority requirements on sustainable drainage. Water butts are an easy partial solution for sheds and garages, with an overflow connected to the drain or a soakaway.
If you pave an area near the house, think about the extra runoff. Our patio paving and sub-base calculator helps with the construction, but it is worth making sure the paving falls to a lawn or border and does not send water back to the foundations. For a quick calculation of roof area, gutter and downpipe size, use the domestic rainwater gutter and drainage sizer. Our guide to which projects need building regulations approval shows when drainage work triggers building control.
Flat roofs, valleys and extensions
Flat roofs drain by plan area with no pitch multiplier, but ponding, upstands and the position of outlets matter, and the rules for parapet or box gutters point to BS EN 12056 because an overflow into the building would be serious. An extension that drains onto an existing roof adds to the load on that roof's gutter, so recalculate the old roof as well. A valley gathers water from both slopes, so treat its effective area as the sum of the two portions that feed it.
Maintenance and common problems
- Overflowing at a corner. Often the cause is a blocked outlet or the corner being too far from the downpipe, rather than the wrong size.
- Dripping joints. Check the seals, clean the joint surfaces, and replace perished rubber rather than caulking over it.
- Sagging runs. Add brackets and reset the fall.
- Leaf build-up. Clean at least once a year, ideally in autumn, using a ladder safely or a long-handled tool from the ground. Gutter guards help with leaves but do not increase capacity.
Work at height is the main risk of any gutter job. Use a stable ladder with the correct angle and a stand-off, or a scaffold tower, and consider a professional for anything above first-floor level. This is general information, not professional advice; building control or the gutter manufacturer's technical team can confirm sizes for a specific roof.