What Size Guttering Do I Need?

The standard sizes

Domestic guttering is sold in a few common sizes. The usual half-round gutter is about 112 to 115 mm across the top, paired with a round downpipe of about 68 mm or a square pipe of about 65 mm. Larger half-round and square gutters, often called deep flow or high capacity, are around 125 to 150 mm wide with a larger pipe. Ogee and square gutters come in comparable widths. The actual sizes vary slightly between makers, and parts from different systems often do not fit each other.

The standard size works well on the typical semi-detached or terraced house, where each run is of moderate length and drains a modest roof area. It is the size you will find on most existing houses, and replacing like for like is usually fine if the old system coped. If it overflowed in storms before, copying it will repeat the problem. Our page on why gutters overflow helps you decide whether size is the cause.

What affects the size you need

Several factors push the answer up or down:

• Roof area: the more roof draining into one run, the more water arrives in a shower. A large pitched roof sends far more water to its eaves than a small porch.
• Roof pitch and surface: a steep, smooth roof sheds water fast, so a surge arrives at once. A flat roof draining into a gutter, or a conservatory roof, adds a load at a single point.
• Length of run: a long run with a single outlet at one end has to carry water a long way. Adding an outlet in the middle, or at both ends, halves the distance.
• Number and size of downpipes: one pipe cannot discharge more than it can carry, however large the gutter.
• Exposure: houses on a ridge or in a gap between hills see harder, wind-driven rain.

Roofs that gain water from another roof above, such as a dormer or an extension sloping onto a main roof, also need extra capacity at that run.

Fall, outlets and fixing

Size is only part of it. A gutter needs a slight fall towards the outlet, so water keeps moving. A flat gutter holds standing water that fills with silt, and a steep one makes the end overshoot. Brackets need to be spaced evenly, with no long unsupported spans, because a full gutter is heavy. Outlets should sit where the water is, not at the far end of a long run.

If your old system sags or overflows at one point, the fault may be the fall or the brackets, not the size. Our page on why gutters sag covers bracket spacing and fall. For profile choice, see square, half-round or ogee guttering, which explains how the shape affects capacity.

Downpipe position is worth planning. A downpipe set in the middle of a long run lets both halves slope towards it, and the water travels half the distance. A pipe at the end of a long run forces the whole flow along one length. Where a downpipe has to land in an awkward place, such as over a conservatory or next to a door, an offset or a hopper head can move it without cutting capacity. Each bend slows the flow, so keep them to the minimum.

Old houses sometimes have gutters that look small by modern standards but have worked for a century, because the roofs were small and the pitch steep. A change in the roof, such as an extension that adds a large slope draining onto an old run, can overload what was fine before. That is the usual story behind a gutter that began to overflow after building work.

How to decide for your house

Measure nothing yourself. A roofer works out the plan area of roof draining into each run, looks at the pitch and the exposure, and picks the profile, size and outlet positions to suit. If a standard system is enough, they say so. If your run is long, your roof large or your extension adds water, they propose a deep-flow gutter or an additional downpipe. Specifying properly at the start costs little beside redoing a run that overflows.

Materials affect the choice as well: the comparison of uPVC, aluminium and cast iron compares what each offers in size and strength. For budget, see gutter replacement cost.

When we survey, we photograph each run, note the roof area and the outlets and tell you if the existing size is right. You receive fixed written figures. The guttering, soffits and fascias page describes the service in full.