A gutter only collects water. The downpipe is the part that gets rid of it, and a gutter system is only as good as its slowest outlet. A long run with a single downpipe at one end can look fine in a drizzle and overflow in a storm, because the pipe cannot pass the water as fast as the roof delivers it. How big, how many and where each pipe goes are therefore design decisions, made before a single bracket goes up. Below the pipe, the water meets a gully, a drain or a soakaway, and a blockage there backs up the whole system. This guide explains how downpipes are chosen and placed, what the connection at the foot involves, and why they block. It covers cast iron and plastic, terraces and detached houses. The gutters that feed the pipe are dealt with from the main gutters hub.
What a downpipe actually has to do
Think of the roof as a catchment. Rain landing on a slope runs to the eaves, drops into the gutter and flows along it to an outlet, where it falls through a stub into the downpipe. The pipe carries it to ground level. At each stage the rate is set by the narrowest point, and the narrowest point is nearly always the outlet and the pipe beneath it, not the gutter itself.
So judge a downpipe by what it drains, not by how it looks. The relevant measure is the area of roof that feeds each outlet. Add a second outlet to a long run and each pipe takes half the load. Move an outlet from the end of a gutter to the middle and the water travels half the distance. Neither is a cosmetic change.
Wind adds to the load. Rain driven against a wall or across a slope in a gale arrives faster and in different places than a vertical shower, and a roof with a tall wall behind it sends extra water down onto the slope below. A valley or a roof meeting a higher wall concentrates flow into a small stretch of gutter, which is why those spots overflow first.
A downpipe also has to stay attached. Full of water it is heavy, and in a freeze it can split. Brackets hold it off the wall, a short distance out, and the fixings have to bite into sound masonry or timber. Pipes that have worked loose and lean away from the wall leak at the sockets and let water behind them.
So the questions to ask of any system are straightforward. How much roof drains to each pipe? Is the pipe big enough for that, and does the outlet match? Does the foot discharge somewhere sensible? Everything below follows from those three.
How downpipes are sized
Sizing balances four things: the roof area an outlet serves, the gutter's capacity, the outlet and pipe diameter, and the exposure of the site. Manufacturers publish tables that match gutter profile to roof area and show how many outlets a run needs. They are the right reference for a given product, because capacity differs between a half-round gutter and a deep box section, and between a small domestic profile and a large one.
For most houses the common domestic sizes cope, but it is not a reason to assume. A large single-storey extension roof draining to a short length of small gutter and a single small pipe is a classic weak point. So is a roof that has been extended or altered so that more area now drains to a gutter that was sized for less. A new dormer, a loft conversion or a rear extension can all change the catchment.
The shape of the roof helps determine pipe count. A simple gable has two eaves lines and usually a downpipe at each end of the front and back runs, or at one end if the gutter is short. A hipped roof has a continuous eaves and may need pipes at the corners. A terrace shares eaves with its neighbours and often drains into a single pipe at the party wall, or into a hopper head that serves two houses.
Fall is part of the answer. Gutters are laid with a slight fall towards the outlet, and a FloPlast installation guide for its small gutter range gives about 1 in 350. A gutter laid dead level holds water, and one laid with too much fall can send water past the outlet at the end of the run in heavy rain. The figure differs by system, so use the maker's instructions rather than a rule of thumb.
When one pipe is not enough
A long run or a large roof section may need a second outlet. Tell-tale signs are a gutter lip that brims at one spot in every storm, a downpipe that is full to the brim at the top, or staining on the wall below an outlet. Adding a pipe is usually cheaper than replacing the gutter, and it can fix a problem that cleaning will not.
Where downpipes go: position and appearance
Position affects drainage, appearance and access. Hydraulically, the best place for an outlet is where water collects: the low point of the fall, or the middle of a long run. Practically, the pipe also has to find a route down the wall, past windows, doors, string courses and cornices, to a place where the foot can drain.
On an older house the pipes are part of the composition of the façade. A pair of pipes at each end of a symmetrical front, or a single pipe at the party wall between terrace houses, is a deliberate choice. Moving them changes how the building reads. Where the house is listed, moving or replacing a pipe may need consent, and our page on gutters and downpipes on listed homes explains how the council weighs changes to these parts of a listed elevation.
Access matters in two directions. You must be able to reach the pipe to clear or replace it, and a pipe should not run behind a fixed feature that blocks a ladder. Behind a climbing plant, a pipe is hidden but the leaf litter finds its way into joints. Beside a boundary, it may be on the neighbour's side of the line.
Offsets and bends
Where a pipe has to step round an eaves projection, it uses an offset, sometimes called a swan neck, which is two bends joined by a short length. Every bend slows flow and catches debris, so keep them to a minimum and make sure the short length between the bends falls steeply enough to clear. A tight offset with a flat middle is one of the most frequent blockage points we find on pitched roofs with deep eaves.
Hopper heads, common on Georgian and Victorian houses, are open boxes at the top of a pipe that receive water from a gutter or a parapet outlet. They also act as a visible overflow if the pipe backs up. They are a feature of the building, and, where original, worth keeping.
From the foot of the pipe to the drain
What happens at ground level is where many problems begin. Roof water can go to three places. It can enter a combined drain that carries foul and surface water together, a separate surface water drain or sewer, or a soakaway, which lets it filter into the ground on the property. Which one applies depends on how your house was built and drained, and there is no way to guess it from the pipe.
The foot of the downpipe usually discharges into a gully, a trapped grating at ground level set into a short drain. The trap holds a seal of water that blocks smells from the drain. A leaf-clogged gully, or a broken one, spills water onto the ground beside the wall and can leave it standing against the base. If a drain is partly blocked, the symptom is often an overflowing gully, not a blocked pipe.
Some downpipes discharge onto a splash block or a hard surface and let the water run away across the ground. That works in an open garden with a fall away from the house. It is a poor arrangement against a wall, on a slope towards the building, or on a terrace where water crosses a neighbour's yard.
If you are changing where roof water goes, for instance connecting a new pipe to a drain, adding a soakaway or diverting a pipe, ask your building control body first. Rules about drainage and connections are separate from the rules about the roof, and an informal arrangement can cause trouble if it is found later. A downpipe that feeds a water butt needs an overflow route back to the drain, as set out in our guide to rainwater harvesting from your gutters.
On a shared terrace, the drain beyond the pipe often serves several houses. A blockage can show on your side even when the cause lies next door, which is a question of who owns the length of drain, and not simply a question for a roofer.
Why downpipes block, and what to do
Debris is the usual cause. Leaves, moss, grit that washes off the roof covering, nesting material and the occasional ball all drift to the outlet, where the pipe narrows. A grating or cage over the outlet catches the larger items, but it also holds them, so it needs clearing. Older pipes build a coating of sludge and rust scale inside, which narrows them too.
Frost can block a pipe in a different way. Water standing in a bend or at the foot freezes and forms a plug, and the pipe above fills and overflows. If the ice splits a plastic section or a cast iron socket, the leak shows later, as damp patches on the wall.
Cast iron has its own pattern. Over many years the inside rusts, flakes build up and eventually fall to the bend at the foot, where they restrict flow. The outside tends to rot at the sockets, where water has lain, and the pipe can fail from the inside out without showing much on the surface. A tap with a screwdriver handle along the pipe, listening for a dull sound, is a reasonable first check, although a roofer will tell more from a close look.
To find a blockage, start at the top. Clear the gutter and outlet, then run a hose into the pipe and watch the foot. If the water arrives slowly or not at all, the blockage is somewhere between. If the water backs up at the gully, the fault is in the drain. Where a pipe cannot be cleared by flushing, the sections can be taken apart and rodded, or replaced. Our page on the blocked downpipe goes through those steps in more detail, and the longer-term damage a neglected run causes is covered in blocked gutter damage.
Cleaning at least once a year is the sensible baseline, and more often if trees overhang the roof. A gutter guard cuts the volume of leaves, though it still wants an occasional look.
Repair, replace and what a survey records
Many downpipe faults are minor. A loose bracket can be refixed, a perished seal at a plastic socket can be renewed, and a cracked section can be swapped for a matching length. A downpipe that has lost its line because the fascia or wall behind has failed is a more involved job, and the sagging gutter above it usually has the same cause. How to tell is set out under sagging gutter.
Replacement is worth considering when a cast iron pipe is rusted through at several sockets, when a plastic pipe has gone brittle and cracked from sun and frost, or when the whole system is undersized for the roof. New pipes in the same material are simplest to agree on an older house, and our guide to soffit vents shows the other parts of the roofline that often get dealt with at the same time.
Hiring access towers or scaffold takes a real share of the cost on a pipe that runs the full height of a three-storey Bath house. MyBuilder's gutter, fascia and soffit guide (2026) reports typical UK guttering at £30 to £70 per metre and scaffold adding £20 to £40 per square metre. That is a guide to how the cost behaves, and the figure for your house is set after we have measured it.
When we survey downpipes we note how many outlets each pipe serves, the condition of each socket and bracket, what the foot discharges to, and whether any run crosses a boundary. We photograph the pipes on your own house, because a clear picture of a rusted socket or a leaning pipe saves a long conversation. The written fixed price that follows itemises the repair or replacement, line by line.