Downpipes: Round vs Square, Swan-Neck Offsets, and Where the Water Goes

A downpipe that stops a foot above the ground and dribbles against the brickwork looks finished from the street, but the wall behind it stays damp every time it rains. Within a couple of winters the bricks are spalling, the plaster inside is tide-marked, and the cause is a cheap plastic shoe nobody bothered to fit. The downpipe is the cheapest part of the roofline, and the part most likely to be done carelessly. Getting it right is mostly about two things: bringing the pipe back to the wall cleanly under the eaves, and making sure the water it carries actually reaches a drain.

What it is and what it's for

A downpipe is the vertical pipe that takes the water collected by the guttering and carries it down to drainage at ground level. The gutter runs along the eaves and collects everything that sheds off the roof. At one or both ends, or somewhere along the run, there's an outlet: a hole in the bottom of the gutter where the water drops out. The downpipe bolts onto that outlet and takes the water down the wall to a gully, a soakaway, or a surface-water drain.

You'll also hear it called a rainwater pipe, a drainpipe, a fall pipe, or a "down pipe" as two words. They all mean the same thing. On a single-storey extension you might have one downpipe; on a larger roof you could have two or three, one per gutter outlet.

The pipe itself is uPVC (the same rigid plastic the guttering is made from), and it comes in 2.5m lengths that push together with socketed joints. It runs down the wall held off the brickwork by pipe clips, and at the top it has to dog-leg back to the wall because the gutter sits out at the edge of the eaves while the wall is set back underneath. That dog-leg is the swan neck, and it's where most of the thought goes.

How many downpipes you need

The number of downpipes is not a free choice. It follows from where the gutter outlets sit, and the outlets follow from the roof area the gutter has to drain. A gutter can only carry so much water before it overtops, and the further the water has to travel along the gutter to reach an outlet, the more the gutter has to hold at any moment. So a long gutter run, or one draining a large pitched roof, needs more than one outlet, and every outlet needs its own downpipe.

A rough working rule for a domestic half-round or square-line gutter is that one 68mm or 65mm downpipe drains the roof area served by a single outlet, and a single outlet placed at one end of the gutter serves a shorter run than an outlet placed in the middle, because a centre outlet has water arriving from both directions. On a small single-storey extension with a modest roof, one downpipe at one end is usually enough. On a wider rear extension, or where the gutter feeds a long uninterrupted run, the gutter is split with two outlets and two downpipes so neither half is overloaded in a heavy downpour.

If your roof area is large and you are stuck with a single downpipe position, the answer is not a bigger gutter, it is a second outlet and a second downpipe, or stepping up to a deeper high-capacity gutter profile. A single 68mm pipe trying to clear a large roof in a cloudburst will let the gutter overtop and pour down the wall, which defeats the point of the whole system. Where the roof feeds one tight position only, some schemes use a 110mm soil-grade pipe as an oversized downpipe to shift the volume, but for most extensions the cleaner answer is to add a second outlet.

Round, square, and matching the house

There are two profiles in common use, and the choice is almost entirely about appearance and matching what's already there.

Round is the traditional profile, 68mm diameter being the domestic standard. It pairs with half-round gutter, the curved gutter profile found on most older and standard houses.

Square (strictly, rectangular) is 65mm and pairs with square-line gutter, the flat-fronted profile common on houses from the 1980s onward. Square downpipe sits flatter against the wall and some people prefer the look on a modern extension.

In capacity terms the two are close enough that on a domestic roof the choice is about looks, not flow. A 68mm round pipe and a 65mm square pipe each handle the run from a single standard outlet comfortably; neither has a meaningful advantage in everyday rain. The decision is made for you by the gutter profile already on the house, because the downpipe has to connect to the gutter outlet without an adaptor.

The rule that matters: the downpipe profile and colour must match the guttering, and ideally match the rest of the house. A round downpipe will not connect to a square gutter outlet without an adaptor, and a white pipe next to anthracite guttering looks like a repair, not a finish.

Anthracite grey and black have become the standard choice on new extensions, and they carry a price premium over white of roughly 40 to 70 per cent. Buy the pipe, the bends, the clips, and the shoe all in one colour and one brand so they fit together and match. Mixing brands risks joints that don't seat properly.

The swan neck: why the eaves overhang sets the offset

The gutter sits at the outer edge of the roof, beyond the face of the wall. The eaves overhang (the distance the roof and gutter project past the wall) is typically anywhere from 50mm on a tight modern detail to 300mm or more on a generous traditional eaves. The downpipe needs to run flat against the wall, so it has to step back from the gutter outlet to the wall face before it drops. That step is made with two offset bends connected by a short length of pipe, and the whole assembly is the swan neck.

The geometry is worth understanding because it decides what you buy. The swan neck is two bends working in opposite directions: the top bend leans the pipe in towards the wall, the bottom bend straightens it back to vertical, and a short piece of pipe between them bridges the horizontal distance, which is the overhang less a little for clearance behind the pipe. uPVC offset bends come in fixed angles, commonly 112.5 degrees (a shallow bend, 22.5 degrees off straight), and the shallower the bend angle the longer the connecting pipe needs to be to cover a given overhang. There are two ways to build the offset. A pre-formed offset is a single moulded piece giving a fixed projection, quick to fit but only right when the overhang matches it. A made-up offset is two loose bends plus a cut length of pipe, which the fitter sizes on the wall to the exact overhang.

To measure the overhang, drop a plumb line or a long spirit level from the gutter outlet to the wall and read the horizontal gap from the wall face to the centre of the outlet. That gap, less around 25mm to 40mm so the finished pipe clears the brick on its clips, is the projection the swan neck has to make. A tight 60mm overhang takes a single pre-formed offset; a 250mm overhang needs two bends and a cut connecting pipe of a couple of hundred millimetres to reach the wall.

Pipe clips and fixing spacing

The downpipe is held to the wall by pipe clips (also called brackets or pipe brackets), each one screwed into the brickwork with a plug and screw. The clip holds the pipe a small distance off the wall, the standoff, so air can circulate behind it and the pipe doesn't stain the brick.

That standoff is not just for looks. A pipe held tight against the wall traps a film of damp behind it, the wall never dries where the pipe touches, and over time a dark stain and then green growth tracks down the brickwork behind the run. The small gap a proper clip leaves keeps the pipe off the masonry and lets the wall breathe.

Spacing matters too. There should be a clip near the top of the run, a clip near the bottom, and a clip at every socket joint between pipe lengths, plus intermediate clips so that no clip is more than about 1.8m to 2m apart on a vertical run. uPVC expands and contracts with temperature, and a 2.5m pipe can move several millimetres between a frosty morning and a hot afternoon, so the clips also need to allow the pipe to move slightly rather than gripping it rigidly. Most clip designs handle this by leaving the socket end free to slide, which is why you fix the clip near the bend or fitting and let the plain end float in the socket below.

Where the base discharges

The bottom of the downpipe has to deliver its water into the drainage system, not just spill it onto the ground. There are several standard arrangements, and which one you use depends on what drainage runs past the foot of the wall.

Into an open gully. The most common on older houses. The pipe terminates in a shoe (a curved fitting that directs the flow forward) over an open gully, which is a small grated chamber that takes the water into the underground drain. The grating keeps leaves out and the trap below blocks smells.

Into a back-inlet gully. A back-inlet gully has a sealed socket on its side so the downpipe connects straight into it below the grating, instead of discharging over an open top. This keeps the connection neat and stops the pipe blowing water past the gully in wind. The gully still has a grated rodding eye on top for clearing blockages.

Into a bottle gully. A bottle gully is a compact trapped chamber, named for its shape, that connects the downpipe straight into the underground surface-water drain while blocking smells and debris. It is common on modern extensions where the pipe needs a sealed connection and the access cover sits flush in the paving.

Into a soakaway. Where there's no surface-water sewer to connect to, the downpipe feeds an underground soakaway: a pit filled with rubble or, more usually now, plastic crates, that lets the water disperse slowly into the surrounding ground.

Direct to a surface-water drain. Where the connection is sealed below ground, the pipe connects straight into the surface-water system through an adaptor, without an open gully at all.

To a water butt. A rainwater diverter fitted into the downpipe sends water sideways into a butt and lets the overflow carry on down the pipe once the butt is full. This is a supplement to a proper base discharge, not a replacement for one.

Foul versus surface water, and combined systems

Building Regulations Part H sets the order of preference for where roof water goes, and it is worth knowing because it decides the whole base detail. The order is: a soakaway or other infiltration first, then a surface-water sewer or watercourse, and only as a last resort a combined sewer. The reason for the preference is volume. Foul sewers and treatment works are sized for the flow from sinks and toilets, and dumping the rain off a roof into them in a storm is exactly what causes drains to back up and combined sewers to overflow.

Some older areas have a combined system, where a single sewer takes both foul and surface water, and in those areas your downpipe may legitimately end up in the same drain as the foul. That is the exception, set by what the street was built with, not a free choice. On a new extension the inspector will want to see that roof water goes to a soakaway or surface-water drain wherever the ground and the existing drainage allow it. Get this wrong and it shows up at the building control inspection of the below-ground drainage, not after the roofline is finished.

Rodding and access at the base

At the base, a shoe or an access fitting (a shoe with a removable cover) lets you rod the pipe if it blocks. Leaves and grit wash down the downpipe and collect in the gully or the bend at the bottom, and an access point means a blockage can be cleared without digging. On a new extension it's worth specifying an access shoe rather than a plain one for exactly this reason. The gully itself should also have a rodding eye or removable grating so the underground run can be cleared from the surface.

Connecting to the underground drainage

The handover between the roofline and the below-ground drains is where the roofer's work meets the groundworker's. The downpipe is uPVC rainwater-goods pipe; the underground drain is heavier-walled drainage pipe in a different diameter, usually 110mm. They do not push together directly. A rainwater adaptor or a back-inlet gully bridges the two: the downpipe drops into the adaptor or gully socket, and the gully connects on to the 110mm underground run. Agree this junction with the groundworker early, because the gully position has to be set when the drains are laid, long before the downpipe goes up. A downpipe finished before anyone has thought about where its gully sits is the usual reason a pipe ends up discharging onto the ground.

How much do you need

Working out a downpipe order is straightforward. Measure the height from the gutter outlet to the ground, that gives you the run length. Then add the fittings.

For a typical single-storey extension with a 2.6m eaves height:

• One or two lengths of 2.5m pipe (a single length usually covers a single-storey drop once the swan neck takes up the top)
• Two offset bends (or one pre-formed swan-neck offset) sized to the eaves overhang
• A connecting piece of pipe for the swan neck if the overhang is deep
• Three pipe clips minimum (top, middle, bottom)
• One shoe or access fitting at the base
• A gutter outlet at the top if not already part of the gutter run

Add a length of pipe over your measurement for cutting waste, and buy a spare clip. The whole lot for one single-storey downpipe is a small materials cost, so the temptation to skimp on a clip or skip the shoe makes no sense.

A worked example makes the order concrete. A 4m wide rear extension with the gutter draining to one end has a single outlet, a 2.6m eaves and a 150mm overhang. That needs one 2.5m length of 65mm square pipe, a made-up swan neck of two offset bends plus a short cut connecting piece to cover the 150mm, three clips, and an access shoe over a back-inlet gully that the groundworker has already set in the path below. If the same extension were 6m wide with a long gutter run, you would split the gutter with two outlets and order the whole list twice, one downpipe per outlet.

Soakaway sizing in brief

Sizing a soakaway is the groundworker's or engineer's job, but the principle is simple enough to sanity-check. The pit has to hold the water from a heavy storm long enough for it to soak away, so its size depends on the roof area draining into it and how fast the ground absorbs water. That absorption rate is found by a percolation test: dig a test hole, fill it with water, and time how long the level takes to fall. Free-draining sandy ground takes a small soakaway; heavy clay drains slowly and needs a much larger one, or sometimes will not take a soakaway at all, in which case the roof water has to go to a surface-water drain instead.

Cost and where to buy

uPVC rainwater goods are stocked by every major UK merchant. uPVC fascia and downpipe usually come from the same suppliers, so it's easy to order the whole roofline together.

Round 68mm and square 65mm pipe sit in the per-length ranges shown in the table above, and the fittings (offset and swan-neck bends, pipe clips, and a shoe or access fitting) are each only a few pounds. Anthracite and black add 40 to 70 per cent over white across all of these, so a coloured scheme costs noticeably more than a plain white one.

Screwfix, Toolstation, Wickes and B&Q carry the common round and square ranges off the shelf in white, black and anthracite for small jobs. Travis Perkins and Jewson stock the full range plus the less common profiles and can order in cast-iron-effect aluminium. For a whole-house colour-matched scheme, buy everything from one brand (Floplast and Brett Martin are the widely stocked names) so the pipe, bends, clips and shoes all fit and match. Rainwater goods are light and bulky, so for a single downpipe it is easiest to collect from a trade counter or order alongside the rest of the roofline; the gully and underground adaptors usually come on the groundworker's drainage order rather than with the pipe.

Alternatives

The main alternative to standard uPVC is cast-iron-effect aluminium, used on period properties and higher-end schemes where the heavier, traditional look is wanted. It costs considerably more, comes from specialist suppliers, and is rarely the right call on a standard extension where matching the existing uPVC roofline is the priority.

There's no real alternative to having a downpipe at all. Every gutter outlet needs one. The only genuine choice is profile (round or square) and material (uPVC or aluminium), and on a standard extension that choice is made for you by what's already on the house. At the base, the choice between a gully, a soakaway, a surface-water connection or a water-butt diverter is the one that actually varies, and that is set by the drainage in the ground rather than by preference.

Where you'll need this

• Roof covering - the downpipe completes the rainwater system once the gutters are hung, carrying roof water down to ground-level drainage

Downpipes are part of the rainwater goods on every pitched or flat roof, so they appear on any extension, re-roof, or roofline renewal. They connect the guttering above to the drainage below, and the same rules apply whatever the project: match the profile and colour, size the swan neck to the overhang, clip it properly, and make sure the base discharges into a drain rather than onto the wall.

Common mistakes

The same handful of downpipe faults turn up again and again, and all of them are visible from the ground after the work is done.

The pipe discharges onto the ground or against the wall. A downpipe that stops short of a gully, or has no shoe to direct the flow, soaks the wall base every time it rains. This is the cause of most damp problems traced to the roofline. The pipe must terminate over a gully or into a sealed drain connection, not into thin air.

The swan neck fouls the soffit. A swan neck cut too short, or the wrong fixed offset for a deep eaves overhang, leaves the pipe jammed against the soffit or standing proud of the wall. It looks wrong and the joints don't seat cleanly. Measure the overhang first.

Too few clips, or the pipe held tight to the wall. A run held by one or two clips bows away from the wall, the push-fit joints work loose, and in wind the pipe rattles and eventually pulls free. A pipe clipped flat against the brick with no standoff stains the wall behind it. Clip at the top, the bottom, every joint, and at intervals no greater than about 2m, with the proper gap behind the pipe.

Too few downpipes for the roof area. One pipe trying to clear a large roof lets the gutter overtop in heavy rain and pour down the wall. Where the roof area is large, split the gutter with a second outlet and fit a second downpipe rather than relying on one.

Rainwater into the foul drain. Connecting a downpipe to the foul system where a separate surface-water drain exists overloads the foul sewer and breaches Building Regulations Part H3. Confirm which drain the gully feeds before connecting, and prefer a soakaway or surface-water route.

No access at the base. A plain shoe with no removable cover and a gully with no rodding eye means any blockage has to be dug out. Specify an access shoe and a gully you can rod from the surface.

Mismatched profile or colour. A round pipe forced onto a square outlet, or a white pipe against coloured gutter, looks like a botched repair. Match the profile, the colour and ideally the brand throughout.