Guttering: Profiles, Sizing to Your Roof, and Matching the House

The roof tiles go on, the scaffold comes down, and the first heavy autumn shower sheets straight off the eaves and down the back wall because the gutter was sized for a smaller roof, hung with too few brackets, or set dead level so it ponds and overflows at the join. Guttering looks like the simplest part of the roofline and gets treated as an afterthought, which is exactly why it's one of the most commonly botched details on an extension. Get the profile, the capacity, and the fall right and you never think about it again. Get them wrong and you get damp brickwork, a soaked patio, and a return visit.

What it is and what it's for

Guttering is the channel clipped along the bottom edge of the roof that catches the run-off and carries it sideways to an outlet, where it drops into a downpipe. Without it, every drop of rain that lands on the roof falls straight onto the ground at the eaves, splashes back against the wall, and over time soaks the brickwork and undermines the foundations. The gutter's whole job is to collect that water and move it to a controlled point of discharge.

The channel is fixed to the fascia, the flat board running along the eaves that the gutter brackets screw into. On most extensions the fascia is uPVC as part of the wider soffit and fascia roofline. The gutter sits just below the tile edge so water running off the tiles drops cleanly into the channel rather than overshooting it.

uPVC (the white or coloured plastic used for almost all modern domestic rainwater goods) is the standard for extensions. It's cheap, light, doesn't rust or rot, and clips together without tools. The system is sized to the roof area it has to drain and laid to a slight fall towards the outlets so gravity does the work. The water leaves through the downpipe and goes somewhere it can soak away or be carried off, usually a bottle gully at the base of the wall that connects to the drainage system.

Rainwater goods are covered by BS EN 12056-3, the standard for gravity drainage inside and around buildings. In plain terms it tells you how to size a gutter and downpipe so they cope with the rain a roof actually sheds, rather than guessing. You don't need to read the standard to fit an extension gutter, but the principles behind it are worth understanding, because they explain why a gutter that looks generously sized on the shelf can still overflow once it's on the wall.

Profiles and which to choose

The profile is the cross-section shape of the gutter channel. It matters for two reasons: capacity (how much water it can carry) and compatibility (whether it connects to your existing run). The four you'll meet on UK domestic work are half-round, square, deepflow, and ogee.

Half-round is the workhorse and what most 1950s-onwards houses already have. Square (also called squareline) gives a sharper, more modern look and holds more water for the same nominal width because the box section has more cross-sectional area. Ogee has a moulded front face and is chosen on aesthetic grounds, often to match the look of older cast-iron guttering. Deepflow is the one to reach for when capacity is the problem rather than appearance: a deep channel shifts far more water and is the right call on a large or steeply pitched roof.

Within half-round there's a width to watch, because not all "half-round" is the same size. The common domestic widths are 112mm and 115mm, and they look almost identical on the shelf but belong to different systems. The 112mm is the older, smaller half-round still found on much post-war housing; 115mm is the more usual modern size and carries slightly more water. Deepflow systems are typically around 115mm to 117mm wide but far deeper than standard half-round, which is where the extra capacity comes from: it's the depth, not just the width, that determines how much water the channel can hold and move. Reading a width off a merchant's listing without checking which system it belongs to is a quick way to order fittings that won't clip together.

Sizing to your roof

A gutter that's too small overflows in heavy rain no matter how well it's hung. Sizing comes down to the area of roof draining into it and how fast the water arrives.

The figure that matters is the effective roof area, not the footprint on the plan. You take the plan area of the slope feeding the gutter and add a pitch allowance, because a sloping roof throws water into the gutter faster than a flat one and a vertical face shed by a wall above can drain onto the same roof too. A common rule of thumb is to take the plan area of the slope and add half the area of the vertical height of the slope, which has the effect of treating a steeper roof as a larger collecting area. A 35-degree slope behaves like a noticeably bigger flat area than its footprint suggests, which is why a gutter sized off the floor plan alone tends to come up short.

The second part of the sum is rainfall intensity: how hard it rains in a given storm. UK design figures sit around 75mm per hour for most of the country for ordinary domestic work, with higher figures in the wetter west and for buildings where an overflow would cause real damage. You multiply the effective area by the intensity to get the flow the gutter has to carry, then pick a profile and outlet arrangement that can shift that flow. You don't have to do the arithmetic by hand for a normal extension, but it explains the logic: more area, steeper pitch, or harder rain all push you towards a deeper profile or a second outlet.

Worked through with an example, take a single-storey rear extension with a roof slope roughly 6m wide and 3.5m up the slope, pitched at about 30 degrees. The plan footprint of that slope is around 18 square metres, but once you add the pitch allowance the effective area is closer to 22 to 24 square metres. That's comfortably more than a standard 112mm half-round wants to handle off one outlet in a heavy storm, which is why an extension of that size usually ends up on square, deepflow, or half-round split across two outlets. As a quick orientation: a small lean-to or porch is fine on standard half-round, a typical single-storey rear extension is comfortable on square or deepflow, and a large roof or one with a steep pitch should be on deepflow or split between more outlets.

The other lever is outlet spacing. Every outlet (the fitting where water drops into the downpipe) drains a portion of the run. More outlets means each one handles less water, so the gutter can be shallower. One outlet at the end of a long run forces all the water to one point and is the first thing to overflow, because the channel has to carry the entire flow to that single low corner. A centre outlet feeding two falls drains twice the length for the same channel depth, which is why it's worth thinking about outlet position as part of capacity, not just plumbing. As a rough guide, keep a single end outlet feeding no more than about 6 metres of gutter on a standard pitched roof, and add a second outlet and downpipe on longer or busier runs.

It's worth being clear about what undersizing actually does, because it's not always a dramatic waterfall off the front. More often the channel fills, the water backs up, and it spills over the back edge of the gutter rather than the front. That spill runs straight down behind the fascia and onto the top of the wall, where it soaks the timber and the masonry and is invisible from the ground. Rot in the fascia and a damp patch on the internal wall head are classic symptoms of a gutter that's too small or has lost its fall, and by the time they show, the cause has been quietly running for months.

Matching the existing house run

This is where extensions go wrong most often. The new gutter almost always has to physically connect to the existing house gutter so they drain as one continuous run, or at least sit at the same level and in the same style so the eaves line reads as one.

Three things have to match. The profile has to be the same shape and, critically, the same brand, because cross-brand fittings don't connect. The colour has to match so the two runs don't look like a patch job. And the outlet positions have to work with where the downpipes and drainage already are, so you're not forced to add a downpipe in an awkward spot.

Before ordering anything, get up to the existing gutter (or have your roofer do it) and identify the brand. It's often moulded into the back of the gutter or stamped on the fittings. FloPlast, Marshall Tufflex, Brett Martin, Hunter, and Polypipe are the common UK brands, and each has its own jointing system. Match the brand and you get a clean mechanical connection. Guess, and you're cutting and bodging two incompatible systems together.

If the existing run is a brand or profile you genuinely can't get hold of, the answer is an adaptor. Most ranges include a union adaptor that joins, say, a half-round to a square or one brand to another, and these are the right way to bridge between systems where matching isn't possible. The other detail is level: where the new gutter meets the old, the channels have to sit at the same height and the new fall has to carry on the direction the old water already runs, or you create a low point at the join that ponds and overflows exactly where the two systems meet. Set the new run off the level of the existing outlet, not off a fresh datum, so the water keeps moving the way it always has.

How it's fixed and jointed

Guttering hangs from brackets screwed into the fascia, or in some systems from a moulded clip on the back of each fitting. Bracket spacing is the single biggest factor in whether a gutter sags. The channel is flexible plastic, and unsupported lengths droop under the weight of water, then pond and overflow at the low spot.

Brackets should sit at roughly 1 metre centres along the run, closer on high-capacity or ogee profiles that carry more weight, and there should be a bracket within 150mm of every joint and outlet. Skimp on brackets and the run develops waves: it sags between supports, water collects in the dips, and the carefully set fall stops working.

There are three bracket types worth knowing. Fascia brackets screw straight onto the face of the fascia board and are the default on any extension with a fascia, which is almost all of them. Rise-and-fall brackets are adjustable, letting you tune the height of each bracket after fitting, which is useful for fine-setting a long run or matching into an awkward existing level. Top-rafter brackets (sometimes called drive-in or rafter brackets) fix over or into the rafter feet where there's no fascia to screw to, as on some traditional or open-eaves details. For a standard uPVC fascia, fascia brackets are what you'll use, but it's worth recognising the others if your roofer specs them.

Modern uPVC systems are clip-fit with rubber seals. Each union (the connector between two lengths of gutter), running outlet, and stop-end has a moulded rubber gasket, and the gutter simply clips in. These cope with thermal movement (the gutter expanding and contracting as it heats and cools) by letting the lengths slide slightly within the seal, which is why you'll see an insertion line marked on the fitting. That line is not decoration: it's the cold-weather setting. You push the gutter into the fitting only as far as the line, leaving a deliberate gap so the plastic has room to expand on a hot day without buckling. Push it fully home in summer and the joint will be forced apart and leak come the next cold snap. Older systems used solvent-weld cement to glue the joints rigidly. Solvent joints are still found on some older houses, but for an extension a clip-fit sealed system is standard, faster to fit, and easier to adjust later.

Setting the fall

A gutter laid dead level doesn't drain. Water sits in the channel, collects debris, breeds the next blockage, and overflows at the joints. The run has to slope gently towards the outlet so gravity pulls the water along to where it drops out.

The fall is slight: around 1 to 3mm of drop per metre of gutter is plenty, which works out to roughly 10 to 25mm of fall over a typical extension run. It needs to be enough to move the water without being so steep that the gutter visibly dips below the tile line at the far end, which looks wrong and lets water overshoot at the high end. The fall is set by fixing the brackets progressively lower towards the outlet, which is why the bracket line is struck before any gutter goes up.

Where the water goes and SuDS

The gutter is only half the job: the downpipe has to discharge somewhere legal and effective. The water can't simply be dumped at the base of the wall, where it soaks the foundations and runs across the neighbour's boundary. It has to reach a controlled point, usually a bottle gully at the foot of the downpipe that connects into a surface-water drain or a soakaway.

On new and extended roofs, Building Regulations push towards Sustainable Drainage Systems (SuDS), which means dealing with rainwater on or near the plot rather than piling it all into the public sewer. In practice that often means a soakaway, a stone-filled or crated pit that lets the water disperse into the ground, sized to the roof area and the soil's ability to drain. Where the ground won't soak (heavy clay, high water table), the discharge goes to a surface-water sewer instead, and you may need building control or the water company's agreement. The point for sizing the gutter is simple: the outlet and downpipe positions have to line up with where that discharge point can actually go, so settle the drainage strategy before you fix the run.

Maintenance, leaf guards, and cold weather

A gutter that drains well when it's fitted still silts up over the years, especially under trees. Leaves, moss washed off the tiles, and grit collect at the low point and in the outlet, and a blocked outlet turns a perfectly sized gutter into a paddling pool that overflows behind the fascia. Clearing it once or twice a year, autumn especially, is the whole of the maintenance.

Leaf guards, mesh or brush inserts that sit in the channel or clip over it, cut down how often you have to clear it, and are worth fitting on a run under heavy tree cover or anywhere awkward to reach. They don't eliminate maintenance, because fine silt still gets through, but they keep the big debris out of the outlet where it does the most harm.

Cold weather adds two issues. Snow and ice can build at the eaves and, as it melts and refreezes, form a ridge that backs meltwater up under the lowest tiles. A correctly hung, free-draining gutter set at the right depth below the tile line is part of the defence, because it gives the meltwater somewhere to go. The second issue is brittleness: uPVC stiffens in hard frost, so leave clearing and any adjustment for a milder day rather than risk cracking a cold fitting.

Common mistakes

The failures here are consistent and all visible from the ground after the first heavy rain.

Mismatched profile. Ordering a half-round system to tie into an existing square run, or the right profile from the wrong brand, so the new gutter won't connect to the old one. The fix is always to identify the existing brand and profile before ordering, and use an adaptor where matching isn't possible.

Too few brackets. The run sags between supports, ponds in the dips, and overflows. Brackets at roughly 1m centres with one near every joint is the standard. A gutter that visibly waves along its length is under-bracketed.

No fall, or the wrong fall. Set level or sloping away from the outlet, the gutter holds water and overflows at the joints. Set the fall towards the outlet before fixing the brackets.

Wrong outlet position. Putting the outlet where it suits the gutter rather than where the downpipe and drainage need to go, forcing an awkward downpipe route or a connection that doesn't reach the existing gully.

Undersized for the roof. Standard half-round on a large or steep extension roof that throws more water than the channel can carry. Step up to square or deepflow, or add a second outlet, rather than fitting the cheapest profile and hoping. Remember that an undersized gutter usually overflows backwards behind the fascia, not off the front, so the rot can run for months before you see it.

Joints pushed fully home. Ignoring the insertion line and ramming the gutter into the union in warm weather, so the joint splits and leaks when the plastic contracts in the cold. Push only to the marked line.

Colour and finish

White is the default and the cheapest, and it's what most houses already have. Anthracite grey and black have become popular on contemporary extensions, particularly paired with dark windows and dark fascia, and typically cost around 30 to 60% more than white for the same profile. Brown is still common on older housing stock. Whatever you pick, match it to the existing run and the fascia so the roofline reads as one.

For period and traditional properties, cast-iron-effect aluminium guttering mimics the look of original cast iron without the weight, the rust, or the painting, but it costs several times more per metre than uPVC (see the table above). It's a deliberate aesthetic choice for a heritage look, not a default for a standard extension.

Cost and where to buy

uPVC guttering is sold in 2m, 3m, and 4m lengths plus separate fittings: unions, running outlets, stop-ends, angles, and brackets, each only a few pounds. The profile and fitting prices in the table above are per 2m length (or per fitting) in white. Anthracite and black carry a 30 to 60% premium, and cast-iron-effect aluminium is in a different bracket entirely.

For a single extension run, Screwfix, Toolstation, and Wickes stock the common FloPlast and Marshall Tufflex half-round and square systems off the shelf, which is the easiest way to buy a small quantity. For larger orders or to match a specific existing brand, Travis Perkins, Jewson, and specialist online merchants like Drainage Superstore and Roofing Megastore carry the full range across brands and can confirm fitting compatibility. Buy the gutter, the matching unions and outlets, the brackets, and the stop-ends together as one system from one brand, because mixing brands is where the leaks start.

A worked order for the example extension above, a 6m run on square profile draining to one end outlet, comes out at roughly three 2m lengths plus a small allowance for cutting, around seven or eight fascia brackets at 1m centres, two unions to join the lengths, one stop-end, one running outlet, and the downpipe and a bottle gully separately. Add a length or two of spare, because cutting to fit the fall and trimming at the ends always eats a little more than the bare run length. Order all of it in one brand and one colour in a single go, so you're not waiting on a missing union mid-fit.

Spec checklist

Before you order, confirm: the existing brand and profile (read off the back of the gutter); the colour to match the fascia and existing run; the effective roof area including the pitch allowance, not just the footprint; the profile and width that comfortably carry that area; the number and position of outlets and whether they line up with the downpipe and gully; the bracket count at roughly 1m centres plus one near each joint and outlet; and an adaptor if you're bridging to a different existing system. Get those settled on paper and the fit is straightforward.

Where you'll need this

• Roof covering - the gutter is fixed to the fascia at the eaves once the roof covering is on, sized to the new roof area and tied into the existing house run

Guttering goes on at the end of the roofline work on any pitched or flat roof extension, after the tiles or covering are down and the fascia is fixed. It feeds the downpipe, which carries the water to a gully or soakaway, and the principles of profile matching, capacity, bracket spacing, and fall are the same whatever the project type.