A centre-pivot roof window fitted in a natural slate roof with a neat flashing kit

Skylights and roof windows in Bath

We fit, replace and repair roof windows and rooflights on houses and commercial premises in Bath and the towns and villages nearby. The glass is the easy part. The flashing, the trimmed rafters and the lining decide whether the window stays dry and warm.

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Consent and Building Regs explained

What we do

What our roof window work covers

A roof window job is a repair, a straight swap in the same opening, or a new opening cut between the rafters. After the survey you hear which yours is and what will be disturbed inside and out.

Signs to watch

Signs a roof window needs attention

A brown ring or damp patch on the lining below the frame
Water running down the glass or pooling on the sill
Mist or a film between the two panes of the glazing
A sash that sticks, will not latch or lets in a draught
Cracked or lifted flashing, or slipped slates at the window head

1
Look at the window from both sides
We inspect the frame, seals and flashing from the roof, then the lining, sill and rafters from inside, and take photographs of what we find.
2
Work out the cause
A drip at a corner could be the kit, the glass seal or condensation. We separate the three before anyone orders a window.
3
Agree the opening and the kit
We confirm the window size against the rafter spacing, choose the flashing kit for your slate or tile, and explain any consent needed.
4
Fit, finish and clear
The window is set, flashed and sealed, the lining is made good, the roof is left tidy and the window is opened and shut with you to check it runs freely.

The window, part by part

Inside a roof window: what keeps it dry and warm

A roof window is a window turned to face the weather from above. Whether it stays watertight depends on the flashing kit, the frame, the rafters round the opening and the lining inside, and the Building Regulations add requirements of their own.

Cut-away drawing of a slate roof — 1The window frame

2The flashing kit

3Trimming the rafters

4Inside the opening

5Dormers

6Conservation rooflights

Part 1 of 6

The window frame

The frame carries the glass, the seals and the weather, and it has to move a little without letting water in.

A roof window is a glazed sash hung in a frame, built to sit in the slope of the roof and shed water over its edges. It is not a wall window tilted on its side. The frame is made to be wrapped in flashing and to sit on the rafters, and its outer face is designed so rain runs off rather than lying on it. Most frames are timber, coated for protection, with an outer cover in aluminium or a similar weather-resistant metal. Frames are made in a range of standard sizes, and the size you can have is set by the gap between your rafters.

The sash opens in one of two main ways. A centre-pivot sash turns about a bar halfway up the frame, so the outer face swings inside and can be cleaned from the room. A top-hung sash is hinged at the head and pushes outwards, which gives a better view and more headroom at the bottom edge. Some windows combine the two actions. Fixed windows, which do not open, are common where light matters more than ventilation. Electric and rain-sensing versions exist as well, and they add cost.

Glass, heat and the Part L limit

Almost all roof windows are double glazed, with a coating on one pane that reflects heat back into the room. Many have laminated glass on the inner side, so a broken pane holds together, and toughened glass outside. Where a roof window is replaced or a new one is fitted in an existing home, Approved Document L (2021 edition, with 2023 amendments) sets a ceiling on heat loss in its Table 4.2. A roof window should achieve 1.4 W/m²K or a Window Energy Rating of Band B. The same table sets 2.2 W/m²K for a rooflight, which is the term for glazing in a flat or low-pitched roof, and 0.15 for a roof element. The new unit must not lose more heat than the one it replaces. Ask for the rating on the data sheet before you accept a quote.

Trickle vents and condensation

Most roof windows have a small vent in the head of the frame that lets fresh air in while the sash stays shut. A bedroom under the slope can feel stuffy without it. Condensation on the inner face of the glass on a cold morning is a ventilation matter more than a defect, and moisture that collects on the frame and runs down the lining is often mistaken for a roof leak. Misting between the panes is different. It means the seal around the sealed glass unit has failed, and only a new unit or a new window can fix it.

What we check on the frame

On a survey we look for perished rubber seals, a coating that has worn through at the foot of the timber, hinges that have worn, a sash that no longer closes square and any soft timber at the bottom corners, where water settles first. A frame in sound condition with a failed glass unit may only need a new sash. One with rot at the base usually needs replacing outright, because new flashing cannot make a decayed frame sound.

1.4

W/m²K: the limit for a new or replacement roof window, or Window Energy Rating Band B

2.2

W/m²K: the limit Table 4.2 sets for a rooflight

0.15

W/m²K: the roof element figure in the same table, for comparison

Figures: Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments, Table 4.2

Our guide to roof window sizes explains how the code on the frame relates to your rafter spacing, and the page on roof window glazing covers the options for laminated, toughened and coated glass. If yours is misting, repair or replace lays out how to decide.

Part 2 of 6

The flashing kit

A kit of formed metal pieces is what seals the window into the roof, and it must suit your covering.

Flashing is thin sheet metal, usually aluminium or lead, shaped to carry water from a joint onto the roof covering around it. A window cut into a slope creates a hole with four edges, and each edge sheds water in a different way. The flashing kit is a set of pieces made to deal with all four. Along the bottom is an apron, a wide piece that lies over the covering below the window and sheds water onto it. Down each side run channels, which take the water that comes off the glass and the slope beside the window. Across the top, a flashing tucks under the covering above and diverts water round the frame.

The kit is made for a particular type of covering. There are kits for flat or low profile tiles and slates, and kits for deep profiled tiles whose waves must be bridged by the metal. Each allows for the thickness of the covering, which differs between a thin slate and a deep concrete tile. A slate kit fitted to deep tiles leaves gaps where the metal cannot follow the profile, and a tile kit used with slates sits proud of them. Either way, rain finds a way in. It is a common reason a new window leaks within its first winter.

How the pieces go together

Order matters. The bottom apron goes on first, lapped over the covering. Soakers, small pieces of metal bent to run under one course and over the next, are woven into the courses at the sides, so each course overlaps the soaker below it. The top piece goes in last, tucked beneath the covering above. Water running down the roof meets only overlaps that face downhill, and that is the whole principle. Battens, the timber strips the covering is nailed to, are cut and supported around the frame so the metal has firm timber to sit on. The underlay, the membrane under the battens, is dressed up to the frame and lapped into a collar, to carry out again whatever water slips under the covering.

Why windows leak at the flashing

A kit can fail because it was wrong for the roof, because a corner was folded and not sealed, because a piece was cut short, or because an old kit was reused on a new window. Moss and leaf litter collect in the side channels and hold water where it should drain. On older slate roofs, a slate at the head of the window is often the one that has slipped, and the leak looks as though it comes from the window itself. The same detail turns up at the foot of a dormer cheek or against a chimney, and metal work beyond the kit, such as lead flashing, is covered on our lead roofing page.

We do not seal a joint with mastic and call it repaired. Sealant is a short-term patch at best, and it hides the real fault until the next heavy rain. When the kit has failed we strip it back, check the battens and underlay beneath, and fit the right kit with a good lap.

Myth

A window that leaks needs replacing.

Fact

The glass and frame are often sound; the flashing, a slipped slate above the window or condensation is the likelier cause, so we trace the water before anything is ordered.

Our page on roof window flashing kits shows which kit goes with which covering, while why a roof window leaks takes you through the causes in the order we check them. Fitting into slate calls for special care, and roof windows in slate roofs covers it.

Part 3 of 6

Trimming the rafters

A new opening only works if the timbers around it are cut, supported and framed correctly.

A roof is held up by rafters, sloping timbers running from the wall plate at the eaves to the ridge. In most Bath houses they sit at regular spacing, with battens fixed across them to take the covering. A roof window sits between rafters when it can. When a window is wider than the gap, or does not line up, one or more rafters must be cut, and that changes how the roof carries its weight.

The cut rafters are held by trimmers: short, stout timbers fixed horizontally between the uncut rafters on either side, above and below the opening. They pick up the load of the cut rafters and pass it sideways to the full-length ones. The trimmers are often doubled up, and the joints are made with proper fixings rather than skew nails. The uncut rafters either side then carry more than they did before, so they need to be sound, and we look at them first. A rafter with rot or insect damage next to the opening is not a safe place to hang a trimmer.

Like for like, or something different

Replacing a window in the same opening needs no rafter work. The frame comes out and the new one goes in, sized to the existing opening. Choosing a larger window, a different shape or a second window alongside changes the picture. The extra width can mean a new trimmer, a new batten layout and a larger gap in the covering. If the work goes past a simple like-for-like swap, ask your building control body whether it wants to see the framing. For homes in the city, that means the council's building control team at B&NES or a registered approver.

Covering, battens and weight

The covering is cut back to the new opening and the battens are trimmed and supported on the new timbers. On a slate roof, slates round the opening are lifted carefully and refixed, so the cut edges end up neat and nailed. A new frame also weighs something, and a large glazed unit puts load onto timber that was not designed for it. We take that into account when we choose the position, and we do not move an opening to a spot where the timbers underneath are weak.

Pitch matters as well. Roof windows are designed for a range of roof pitches, and a very shallow slope can be unsuitable or need a different product. The window maker's data sheet states the permitted range, and we check it before we price. A roof window set in a low pitch may need a taller flashing upstand, a different kit or a flat rooflight in its place.

Weather planning is part of the job. We open only as much of the roof as can be closed again, and the opening is covered if the day ends before the flashing does.

Roof window jobs and what each one involves

Job	Timber work	What we check first
Same size, same opening	None	Rot at the frame foot and condition of the old kit
Larger than before	New trimmers, possibly a cut rafter	Sound rafters either side and the pitch range
New opening between rafters	Battens cut and supported	Rafter spacing against the window size
New opening cutting a rafter	Doubled trimmers, load passed sideways	Whether building control wants to inspect
Two windows side by side	Wider trimmed opening	Weight on the remaining rafters

Read replacing an old roof window for the steps in a swap, or minimum pitch for roof windows if your roof is shallow. The page on roof windows and Building Regulations sets out when the rules apply to the framing.

Part 4 of 6

Inside the opening

The lining, the light well and the escape rules are decided from inside the room.

Seen from the room, a roof window is a sash set into an opening in a sloping ceiling. The opening needs a lining, which is the finished surface that runs from the edge of the frame back to the ceiling or wall. The lining sits in a shaft called the light well, and its shape controls how much daylight enters the room. A lining cut with a vertical top and a horizontal bottom, so the sides are plumb and the top and bottom are level, lets in more than one that follows the roof slope. We form it with insulation behind and a vapour check on the warm side, so moist room air does not reach cold timber and condense there.

A gap between the frame and the lining is the most common cause of a cold bedroom under a roof window. Insulation must wrap the frame fully, and the join to the ceiling plaster needs sealing. A rushed job leaves a cold bridge, and the edge of the lining then grows mould where warm air meets cold surface. The same fault is often blamed on a leak. Our spoke on roof window condensation describes how to tell the two apart.

Fitting from inside

Some of the work is done from inside, particularly when a window is replaced like for like. The lining is opened out, the old frame is freed from the rafters, and the new unit is lifted in from the room side with the flashing finished from the roof. Fitting a roof window from inside explains what is practical and what still needs the roof.

Means of escape

If the window is in a bedroom or other habitable room on an upper floor, escape comes into it. Approved Document B says that where an upper storey is no more than 4.5 m above ground, each habitable room, apart from a kitchen, needs either an escape window or door or direct access to a protected stairway. An escape window must have an unobstructed openable area of at least 0.33 m², be at least 450 mm high and 450 mm wide, and have the bottom of that openable area no more than 1,100 mm above the floor. A roof window can serve as an escape only within that 4.5 m height from the ground; above it, different rules apply. A loft room on a two-storey house needs a protected stair.

These measurements govern which window you choose, how high it is fixed and whether a top-hung or pivot sash works. A window hung high on a steep slope can look right and still be too high above the floor. We check the numbers against the room before the opening is cut.

The guides to roof windows as an escape route and Part L and escape rules go through the dimensions and when the building control body steps in. Your own building control body has the final word on how the rules apply to your house.

Part 5 of 6

Dormers

A dormer is a small roof structure of its own, with its own cheeks, roof and joints to leak.

A dormer is a window set upright in a box that projects from the slope of the roof. Strictly it is a structure, not just a window, because it has its own walls, its own roof and several junctions where it meets the main roof. The sides are called cheeks. The front holds the window. The top is a small roof, flat or pitched. Each of those parts is covered and weathered differently, and that is why dormers leak more often than a plain roof window.

Types of dormer

A flat-roofed dormer is a box with a near-level roof, usually lead, GRP or EPDM. GRP is glass-reinforced plastic, a fibreglass laid as a single joint-free sheet; EPDM is a synthetic rubber membrane. A gabled dormer has a small pitched roof with its own ridge, tiled or slated to match the main roof. A hipped dormer slopes back on three sides. The cheeks are usually clad in lead, tile hanging or render over timber framing. Some roofs, especially mansards, have the windows built into the slope itself, so the cheeks are part of the roof structure.

Bath's mansards and integrated dormers

A mansard roof changes pitch part way up, with a steep lower slope under a shallow top. Writing about the terrace at Lambridge Place, Bath Preservation Trust noted that the mansard and its built-in dormers show more from the street than a roof recessed behind the parapet line. That visibility is a reason to keep the original proportions and materials when a dormer is repaired. It is also the reason a council may care about the details.

Where dormers leak

The roof junctions fail first. The top of each cheek meets the main roof at a valley-like corner, where lead soakers or a stepped flashing have to be right. The flat roof on top can crack, split at an upstand or pond. The window head can lose its flashing. Timber behind the cheek lining can rot unseen for years, and the first sign may be a stain on the bedroom wall. We trace the water to its source before we quote, and we fix the roof around the window as well as the window.

A dormer cannot be judged from the glass alone. A sound-looking bedroom window can hide a failed cheek, a split flat roof and rot in the framing. A repair that stops at the window only delays the next leak.

Consent

Work that changes a listed dormer calls for listed building consent from B&NES, and that is not the planning permission route. Speak to the council before you start.

Fix the structure first

On a leaking dormer, repair the cheeks, junctions and flat roof before the window, since a new window set into rotten framing leaks again.

Our guide to dormer windows describes the types and the repairs, and why dormers leak lists the failure points. If you are weighing a new one, dormer cost and dormer planning permission explain what drives price and consent, and dormers on listed buildings is for protected homes.

Part 6 of 6

Conservation rooflights

A flush rooflight sits level with the slates and suits period roofs where a raised box would not.

A conservation rooflight is made to sit almost level with the covering. It has a slim metal frame, often with a single glazing bar, and a low profile that does not stand proud of the slates. The glass lies in nearly the same plane as the roof, so from the street it reads as a small break in the slope rather than a box on top of it. The idea is to let in daylight without changing how the roof looks, which is why planners and conservation officers prefer it on period buildings.

They are made as a fixed unit or with an opening sash. Installation is a skilled job. The frame must be set into a roof of slate, stone or clay, with the covering cut tight to its edge, and the flashing must be shaped to the covering so that water cannot track under the metal. In a flat or low roof the same product is a rooflight, and the 2.2 W/m²K limit in Table 4.2 applies to it. Where it has to meet a roof window standard, a manufacturer can tell you what a given model reaches.

Bath's rules in everyday words

The whole of Bath is a World Heritage Site, but its city-wide conservation area covers only about half of it. That makes a difference. In a conservation area, planning rules for alterations to a roof are stricter, and some streets are subject to Article 4 directions that remove permitted development rights. Listed buildings need consent from B&NES before alterations that affect them, and the council's advice is to speak to it first. Permitted development rights for roof windows are set out in Class C of the General Permitted Development Order, and have limits; our roof window planning guide covers them in one place.

Choosing and fitting one

A heritage roof needs the right proportions, so we start with the glass size and position, working from the rafter layout and the courses of the covering. Where a window can sit in line with other features, such as a chimney or a dormer, it looks more natural. Then we choose the frame colour, which is usually a dark metal finish, and a flashing detail that fits the covering without stepping up from it. Slates are cut and refixed tight, and a lead apron is dressed to the slates beneath.

We also advise you on what to ask the council. A drawing showing the size, position, frame and covering helps the conservation officer, and we can supply the technical description for it. It is far better to have that conversation before an order is placed than after a unit has arrived.

Words you will hear

Rooflight: Glazing set in a flat or low-pitched roof, rated separately from roof windows in Part L.
Conservation rooflight: A slim, low-profile rooflight that sits nearly level with the roof covering.
Glazing bar: A thin bar dividing the glass, used to echo the lines of old roof lights.
Mansard: A roof with a sharp lower pitch and a gentler top pitch.
Article 4 direction: A council order removing certain permitted development rights in a defined area.

The guides to conservation rooflights and rooflights in a conservation area go through sizes, frames and the consent route. For a general introduction, rooflight or roof window compares the two. Our conservation area roof rules page covers consent more widely.

Typical costs

What roof windows typically cost

Typical UK ranges, as reported by the MyBuilder cost guide for Velux-type roof windows (2026). Window size, a new opening, electric operation and access each push a job towards one end of the range or the other.

Job	Typical UK range
Installing a standard roof window (labour and fitting, window extra)	£350 to £600
A basic roof window supplied (the window only)	about £305 and up
Scaffolding, where the roof needs it	about £500
Electric operation, added to the window cost	£200 to £500

These are national guide figures, not our prices. A fixed written price follows the survey, once we know the size, the covering and whether the opening is new or existing.

Guides and answers

Roof window and dormer guides

Quick answers

Questions

Roof windows and dormers in Bath: the questions that come up first

Can a roof window be fitted into an old Bath slate roof without breaking the slates?

Usually, yes. Slates around the opening are lifted by cutting their nails, kept in order and refixed round the new flashing. Slate that is soft or already cracked will not survive the handling, so we allow for matching replacements. Brittle, thin slate on an old roof is the main risk, which we judge on the survey.

Why does my roof window drip at the sill on cold mornings when it has not rained?

That points to condensation rather than a leak. Warm room air meets the cold glass and frame, and water collects at the lowest point. Improving ventilation, checking that the lining is insulated and sealed, and keeping the trickle vent open usually help. A stain that grows after rain is a different thing and needs a look at the flashing.

Will a roof window in a loft bedroom count as the escape route?

Only if the room is within 4.5 m of the ground and the window meets the size and height rules in Approved Document B. That means at least 0.33 m² openable, 450 mm each way, and the bottom of the opening no higher than 1,100 mm above the floor. Higher up, different rules apply. Check with your building control body.

Do I need consent to change a roof window in a listed house in Bath?

Probably. The consent comes from B&NES and is separate from planning permission. B&NES asks owners to raise it with the conservation team ahead of any works on a listed property, and work done without consent is an offence. Replacing a window like for like may still count, so ask the conservation team first.

Why do flat-roof rooflights have a different heat-loss limit from roof windows?

Approved Document L treats them as different products. Table 4.2 gives 1.4 W/m²K for a roof window, or Window Energy Rating Band B, and 2.2 W/m²K for a rooflight. The new unit must not perform worse than the old one. Check the stated figure on the data sheet before you choose.

Can the flat roof of a dormer be recovered without taking the dormer apart?

Often. The flat roof can be stripped, the deck and insulation checked, and a new membrane or lead laid, with the upstands and junctions re-formed. If the cheeks or framing have rotted, those come first. We look at the timber from inside before deciding how much needs to come off.

Book a roof window survey

Photos of the window and the ceiling beneath it help. Post them through WhatsApp or the enquiry form, or call 01225 285181; a surveyor then looks at the frame, flashing and rafters in person. The report comes with pictures and a fixed written price for repairing, replacing or fitting your window.

Get a free survey

Get your free survey

Get your free roof survey

Tell us what's happening and we'll call you back to arrange a visit. Prefer to talk? Call 01225 285181.

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