Misted and Failed Rooflights Replaced in Bristol Clear glass back overhead, and a vent bar to keep it that way

This page is for people who have looked up at a rooflight over the landing, the kitchen extension or the loft bedroom and found the view gone. We replace the failed glass, or the whole unit where that is the better answer, and we put the new one in so it opens or stays fixed as you want. Because we are in the business of fresh air through the roof, we also use the visit to ask whether the old unit should have been an opening one all along.

What we are looking at on the day

The first job is to tell a failed seal from the other things that make glass look poor. Dirt on the outside, a film of grime on the inside of a bathroom unit, a scratch, and condensation on the room-facing pane in cold weather can all look like a fault. Condensation on the room side is ordinary moisture from the house meeting a cold surface, and it clears when the room warms or is aired. Fog between the panes does not clear for good. It may thin in dry weather and return in the next damp week, but it never goes.

The second job is to look at what surrounds the glass. A sealed unit sits in a frame of aluminium, timber, PVC or coated timber, and the frame may be fixed to a roof or kerb with its own flashing. If the frame is sound and the glazing bead can be taken off, a new unit can go in without disturbing the roof. If the frame itself has decayed, or its glazing method makes a clean swap impossible, then the whole rooflight or roof window is replaced.

Around Bristol the units we meet are of every age. A Victorian terrace in Bedminster may have a fixed timber rooflight over the stair, reglazed once in the nineties. An interwar semi in Henleaze tends to carry a roof window put in with the loft room. A Georgian house in Clifton can have a lantern over a rear stairwell. Newer estates have flat-roofed extensions with a pair of aluminium rooflights over the kitchen. Each one fogs for the same reason, but each asks for a slightly different method, so the chapters below follow the order in which you will face the choices, ending with roof windows and what a replacement costs.

Other signs worth noting

Misting is the loudest sign, but a unit can be on its way out earlier. A rooflight that has gone slightly cold to the touch in winter, a pane that looks faintly rainbow-tinted in sunlight, or a room beneath that seems chillier than it did all point towards a seal under stress. None of them is a verdict. They are reasons to have someone look before the glass clouds over.

It also helps to check what is around the glass. Stained plaster on the reveal, softened paint on a timber frame, or a trickle line on the wall below suggest the trouble may be at the flashing or the kerb rather than in the glass. Water getting past the frame is a separate matter from a failed seal, and it changes the job. We look at the roof junction at the same visit so that a new unit does not go into a frame that is letting moisture in.

Photographs help a lot

If you send two photos before we visit, one of the whole rooflight from inside and one close to the corner where the haze is worst, we can say a good deal over the phone. Note the make if it is printed on the frame or the spacer (a small label is sometimes stuck on one edge of a roof window), and measure the visible glass if you can. A tape across the opening, width by height in millimetres, is more use than a guess.

We then visit for the survey. The visit costs nothing, you receive an itemised written quote, and you are not committed to anything. The quote shows each option beside the others: a new sealed unit in the existing frame, a new unit with different glass, or a complete replacement. If the window might be improved by an opening vent or a motor, that goes on the page as well, because opening options appear on every quote we write.

Some people ask whether they can wait. A foggy unit does not stop working at once, and nothing about the fog itself harms the room. The gradual loss is in the view, in the daylight, and in how well the glazing insulates, because the gas has escaped and the gap is full of damp air.

Swap the sealed unit only

If the frame is straight, sound and holds the unit with a bead you can lift, the old unit is taken out and a new one made to the exact size goes in. The kerb, the flashing and the roof covering stay as they are. This is the quickest route and the least disruptive, and a dry room is kept dry throughout. The new unit can carry better glass than the old, such as a low-emissivity coating and a warm-edge spacer, as long as the frame can hold the thickness.

Fit a new glass unit with a change of spec

A swap is a chance to improve the glazing. A south-facing loft that overheats might take a solar control coating. A bathroom rooflight might take obscure glass for privacy. Over a stair where people pass beneath, a laminated inner pane (one with an inner film that keeps a cracked pane in one piece) is worth asking about. Part K sets no such rule for a house roof, so it stays a recommendation. We list it on the quote with its price beside it so you can see the difference.

Replace the whole rooflight or roof window

If the frame has decayed, if the unit is bonded in so it cannot come out without breaking the frame, or if the roof window is old enough that its parts are no longer made, then the whole thing is replaced. A new frame and glass go in together, usually in the same opening, with new flashing. That counts as notifiable building work, so the paperwork is ours: Building Control is told, and you receive the completion certificate.

Whichever route suits, the new glazing can be a fixed pane or part of an opening one. If the existing frame is fixed and the room could use air, we price an opening version alongside, so you see the extra for a vent bar or a motor next to the cost of a like-for-like swap.

Why we do not guess from the ground

We have to see the frame to say which route fits. Two rooflights side by side on the same roof can need different answers, because one frame has been painted every few years and the other has not. We open the bead, check the rebate (the groove the glass sits in), and look at the condition of the timber or the finish on the metal. Only then do we write the quote. If a swap would put a new unit into a frame with a short life left, we say so, and let you choose between a cheaper swap now and a full replacement that lasts.

It is also worth asking why the unit failed. Units that sit in a bathroom full of steam for years, or on a roof that bakes in afternoon sun, may fail sooner than ones that live in calm conditions. A new unit with a better edge seal, and a vent or a fan that moves steam out of the room, can help the next one last longer.

1. Sun and heat. A south or west roof bakes the seal each afternoon, and dark frames hold more heat than pale ones.
2. Steam. A bathroom or kitchen rooflight sees damp air every day, which works on the edge seal from the inside.
3. Movement. A frame that flexes with the roof, or a timber frame that swells and shrinks, loads the seal along its length.
4. Quality of the unit. A warm-edge spacer and a double seal outlast a basic aluminium spacer with a single seal.
5. Fit. A unit sitting on poor packers, or with water standing in the rebate, ages early.

Keeping a unit healthy

You cannot make a seal immortal, but you can avoid shortening it. Keep the drainage holes in the frame clear, because standing water in the rebate attacks the seal. Wipe condensation from the glass in a steamy room and air the room afterwards. If the unit opens, open it after a shower. A vent bar, which holds a roof window a finger-width ajar, gives overnight air without a draught. Over time these small habits keep the glass drier, and a drier pane means less strain on the seal.

Check a timber frame's paint every few years, since flaking at the bottom edge lets water in.

Guarantees on old and new

For the new unit, the maker's guarantee covers the product, and our own 10-year workmanship guarantee covers how it was fitted. The two are separate. A product guarantee is about the glass, and the workmanship guarantee is about the fitting, the sealing round it and the finish. Both are written down, and you get them with the paperwork when we finish. Details are on our guarantee page.

It is also fair to say when a swap does not pay. A unit in a frame near the end of its life, or one that has failed twice, tells you the frame or the conditions are the real trouble. We would rather price a full replacement than have you pay for a new unit that fails again. The next chapter weighs this properly.

The cost of waiting

Costs do not stay still while a unit is left to fog. The glazing insulates less, a cold pane draws more condensation onto the reveal, and the fog itself spreads. A room that depends on the rooflight for daylight, such as a stair with no other window, gets gloomier. None of this is urgent in itself, and we do not suggest it is. It is a reason to book a survey while the choice is open, because a measured unit can take a few weeks to arrive.

If a whole roof window or rooflight has reached the end of its life, our main page on rooflight replacement in Bristol covers the full job, from survey to a roof that is weathertight every evening.

VELUX is a brand, and its windows are labelled with a size code on a sticker on the frame, found on the inside of the sash edge or on the frame side. In a code like MK04, read the letters as the width and the digits as the height. With that code and a photograph of the label, we can tell which unit and glazing you have before we visit, and whether a matching pane is still made. Other leading makers have their own codes and their own parts, and we deal with them in the same way.

A choice that matters in a loft

If a roof window in a loft room has failed, ask whether it opens the way you want. An older window may swing on a pivot bar across its middle. A top-hung one tilts outward from the top. Either can take a vent bar. When the glass is being renewed anyway, it is the cheapest moment to ask for an electric or solar sash, which opens and closes at a switch and shuts itself when the rain sensor feels water. The page on electric opening rooflights shows the options.

Whichever you pick, the sum of the work is the same shape: measure, order, fit, make weathertight, tidy. A glass swap on a roof window means working from inside and out, with a ladder or scaffold at the eaves. Where the window is high on a steep slope we use a proper access system, not a ladder alone.

Is it worth it?

The honest question behind these jobs is whether to renew the glass or the whole window. The guide on whether a misted rooflight is worth replacing sets out the sums. In short, a sound frame with a failed unit is the best case for a swap. A tired frame with a failed unit leans towards a full replacement. Price follows the unit's size, spec, access and whether the sash opens. You will find broad UK market figures on the costs page, and after the survey you receive our own itemised quote in writing.

Where we work

We come to you, wherever you are within 30 road miles of Bristol. That covers terraces in Totterdown and Southville, semis in Hanham and Kingswood, Georgian homes in Clifton and Bath, and estates beyond the city edge. Councils differ, so we name the right one on the quote: Bristol City Council, Bath and North East Somerset Council, North Somerset Council or South Gloucestershire Council. The 10-year workmanship guarantee comes with every fitting, in writing.

If you are comparing a glass swap with moving to a fresh-air design, read fresh air rooflights for the rooms where a sash that opens earns its keep, and those where fixed glass wins.