Rooflight above a white sofa with a hazy, clouded pane

Misted Rooflights in Derby

Grey glass over the stairs, swapped for clear daylight.

A rooflight that has fogged between the panes has lost its seal. We survey it, then replace the glass or the whole unit.

Book a free survey

Free survey
Fixed written quote
10-year workmanship guarantee

Where clear glass is missed most

Timber roof window with condensation fogging the glass — On a timber roof window where condensation has fogged the glass.

Roof window in a white loft ceiling with misting across the lower pane — On a white loft ceiling, where mist spreads across the lower pane.

Old roof window with a worn frame and cloudy glass in a plastered ceiling — On an old roof window, its worn frame holding cloudy glass.

Ten years on our workmanship. In writing.

Surveyed before we price
A fixed written quote that does not change on the day
Building Control notification handled

How the guarantee works

Roof window seen from inside with a tide line of moisture between the panes

The full guide

Misted rooflights, from the grey patch down

A rooflight that has misted between the panes has not got dirty. Its seal has failed, and moisture is now trapped inside the unit. This guide explains what has happened, what can be replaced, and how we decide between new glass and a new rooflight for a house in Derby or the towns around it.

It starts with the spot underneath, because that is where you notice the problem first.

Grey patch
New glass
Frame and flashing
Roof types
Daylight back
Safe access

Callout 1

Finding the misted rooflight in Derby that needs attention

What we look at from the landing up

The spot below: marked on the floor plan
Where the mist sits: inside, outside or both faces
Glass type: double, triple or single sheet
Frame and flashing: checked from the roof side
Opening size: measured to the millimetre

People rarely ring about a rooflight. They ring about the landing that has gone gloomy, or the kitchen worktop that never gets the sun it used to. Then they look up and see a milky smear in the glass, and it does not wipe away. That is the usual first sign of a misted rooflight in Derby, and it is almost always a sealed glass unit that has stopped being sealed.

A sealed glazed skylight unit is made from two or three sheets of glass held apart by a spacer bar around the edge. The gap between them is filled with a dry gas or dry air, and the edge is sealed to keep moisture out. Over time that edge seal can fail. Damp air gets into the gap, the desiccant in the spacer fills up, and the moisture starts to condense on the inside faces of the glass. You see it as fog, droplets, or a hazy film that comes and goes with the weather.

The first job is to be certain that is what you are looking at. A dirty skylight looks the same from underneath until you go outside, and grime on the outer face washes off. Mist between the panes stays put. If you can reach the glass from inside, run a cloth over the underside. If the cloudiness is still there, it is inside the unit. If the pane is a single sheet of glass, or a plastic dome, the cause is different and we cover that in replacing a cracked or yellowed polycarbonate dome.

What the spot tells us

We start from the place in the room that has lost its light. A rooflight over a hob has different needs from one over a bed. Fogged glass over a stair landing is a nuisance. Fogged glass over a desk, where you read and type all day, changes how the room is used. When we visit for the free survey we mark that spot on the floor plan, look up at the opening, and then go out on the roof to see what is above it.

The survey looks at the glass, the frame it sits in, the flashings around it and the way water leaves the roof at that point. A fogged unit is the visible problem. We want to know whether it is the only problem, because that decides what to quote.

How a seal fails

Seals fail for ordinary reasons. Sun heats the unit, the glass and the spacer expand at different rates, and the edge moves a little every day for years. Water that pools around the bottom of a frame will sit against the seal and work its way in. Older units were sometimes made with simpler sealants. A unit that took a knock during fitting or a later roof job may have a weakness from day one. None of this is anyone's fault, and none of it is unusual.

If you want the longer explanation, why has my skylight gone cloudy between the panes goes into the causes. If you want to know whether the fog can simply be removed, can misted skylight glass be cleared gives a straight answer.

Roof window or rooflight

The word matters for what comes next. A roof window is usually the openable, hinged kind that sits in the plane of a pitched roof, and VELUX is the best-known make. A rooflight is the broader trade term and includes fixed glazing, flat-roof units and lanterns. Homeowners say skylight for all of them, and that is fine. We use the trade words so the quote is precise. A roof window that has misted often has a replaceable glass pane. A flat-roof rooflight or a lantern with misted glass has a different construction and may be treated differently, as the later chapters explain.

Mist you can't wipe away is inside the unit, and the seal that kept it out has gone.

Callout 2

Replacement glass or a whole new unit

When the sealed unit has failed, there are two broad routes. One is to replace only the glass. The other is to take out the whole rooflight and fit a new one. Both clear the fog, because both put in a unit with a working seal. The right route depends on what the rest of the rooflight looks like.

When new glass is enough

If the frame is sound, the flashings are tight and the opening is a standard size, a new sealed glass unit can be made to fit the existing frame. The old unit comes out, the new one goes in, and the daylight comes back. This is the lighter job. It disturbs less of the roof and it usually takes less time. On a roof window, the glazed sash is often the part that is replaced, with the outer frame left in the roof.

We measure the old unit carefully, including the thickness, the type of spacer and the build-up of the glazing, because the new one must sit in the same rebate and seal against the same gaskets. A pane that is a few millimetres out will let in water. That is why measuring is a task for the survey and not a guess from a photograph.

New glass also lets us improve the performance. A modern sealed unit with low-emissivity coating and a warm-edge spacer loses less heat than a unit fitted a couple of decades ago. Where the frame can carry it, we can specify the replacement to do better than the original. If you want to compare the options in more detail, energy efficient rooflights explains how U-values work.

From grey glass to clear, in order

Survey
We view the spot, the unit and the roof, and measure the opening.
Fixed written quote
The chosen route and the reasons for it, in writing, fixed.
Order and make
The new glass or rooflight is made to the measured size.
Fit
Old unit out, new one in, flashings and seals checked.
Sign-off
We tidy the roof and room, and handle the Building Control notification.

When the whole rooflight should go

Sometimes the glass is only the first thing to fail. A timber frame that has been wet for years may be soft. A powder-coated aluminium frame may have corroded at its corners. Seals and gaskets may have hardened. Flashings may have lifted. In those cases a new pane in an old frame would only postpone the next problem, and the new glass would be sitting in something that cannot hold it properly.

A whole new rooflight then makes more sense. It brings a new frame, new weather seals, a fresh set of flashings and a current glazing specification, all fitted and tested together. It also gives the chance to change the size or the type. The spot that was lit by a fixed pane could be lit by an opening unit that also lets steam out. The kitchen worktop could do with a lantern instead of a flat pane. For that reason we treat a misted unit as a good moment to ask what the room needs, as well as what the roof has.

We set both options out in new glass or a whole new rooflight, with the signs that point each way.

Energy and Building Regulations

Swapping one rooflight or roof window for another is still regulated. Part L of the Building Regulations counts it as a controlled fitting. The limits sit in Approved Document L, Table 4.2. A roof window is held to the window figure, a U-value of 1.4 W/m²K, though band B on the Window Energy Rating scale also qualifies. A rooflight, lantern included, is held to 2.2 W/m²K. Whatever glazing we choose has to clear whichever figure applies. Where the job needs notifying, the Building Control paperwork is ours to deal with, so no certificate needs chasing afterwards.

Replacing only the glass in an existing frame is a different question from replacing the whole unit, and which rules apply depends on what is being changed. We confirm that at the survey and put it in the written quote.

A new pane in a tired frame only moves the problem a few years down the line.

Callout 3

What the frame and flashings are telling us

A misted unit is the symptom people notice. The frame, the kerb and the flashings are where the cause often sits, and they decide whether a simple glass swap is wise.

Signs that point to the frame

Look for staining below the rooflight on the ceiling or the lining. A brown ring or a bubbled patch of paint means water has been getting past something. Check the timber for dark patches or softness. Check metal frames for white powder or pitted corners. Check whether the rubber gaskets are cracked, flattened or pulled away from the glass. Any of these tells us the glass was not the only thing under strain.

Water that has been sitting in the bottom channel of a frame is one of the usual reasons a seal fails in the first place. If the channel is blocked, or the sill is flat, rain collects and presses on the edge seal for weeks at a time. Fit a new unit into that frame without fixing the drainage and the new seal will have the same fate.

Flashings

Flashings are the metal or lead strips that make the junction between the rooflight and the roof covering watertight. On a pitched roof they step around the frame and tuck under the tiles or slates. On a flat roof the junction is usually an upstand or kerb, covered with membrane. When we replace a whole rooflight we renew the flashings with it, because old flashings are cut to the old frame and rarely suit a new one. When we replace only the glass we still inspect them, and we say in the quote if they need attention.

What changes the quote for a misted unit

Glass only or whole unitA new rooflight brings frame, seals and flashings with it.
Roof typePitched, flat and lantern roofs need different access and finishing.
Opening sizeLarger units are heavier and need more hands and lifting gear.
Condition of the frameSoft timber or corroded metal may mean a full replacement.
Glazing specificationCoatings, tints and extra layers change the make-up.
AccessScaffold, or a clear approach to the roof, affects the set-up.

This matters for another reason. People often assume a leak is the roof when it is really condensation inside a failing unit. And people often assume fog is the glass when moisture is also coming round the edge. Neither is certain until someone looks from both sides. That is the reason we go on the roof during the survey and not only into the room.

Pitched and flat roofs

On a pitched roof, a roof window sits between the rafters. The opening is framed with trimmers, and the unit is held by brackets fixed to the timbers. A new unit matching the old opening leaves the structure untouched. If you want a larger one, a rafter has to be cut and trimmed, which is structural work and falls under Building Regulations.

On a flat roof, a rooflight usually sits on a kerb, which is a small upstand built up from timber or a manufactured section. The kerb is insulated and covered by the roof membrane. A misted flat-roof rooflight tells us to inspect that kerb. If it is damaged or too low, the replacement may need a new one. A flat-roof extension with a lantern has the same issue at a larger scale. Our page on flat roof rooflights explains how those are built, and roof lanterns covers the taller, glazed kind.

Planning and conservation

A like-for-like swap rarely changes what the roof looks like from the street. Where it does, for instance when moving from a conservation-style unit to a modern one, the planning position matters. Houses generally get their rooflights through permitted development, with conditions attached, and our planning guide is where those are laid out. Flats and maisonettes sit outside householder rights altogether. For a listed building, most alterations need listed building consent. Inside a conservation area, permitted development can be withdrawn by an Article 4 direction, so ask Derby City Council, or the council for your own town, before anything is ordered.

Callout 4

Glass, polycarbonate and GRP, and what each does when it ages

Not every cloudy rooflight is a sealed glass unit. Roofs in and around Derby carry several kinds of glazing, and each ages in its own way. Knowing which one you have saves a lot of confusion.

Sealed glass units

Double or triple glazed units are the standard in roof windows and in most modern rooflights and lanterns. They fail by misting, as described above. The glass itself stays clear, the fog is inside, and the unit is replaced as a whole sealed assembly.

Polycarbonate domes

Plastic domes are common on older flat roofs and extensions. They are light, cheap to make and easy to fit, so they were widely used. Polycarbonate and acrylic do not mist in the same way, but they discolour. Sun makes them yellow or grey over the years, the surface crazes into fine cracks, and the material becomes brittle. A dome that has gone brittle can crack in cold weather or when someone walks nearby. A twin-skin dome can also take in moisture between the skins, which looks like mist.

With a dome, replacement is usually the sensible answer. The material has aged all the way through, so cleaning or recoating would not bring back the strength. We cover the details in replacing a cracked or yellowed polycarbonate dome. A new dome is not the only choice. A glass rooflight on a proper kerb will perform better for heat and last longer, and we can quote both so you can compare.

GRP and multiwall sheets

GRP (glass-reinforced plastic) sheets and multiwall polycarbonate panels are used in some older lean-to roofs, conservatory roofs and small commercial buildings. They cloud as the surface weathers, and multiwall sheets can collect algae and damp in their channels. Because they run in large panels, the fix can be a full re-roof of the glazed area and not a simple swap. We do take small commercial jobs, and we apply the same survey first approach.

Why the roof shape still matters

Whichever material is up there, the roof shape decides how we get to it and how the new unit is finished. A steep pitched roof needs safe access and fall protection before anyone works near the edge. A flat roof needs a clear route up and a plan for the membrane. A lantern on an extension needs lifting gear. The table below puts the common types side by side so you can see which row looks like your roof.

Domes yellow and crack. Sealed glass fogs from the inside. The cure is different for each.

Which glazing do you have?

How each kind of rooflight glazing shows its age.
Glazing	Sign of age	Usual fix	Roof
Sealed glass unit	Fog between panes	New glass or unit	Any
Polycarbonate dome	Yellow, crazed, brittle	New dome or glass	Flat
Twin-skin dome	Damp between skins	Replace the dome	Flat
GRP sheet	Cloudy, fibres showing	Replace the sheet	Lean-to
Multiwall panel	Green algae in channels	Replace the panel	Roof
Lantern glazing	Fog in bars or panes	New panes or lantern	Flat

Callout 5

What clear glass gives back to the room

It is easy to think of a misted rooflight as a cosmetic fault. It is more than that. A hazy unit diffuses and absorbs daylight, so the spot below gets less of it and the light that does arrive looks flat. A kitchen worktop under a clear pane is bright enough to chop vegetables by. Under a fogged pane it often needs the lights on at ten in the morning.

What clear glass does for natural light and brightness

Daylight from above is the best light a room can have. It falls from a high angle, it reaches deep into the plan, and it changes through the day in a way that artificial lighting cannot copy. A landing with a clear roof window feels like part of the house. The same landing with a grey patch overhead feels like a corridor. A bedroom under the roof, with a clean rooflight over the bed, wakes up gently. A study with a clear pane over the desk needs less lamp time.

Many people also say a bright room is easier to live in. We do not claim any measured benefit, and we do not need to. What you can check is simple. Stand at the spot, look at the sky through the glass, and see whether it looks like the sky.

Condensation, mould and what mist can mean

Fog inside a sealed unit is not the same as condensation on the room-facing surface. Condensation on the inner face of a healthy unit comes from damp room air meeting cold glass, which is common in bathrooms and kitchens and is helped by an opening rooflight or a better-glazed unit. Mist between the panes is a seal failure. If you are unsure which you have, is a misted rooflight urgent helps you sort it out and says what to watch for in the meantime.

Where a failed seal has also let water get to the timber, damp patches and mould can follow around the lining. That is another reason to have the unit looked at before the frame has years of damp behind it.

Heat loss and heat gain

A failed unit loses its insulating gas, or the dry air fills with damp air that conducts heat more freely. The rooflight then loses more warmth in winter than it should. In summer, rooflights can add heat to a room, particularly on a south-facing slope. A replacement is a chance to choose glazing that does better on both counts. Solar control coatings reduce heat gain, low-emissivity coatings keep warmth in, and tinted or laminated glass can cut glare. We talk through these at the survey in terms of the spot. A bed under a south-facing roof window has different needs from a desk under a north-facing one.

For a deeper look at how rooflight energy performance is measured and compared, see energy efficient rooflights. If the room is a loft, a roof window is the likely candidate, and roof windows explains the range, including VELUX.

Opening or fixed

Because we are already removing the unit, it is worth asking whether a fixed pane is still right. A fixed rooflight over a landing is fine. A fixed rooflight over a cooking area or a bathroom gives you daylight but no ventilation. An electric opening rooflight can vent the room and close itself when it rains. Our page on electric opening rooflights covers those. Changing the type is a bigger job than swapping glass, and we say so in the quote, but it is the cheapest time to consider it, since the roof is being opened anyway.

Callout 6

The installation process: how the work is done safely, from survey to sign-off

Replacing a rooflight is work at height on a roof, and it deserves to be planned that way. The way we set up the job affects your house, your garden and the people on site.

Access and safety

Before anyone steps on the roof we decide how to reach it and how to keep people safe on it. On a pitched roof that can mean roof ladders, edge protection or a scaffold tower. On a flat roof it means a secure route up and protection around any open edge. For a large lantern, it can mean a crane or hoist. On a house with a loft conversion, we protect the room below from dust and falling debris, because opening a roof is untidy work and the floor under a window is often furnished.

Fall protection is not an optional extra. It is part of how the job is priced and programmed, and it is why access gets discussed at the survey. A single small rooflight on an easy roof is usually done inside a day or two of roof work. Anything bigger or fiddlier runs longer, and the quote says by how much.

Keeping the weather out

A rooflight swap should never leave your house open to rain. We plan the work so that the opening is closed up the same day, with the new unit fixed and weatherproof, or covered securely if something has to wait overnight. When the new unit is in, we check the flashings and seals before we leave the roof. A weathertight finish from the first night is the minimum we expect of ourselves.

Paperwork that is handled

A new rooflight that changes the roof structure is building work. Approval is needed from the local authority or a registered building control approver. Part L classes a straight swap as controlled work, and whatever goes in must reach the U-value for its type. Notification is dealt with by us whenever the job calls for it, and the fitting follows current regulations. Alongside that you receive a fixed written quote, plus a 10-year workmanship guarantee in writing.

Areas covered around Derby

We work in Derby and about 30 road miles around it, measured by road. Areas covered include the city, the Derbyshire towns to the north, the market towns to the west and the Nottinghamshire and Leicestershire towns to the east and south. The survey is free wherever you are inside that radius. Our area pages list the towns we cover.

Homes and small commercial

Most of what we fit goes into houses, from terraces near the centre to semis in the suburbs and newer estate homes on the edge. Small commercial roofs, such as a shop, office or workshop with a failing rooflight, are welcome too. The survey works the same way. Industrial or very large roofs belong with a specialist roofer.

Booking the free survey

The simplest way to start is the survey form. Tell us which room, which spot and what you can see. Photos help. You can also message us on WhatsApp at 07880 284133 or email info@skylarkskylightsderby.co.uk. We visit, look at the spot, the unit and the roof, and then send a fixed written quote. Start with the free survey form whenever you are ready.

The survey comes before any price, and the price you are quoted is the price on the day.

Misted rooflight FAQs

Do I need a whole new rooflight if the glass has misted?

Not always. If the frame, flashings and opening are in good order, the glass unit can often be replaced on its own. If the frame is soft, corroded or leaking, a whole new rooflight is the better choice. We decide at the free survey and set the reasons out in the quote. The page on new glass or a whole new rooflight shows the signs.

Why has my roof window gone cloudy between the panes?

The edge seal around the sealed glass unit has failed. Damp air has got into the gap, and moisture condenses on the inside faces of the glass. It is common with age, with heat movement and with water standing around the bottom of the frame. See why has my skylight gone cloudy for the full explanation.

Can the mist be cleaned out or dried out?

The moisture is inside a sealed gap, so there is nothing on the surface to clean. Once the seal has gone, the unit will mist again. The sensible fix is to replace the sealed unit with a new one. Can misted skylight glass be cleared goes through the options honestly.

Will a new roof window or rooflight leak?

A correctly fitted unit should be weatherproof from the day it goes in. That depends on sound flashings, a good kerb or frame, and careful sealing. We check each of those during fitting, and the work carries a 10-year workmanship guarantee in writing. If the old flashings are tired, we renew them with the unit.

Is there a Building Regulations rule for replacing a rooflight?

Yes, in two ways. A straight swap counts as controlled work under Part L, so the unit going in must reach the U-value for its kind. For a roof window that is 1.4 W/m²K, with Window Energy Rating band B as the alternative route. For a rooflight it is 2.2 W/m²K. Cutting a new opening changes the roof structure and needs sign-off from the council or a registered building control approver. We look after the paperwork.

How long does it take to replace a misted rooflight?

A straightforward single rooflight is usually a matter of a day or two up on the roof. Larger units, lanterns, awkward access or a frame that needs rebuilding can take longer. We state the expected time on the written quote after the survey, so you know before you agree.

Should I wait, or is a misted rooflight urgent?

Fog alone is not an emergency. It does mean the seal has gone, and the unit will not recover. The urgent signs are leaks, stained ceilings or soft timber around the frame. Is a misted rooflight urgent sets out what to watch for.

What if the misted unit is a lantern or a sun tunnel?

Both can be replaced. A lantern has panes in a framed structure, so we may replace individual panes or the whole lantern depending on the condition. A sun tunnel has a dome and a tube, and the dome is the part that tends to age. Look at roof lanterns and sun tunnels for the options.

Book your free survey

Where do you want the light? Tell us in the form: the worktop, the stair, the desk.

No obligation. We survey before we price.
We cover Derby and 30 miles around.