You notice it first on a bright morning. The kitchen rooflight that used to show a clean square of sky now has a grey bloom in one corner, a film that never wipes off because it is not on either outer face. A few weeks later the haze has spread, and on cold mornings there are tiny droplets running down the inside of the glass where nobody can reach them. The unit has lost its seal, damp air has found its way between the panes, and the glass will not clear again by itself.
What happens next depends on the frame the glass sits in, not on the glass alone. If the frame, the kerb and the weatherproofing around them are sound, a new sealed unit may be all the rooflight needs. If they are not, a whole new rooflight is the better answer. The two routes differ in cost, disruption and paperwork, so it pays to know which one you are on before anyone is booked. Below is how we separate them, what each looks like on the day, and when the work has to be notified to building control.
What misting tells you
A double or triple glazed unit is a sandwich: sheets of glass held apart by a spacer bar around the edge, with the gap between them sealed and filled with dry air or gas. The edge seal is the part that fails. Once it does, moist air passes in, the desiccant in the spacer bar (a drying agent) takes up what it can, and when that is full the moisture shows as mist, droplets or a chalky mark between the panes.
That position, between the panes, is the useful clue. Condensation on the room-facing surface comes and goes with the weather and wipes off. Mist inside the cavity does not wipe off from either side, though it may fade on a warm afternoon and return the next cold night. If you can touch the surface and the mark sits behind your finger, the unit has failed. Our page on why a rooflight mists between the panes goes through the causes.
A failed unit also insulates less well, because the sealed gap that gave it its performance has gone. That is a reason to plan the work, though a clouded unit is usually a nuisance for months before it becomes anything else, and the time lets you find out which route you are on. We note where the clouding starts, because a failure that begins at the bottom edge often points to standing water in the frame, which we need to look at before we price new glass.
Checking the frame and kerb
A new sealed unit is only as good as what it sits in. Before recommending one, we look at five things, in an order that matters because the first ones decide whether the rest are worth checking.
- The frame. Aluminium should be straight and free of corrosion at the corners. Timber should be firm when a screwdriver tip is pressed into the underside. PVC should show no warping or cracked welds.
- The beads and gaskets. These hold the glass and carry the weatherproofing. If they are loose, brittle or missing in places, a new unit will not stay weathertight.
- The kerb. The upstand the rooflight sits on should be sound and level. A common minimum is a kerb that rises at least 150mm clear of the roof covering, to the maker's instructions.
- The covering at the base. The membrane or flashing that dresses up to the kerb has to be intact and properly lapped, because new glass cannot make up for an open joint beneath it.
- The glass pocket. The new unit is made to fit the existing frame exactly, so the frame must be square and the measurements taken with care.
Most of this is visible from inside and from the roof edge, and it forms part of the free survey. If the frame is sound and the kerb and covering are in good order, a new sealed unit is a straightforward route. If the survey finds timber gone soft, a frame that has moved or a covering that is failing, we say so in writing, because new glass in a tired frame only moves the problem on.
Frame depth is a point to watch. A frame rebated for a thin unit may not take a thicker, better-performing one without the beads or rebate being altered, and that changes the job. We measure the pocket, record the thickness it can accept and put that in the quote.
| Finding | Usually means |
|---|---|
| Sound frame, sound kerb, clouded glass | A new sealed unit |
| Soft timber or corroded aluminium at the base | A whole new rooflight |
| Frame cannot take the thickness needed | A whole new rooflight |
| Covering at the kerb is failing | Roof work first, then glass |
| Old plastic dome in a thin frame | A glazed replacement |
Sealed unit replacement
Replacing the sealed unit means taking the old glass out of its frame and fitting a new unit made to the same outer size. The frame, the kerb and the covering stay as they are. On a flat rooflight the glass comes out from the roof side, so the fitters work on the deck and the room below needs only the usual floor protection.
The new unit is made to measure. We record the outer dimensions and the depth of the glass pocket, and the maker builds the unit with the spacer, coatings and gas fill listed in the quote. A low-emissivity coating and a warm-edge spacer are the common choices, and where the frame has room the build-up can be better than the original. Because the unit is made for your frame, the order waits until the frame has been checked and the sizes confirmed.
On the day, the fitters remove the beads, lift out the clouded unit, clean the pocket, seat the new unit on fresh setting blocks and tape or gaskets, and refit the beads and capping. Glass is heavy, so lifting gear and enough hands matter on larger panes. The kerb and covering are not disturbed, so the job is quicker and tidier than a full replacement, and a finished extension does not need its decoration touched.
Roof lanterns and multi-pane glazed roofs may carry several units, and not all will have clouded together. It is usually sensible to price them all at once, since the same access and lifting gear serve the whole roof and the clear ones share the age and exposure of the failed ones. Our page on reglazing a roof lantern covers that case.
Whole rooflight replacement
Replacing the whole rooflight takes out the frame and glass together and sets a new unit on the kerb. It is the right route when the survey finds a problem in the frame, when the existing unit is a dated or basic design, or when the opening needs to perform beyond what a new pane in an old frame can give.
A new rooflight brings a fresh frame, new weatherproofing at the kerb and glazing designed together with that frame. It also lets you change things the old unit did not allow: a fixed unit can become an opening one, a dome can become flat glass, and the clear size can alter if the kerb is adjusted. The commonest version of that change is covered in our guide to replacing an old polycarbonate dome.
The work is more involved than a glass swap. The old frame is released from the kerb, and the membrane or flashing dressed to it is cut back and renewed. The new unit is set and fixed, and the roofer dresses the covering up to it again. If the kerb must change in height or plan, that is done first, so the covering finishes against the new frame instead of the old one.
Inside, the lining and reveal may need work. If the new frame differs in size, the plasterer or decorator comes back for the reveal, and that visit sits in the programme so the opening is not left half-finished. Where the unit opens electrically, the electrician's cable is in place before the reveal is closed.
A whole replacement takes longer than a glass swap, but it deals with frame, kerb detail and glass in one visit, with one delivery and one handover. Our costs page sets out how rooflight prices are built up, and the written quote after the survey gives the figure for your own opening.
Which route is notifiable
The two routes sit differently under the Building Regulations in England. Replacing only the sealed unit, with the frame staying in place, is not notifiable. Changing the whole rooflight, meaning frame and glass together, counts as a controlled fitting, so a Building Control notification goes in.
A notifiable replacement has to meet the energy limits in Part L (2021 edition), Table 4.2, for existing dwellings. For a roof window the ceiling is the ordinary window figure of 1.4 W/m²K, and the rating label route is an alternative way to show it. A rooflight, meaning a unit in a flat or low-pitched roof, takes 2.2 W/m²K. Which limit applies follows the type of unit, and the quote states the figure for the one we propose.
We handle the notification where the work is notifiable, and the paperwork is in your hands at handover. The inspector may come from the council, or from a private firm, which since April 2024 is titled a registered building control approver. Our page on Building Regulations for rooflight replacement goes into the mechanics.
A sealed unit swap needs no notification, but it is still work with a specification. Keep the quote and the glazing details together, because a buyer's solicitor or a later builder will ask for them.
Planning is a separate question. Swapping a like-for-like rooflight in an existing opening does not normally raise one, but listed buildings, conservation areas and some National Park land have their own rules. We look up the position for the address and report it, and the council decides. If the size or shape is changing, the planning permission guide explains how to check.
Ordering and timing
Both routes depend on glass made for your opening, so the order matters more than the fitting. The survey takes the measurements and checks the frame and kerb. The written quote then lists the glazing, the production wait and anything needed from your builder. When you accept, the order goes to the maker, counted back from the day the roof needs the glass.
The lead time comes from the maker and is written into the quote, and the delivery date is fixed at the point of order and passed to you and any builder involved. Fitters are booked only once the glass has a date. Our page on how long a rooflight replacement takes explains the stages.
- Measure and check: the survey records the frame, pocket and kerb.
- Quote: glazing, lead time and builder requirements, in writing.
- Order: placed against the day the roof needs it.
- Fit: the roof is made weathertight the same day.
- Finish: reveal work and decoration follow.
Coordinate with other work. If the flat roof is about to be covered afresh, a failing unit should go in at the same time, since the covering is dressed up to whatever sits on the kerb. Our guide on replacing a rooflight during a re-cover describes the sequence. If the covering is sound, the replacement fits any dry spell.
To start, ask for a survey through the misted rooflight replacement service and send a photograph of the clouded unit. The survey is free, the quote follows in writing, and the work carries our 10-year workmanship guarantee.
Smaller questions about clouded glass
- Can the mist be cleaned out from between the panes?
- No. The moisture is inside a sealed cavity, and there is no practical way to open it, dry it and close it again. The unit is replaced with a new one made to the same size.
- Will a clouded unit fail suddenly?
- A clouded unit is a failed seal, not a broken pane, so it still keeps weather out. It insulates less well and looks poor, which is why the replacement is worth planning, without being urgent.
