Misted and failed rooflights replaced across Liverpool

Every double-glazed unit relies on its edge seal for the whole of its life, and a roof is a tougher home for one than a wall. The glass faces the sky, so it takes the full swing of the weather: frost on a clear night, strong sun on a July day, then a cold downpour on the same glass. Each swing makes the panes and the spacer expand and shrink by tiny amounts, and over the years the sealant is asked to stretch with them. Eventually it pulls away from the glass at one corner, and that corner is where moisture starts to creep in.

Standing water makes it worse. A flat rooflight sits on a kerb, and if the roof around it does not drain well, water can lie against the frame for days after rain. Sealants are not designed to live underwater. A rooflight with moss at its upstand or a puddle against one side often shows misting on that side first, and the haze then spreads across the unit.

Sun is the other pressure, especially on units glazed into the bars of a lantern. Dark frames soak up heat and pass it to the edge of the glass, which is hot in the afternoon and cold again overnight. Sealants harden with that treatment and lose their grip. Roof windows meet the same stresses, although the frame often shades the edge of the inner pane a little.

Poor original fitting is a smaller cause but a real one. A unit squeezed into a frame too tightly, or sitting on the wrong packers, is under strain from day one. Drainage channels blocked with paint or debris have the same effect as a puddle on the kerb, because water sits in the glazing rebate and works on the sealant from outside.

We cover the mechanics in full in the guide to why rooflight glass mists between the panes, including where the weak points are on different types of frame. The short version for this page is that nothing you did made it happen, and that waiting does not make the unit recover, because a failed seal does not mend itself.

A misted rooflight still works as a roof, but it no longer works as a window.

That distinction takes the panic out of the decision. A clouded unit rarely leaks, so you have time to arrange a survey, compare the options and choose a sensible date. What you should not do is leave it for years, because the condition that let moisture in at the edge of the glass can also be letting it into the frame.

A word about the people who have this problem. It tends to turn up in the same few places: the flat rooflight over a kitchen extension added to a terrace or a 1930s semi, the lantern on a larger extension, and the roof window that lights a loft room or landing. Each of those has its own pattern, so it is worth knowing which one you have.

The right answer depends on what is on your roof, so we start by naming it. Failed units in Liverpool houses fall into four families, and the choice of route follows from the family.

Flat glass rooflights on a kerb. These are the rectangular units on kitchen extensions, porches and garage conversions, with a flat or very slightly domed glass top on a raised frame. The sealed glass unit sits in a rebate in the frame, held by a bead or a cap. If the frame is still square and sound, the unit can sometimes be lifted out and replaced with a made-to-measure new one. If the frame has corroded or the kerb has failed, the whole rooflight comes off. Our flat roof rooflights page describes the products that go in.

Plastic domes. The domed rooflights that went onto thousands of flat roofs in earlier decades were usually acrylic or polycarbonate, often two skins with an air gap. These do not mist like glass. They yellow, craze or collect dirt between the skins. A sealed glass unit cannot go into a plastic dome frame, so the dome comes off and the choice is a new dome or a glass rooflight on a new kerb. For most kitchens we steer people towards glass, which stays clear and insulates better.

Roof lanterns. A lantern is a glazed roof structure, with bars rising on each face to a ridge, and the glass sits between the bars in separate panes. Because the panes are separate, one misted pane can sometimes be replaced without disturbing the rest, provided the bars and cappings are sound and the new unit can be glazed neatly. The guide to replacing just the glass in a roof lantern explains when that works and when it does not. For a new lantern altogether, see roof lantern installation.

Pitched roof windows. A roof window in a slate or tile slope is a framed sash with the glass held in it. On most makes the sealed unit is not a loose part: the sash and the glass are made as one, so the usual route is a replacement sash where the maker still supplies one, or a new window with its own flashing. If you have a VELUX or equivalent window with misted glass, our VELUX window installation page covers what goes in its place.

Whatever the family, the first look is always at the same thing, which is the surrounding structure. The kerb, the flashing and the covering that surrounds the glass are what keep the house dry for the next ten years, and they are where a careful eye earns its keep.

Under the Building Regulations 2010, England treats a new or replacement rooflight as controlled work. You will not need to contact the council or a private inspector yourself, because the Building Control notice is ours to send whenever the job needs one; the notice can go to the local authority or to a registered building control approver, a title private inspectors have carried only since April 2024. At the end, a completion certificate comes from the council or a final certificate from the approver, and we pass it on to you.

Whether a particular job is notifiable depends on what is being replaced. A new rooflight or roof window going into the opening is clearly work to tell building control about. New sealed glass going into an existing frame is a finer question, and we settle it at the survey, because the answer turns on what is being changed. We would rather ask before the work than after it.

The thermal side is straightforward. Existing dwellings are held to 2.2 W/m²K for rooflights by the 2021 edition of Approved Document L volume 1, in its Table 4.2, while roof windows need Window Energy Rating band B or a U-value of 1.4 W/m²K. Roof lanterns count as rooflights. England moves to the 2026 edition on 24 March 2027; the existing-dwelling figures stay as they are and the table becomes Table 3.2. An old unit from decades ago will probably fall short of these figures, and a new one is chosen to meet them, which is part of why a replacement makes a room feel better to sit in.

Planning is rarely an issue when the new unit matches the old. Under Class C in Part 1 of Schedule 2 of the 2015 General Permitted Development Order, a rooflight in England is normally covered, as long as the limits that apply are respected. A replacement with matching dimensions, at the same height, does not change the roof in a way that raises fresh questions. The exceptions are flats and maisonettes, which do not have those rights, listed buildings, which need listed building consent, and properties under an Article 4 direction. Our guide to planning permission for a skylight goes through the conditions properly, so we do not restate them here.

If your rooflight sits on an older slate slope in a sensitive setting and the unit has failed, you may want to look at the slimmer styles covered on our conservation rooflights page, which sit low in the slope and suit older roofs better than a standard box.

Across Flintshire and Wrexham the same job runs under the Welsh version of the rules, and our surveyor will tell you which applies to your address when we book the visit.

Send us a few photos on WhatsApp, or phone, and tell us roughly where the rooflight is and what you can see in it. We will say what looks likely, book a survey and come to measure. The survey is where we get onto the roof, look at the frame, check the kerb or flashing and confirm the size of the opening. No price is given before that, and none changes afterwards unless you ask us to do something extra.

Once we have surveyed, a fixed quote follows in writing. It names the work, the unit and what happens to the old one, so nothing is left to surprise you later. If a glass-only job and a whole-unit job are both worth considering, the quote shows both side by side, with our recommendation.

When the work is done, the workmanship guarantee runs for 10 years and is given in writing. Our guarantee page explains what it covers. If the job is notifiable we make the Building Control notification ourselves, so there is no certificate for you to chase from the council.

We cover houses of every age around Liverpool, from slate terraces with an old rear rooflight to 1930s semis with a kitchen extension on the back, and our house types guide shows how rooflights sit on each. If misting has made you think about the rest of your roof glass, two other pages may help. Energy efficient rooflights looks at what a better-performing unit can do for a room, and roof window installation covers the pitched side.

Cloudy glass between the panes is a fault in the sealed unit, and the survey establishes what is in the frame before anything is chosen to go into it.

It also helps to tell us how the room is used. A rear kitchen with a lantern over the dining end, a landing roof window that lights a staircase and a utility room with a small flat rooflight all call for slightly different thinking, from the glass specification to how the new unit is opened or ventilated. Tell us what bothers you most, whether that is glare, a cold room or simply the look of the cloud, and the quote can be built around it.

Photos sent our way are all it takes to start. Get a free quote