Misted rooflight replacement in SouthamptonClear glass, back on the kerb.

A sealed rooflight unit is two panes of glass bonded round the edge with a gap between them, and the gap is meant to stay closed for the life of the unit. The edge seal is a flexible bead that has to cope with sun on the surface, cold at night and the movement of the roof as it warms and cools. In time it can lose its grip, and when it does, moist air finds its way into the gap.

The first sign is usually a faint haze that comes and goes with the weather, seen most clearly early on a bright morning. Later the moisture leaves a permanent film, often with a pale water mark along one edge, and the view of the sky turns grey and blurred. At that stage the insulating gas has also escaped, so the unit holds the heat in less well than it did, which you may notice in winter as a cooler area of ceiling.

The moisture is inside the unit, so wiping the glass does nothing. Our answer to why a rooflight mists between the panes explains how the seal fails and how to tell it from the ordinary surface condensation that clears by midday.

Frames and domes

Misted glass is not the only reason a rooflight comes up for replacement. A frame may have warped so the sash no longer closes squarely, the seals round an opening may have hardened, or the surround may be stained from years of weather. Older plastic domes go yellow and brittle, and some have cracked at the fixings.

A failed unit is easier to diagnose with a picture than a description. If you can send photographs of the rooflight from inside and out, with the frame edge visible, we can say what looks likely before we visit, and the survey then confirms it on the roof.

The scope of the job decides whether the Building Regulations are involved. Replacing a whole rooflight, meaning both the frame and the glass, is a controlled fitting under the regulations, so the work is notifiable to a building control body and the replacement must satisfy the present energy limit. Replacing only the sealed unit inside the existing frame is not notifiable.

We deal with notifiable work and tell you exactly what has gone to the building control body. The new unit is specified against Part L for an existing home, with 2.2 W/m²K as the ceiling for a rooflight and the ordinary window limit for a roof window. Our guide on rooflights and the Building Regulations covers the framework, and whether replacing a rooflight needs approval answers the question directly.

Some owners wonder whether they can have the sealed unit changed now and the rest of the rooflight later. That is possible, and the glass-only route does not trigger notification. The point to watch is that a later whole replacement will bring the current energy limit with it, so a frame that is plainly near the end of its life is often better changed at the same time as the glass, with the paperwork done once.

Planning is a separate matter

Building Regulations deal with how the work is done, while planning deals with whether a change may be made at all. A straight swap in the same opening usually raises no new planning question, but a rooflight on a listed building or in a conservation area can be subject to extra controls, and a larger or higher unit than before is a change.

At the survey we look up the planning position for your address and report back. The decision rests with the council, so we never presume the outcome. For the general rules on permission and how to check, see our planning guide.

One more practical point: if your home is sold or remortgaged later, a buyer's solicitor may ask whether a replaced rooflight had its paperwork. A job that has been notified properly is the easiest to answer, and that is why we deal with it at the time and not leave it for you to find out afterwards.

If the flat roof around a failing rooflight is due to be re-covered, the rooflight should be part of that plan. A new membrane is laid up the kerb and tied into the frame, and if the rooflight is then replaced afterwards, that junction has to be disturbed and made good, often by a different firm. It is more sensible to renew the rooflight first and let the roofer dress the new covering to it.

The sequence is simple in outline. The old rooflight comes out and the kerb is checked, rebuilt or insulated as needed. The roofer lays the deck covering and takes it up the upstand. Then the new rooflight is lifted on and fixed, with the frame sealed to the finished kerb, and the roofer returns for any final dressing the manufacturer requires.

On the day, the first job is to protect the room under the opening. We lay dust sheets and clear the floor, since the old unit will release dirt and debris when it is lifted out. For a large or heavy unit we agree the access beforehand, including where the lift will take place and whether scaffolding or a hoist is needed.