Misted and failed rooflight replacement in Cambridge, with clear light again

The clearest sign is moisture you cannot wipe away. Condensation on the room side of the glass on a cold morning is a ventilation matter and clears as the room warms. Misting inside the sealed unit is different. It sits between the two panes, comes and goes with the temperature, and over months leaves a white bloom or tide marks where water has dried and left its minerals behind. Once that bloom appears, the glass will not look clear again. Our guide to why rooflights mist between the panes goes through what is happening inside the unit, and the answer on why a rooflight goes cloudy separates misting from dirt, scratches and weathered plastic.

Other signs point to a unit that has reached the end of its life rather than just a failed seal:

• A cracked pane, whether from a thermal stress crack that starts at an edge or a knock from a falling branch or a dropped tool during other roof work.
• A yellowed, crazed or brittle plastic dome, typical of acrylic and polycarbonate units that have sat in full sun for decades.
• Rusting wire in old wired glass, which swells and splits the glass from the edges inward.
• Staining on the ceiling or the reveal around the unit, or a kerb that feels soft, which suggests water has been getting past a tired frame or a low upstand.
• Frames that have warped, sagged or lost their finish, and opening units that no longer close firmly.

Any one of these is a reason to look at the whole rooflight, not just the glass. The checklist in the signs a rooflight needs replacing is a useful thing to walk round the house with before a survey. If a pane has cracked, our guide to replacing a cracked rooflight explains what to do in the meantime and how the new unit is specified so it does not happen again.

This is the first decision in any replacement, and it is worth getting right, because a new sealed unit in a tired frame only resets one part of the clock. The questions we ask at the survey are practical ones. Can the glazing unit be taken out of the frame without destroying it? Is the frame still straight, with its thermal break intact and its drainage working? Is the upstand high enough, insulated and dry? Is the unit the right size, in the right place, for the way the room is used now? And will a new pane in this frame meet current thermal standards, or is the frame itself the weak point?

Frameless flat rooflights, where the glass is bonded directly to a kerb or a slim perimeter, are usually replaced as a complete unit, because the glass is the rooflight. Framed units vary: some are built so the sealed unit lifts out, and some bond the glass into the frame at the factory. Roof lanterns are often glazed with individual panels between bars, so a single misted panel can sometimes be swapped on its own; the answer on replacing a single roof lantern panel explains when that works and when the colour or coating of a new pane will not match its neighbours.

For the full comparison, including how the thermal performance of each route compares, see glass only or full unit replacement, and the shorter answer to can you replace just the glass in a rooflight? Whichever route suits your rooflight, the written quote says which one we recommend and why, before anything is ordered.

The old unit was probably chosen without much thought about the light. It may be clear double glazing on a south-facing roof that turns the kitchen into a hothouse from May to September, or a heavily tinted dome on a north-facing roof that makes a small bathroom feel gloomy even at midday. Replacement is the moment to put that right, and it is where our approach to planning the light earns its keep. Before we specify the new glass we look at the orientation of the roof, the hours the room is used, the surfaces the light will fall on and any glare problems you have lived with.

On a south or west-facing roof, solar control glass is usually the right call. Clear low-emissivity double glazing lets through somewhere around 60 per cent of the sun's energy; a good solar control coating brings that down to around 30 to 40 per cent while keeping most of the visible light. For a room that faced the afternoon sun under the old unit, that difference is the one you will feel. On a north-facing slope, clear low-E glass is often better, because heat is rarely the problem there and every bit of steady, soft daylight is worth keeping. East-facing rooflights over a kitchen or bedroom get bright, low-angle early sun, which is welcome in a kitchen at 7am and less welcome in a bedroom at 5am in June; that is a case for a blind as much as for a coating.

Every new rooflight we fit has a laminated inner pane. Overhead glass has to stay in place if it breaks, and a laminated pane holds together on its interlayer where old annealed or wired glass would drop. It also takes out much of the ultraviolet that fades floors and fabrics under an old unit.

Heat loss improves in almost every case. Part L of the Building Regulations sets a limiting U-value of 2.2 W/m²K for a new or replacement rooflight, measured in the horizontal plane, and good modern flat rooflights with thermally broken frames do considerably better than that. An old single-skin plastic dome loses heat several times faster. The answer on whether a misted rooflight loses heat explains how much of the difference comes from the failed seal and how much from the age of the unit. Our energy efficient rooflights page covers triple glazing and the trade-off between insulation and daylight.

On a flat roof, the rooflight sits on an upstand, a timber kerb built around the opening and weathered with the roof covering. Guidance for flat roofs calls for the top of the upstand to stand at least 150mm above the finished roof surface, so standing water and wind-driven rain cannot reach the frame. A great many older kerbs were built lower than that, sometimes barely above the felt, and a few have had a new roof covering laid over the top that has made them lower still. A new unit on a low kerb inherits the old problem on day one.

Once the old rooflight is off, we check the kerb timbers for rot or softness, look at how the old roof covering was dressed up the kerb, and measure the fall. A flat rooflight needs a fall of a few degrees so rain runs off the glass; units designed for flat roofs either build that fall into the frame or need a kerb that is taller on one side. If the kerb is sound, it may only need to be raised, insulated and re-dressed. If it is not, it comes out and a new one is built, insulated on its inner face so the cold does not bridge into the reveal and cause condensation on the ceiling around the unit.

The roof covering around the kerb then has to meet the new upstand properly. On a single-ply or liquid-applied roof in good condition, that usually means dressing new material up the kerb face and bonding it into the existing covering. On an old felt roof near the end of its own life, the better answer may be to replace the roof covering and the rooflight together; the guide to replacing rooflights during a reroof covers the timing. For older flat rooflights more broadly, including those set almost flush with the deck, see replacing an old flat rooflight.

On pitched roofs, the equivalent is the flashing kit and the lining. Replacement roof windows are usually made so the new frame fits the existing opening, and matching flashings allow the new window to sit at the right depth in the tiles or slates without disturbing the internal lining more than necessary. Where the old window was fitted with a gap around it or no insulation collar, the replacement is the time to fill it.

Planning first. Across Cambridge and South Cambridgeshire, planning is handled by Greater Cambridge Shared Planning. A replacement that looks materially the same from outside, in the same position, at the same size and projecting no further from the roof, is generally treated as maintenance rather than development. Adding or enlarging a rooflight on a house usually falls under permitted development, provided it projects no more than 150mm from the roof plane and does not rise above the highest part of the roof. Those rights do not apply to flats and maisonettes, and they can be removed in a conservation area by an Article 4 direction. Listed buildings always need listed building consent for changes that affect their character, and that can include swapping one rooflight for another of a different type. The answer to do I need planning permission to replace a rooflight? covers the common cases, and replacing like for like in a conservation area goes into the detail for the city's many conservation areas and the villages around it. If you are unsure, the council's constraints map on the Greater Cambridge Planning website is the place to check.

Building Control is a separate question. In England, replacing a whole window or rooflight in an existing home counts as building work: the new unit has to meet the thermal standard in Part L, the safety requirements for overhead glazing, and fire rules where the roof is close to a boundary. The work has to be either self-certified under a competent person scheme or notified to Building Control. Where the replacement is notifiable, we handle that notification for you, and you receive the completion paperwork at the end. Replacing only the glass in a sound frame is generally not notifiable, though the new unit should still be specified to current standards.

Where the structure changes, because the opening is enlarged or moved, Building Control will want to see how the trimmed joists or rafters are supported. Our guide to Building Control for replacement rooflights sets out which jobs are notifiable and what the inspector looks at.

Leasehold flats add one more layer. The roof, and often the rooflight itself, usually belongs to the freeholder, and the lease will say whose job replacement is and whose consent is needed. The answer on replacing a rooflight in a leasehold flat covers the questions to ask before booking the work.

At the survey we look at the rooflight from inside and, where it is safe to reach, from outside. We measure the opening and the kerb or frame, note the make and model if there is one, check the orientation of the roof and the way the room is used, and look for any staining or softness around the unit. That gives us the route (glass only, whole unit, or a changed opening), the glass specification for the orientation and the access we will need. The written quote sets all of that out with a short light plan, so you can see where the light from the new unit will land and why we have chosen the glass we have.

Access is planned before the day. A single-storey flat roof can usually be reached with ladders and edge protection or a small tower. A rooflight high on a pitched roof or on a taller building may need a tower or scaffold, while many roof window replacements are done from inside. The answer on whether replacing a rooflight needs scaffolding explains how we decide.

On the day itself, a typical whole-unit replacement on a flat roof runs like this:

• Floors and furniture under the rooflight are covered, and a dust screen goes up where the room needs one.
• The old unit is taken off from outside, with the opening covered straight away so the room is never open to the weather.
• The kerb is checked, raised or rebuilt, insulated and weathered into the roof covering.
• The new rooflight is bedded and fixed, and its seals and drainage checked.
• Inside, the reveal is lined, plastered or trimmed and left ready for decoration.

A glass-only replacement or a like-for-like roof window sash can take a few hours. A whole flat rooflight on a sound kerb is usually a day, and a lantern, a rebuilt kerb or a changed opening can take two or three. We pick a dry forecast for the part of the job when the roof is open. People understandably ask about the room below, and the answer on whether replacing a rooflight will damage my ceiling explains how the plaster around the reveal is protected and made good.

The old unit is taken away with us. Glass, aluminium and timber are separated for recycling where the local facilities allow; see what happens to the old rooflight. The installation is then covered by our 10-year workmanship guarantee, alongside the manufacturer's own warranty on the new product.