Replacing a failed flat rooflight

A sealed glass unit is two or three panes bonded to a spacer bar around the edge, with the cavity filled with dry air or argon and sealed against moisture. The seal is doing one job: keeping the inside of the cavity dry. When it goes, water vapour gets in, and the first thing you see is misting on the inside of the glass that you cannot wipe off because it is between the panes. It comes and goes with the temperature at first, worst on a cold bright morning, then becomes permanent as a milky haze or a tide line of mineral deposit. Once a unit is misting, it is finished. There is no bringing the seal back.

Plastic domes and pyramids fail differently. Acrylic and polycarbonate both degrade under ultraviolet light, and a flat roof points its rooflights straight up at the sky, so they take the full dose. The plastic yellows, goes brittle, and develops fine surface crazing that scatters the light and turns a once-clear dome cloudy and grey. Older twin-wall polycarbonate greens up with algae in the flutes and starts to sag. A dome that has reached that state is not going to improve, and it will crack the next time someone has to walk near it.

Then there is the quiet thermal failure, the one that does not look like a fault at all. An older single-skin rooflight, or a thin double unit fitted twenty years ago, runs cold on its inner face all winter. Warm damp air from the kitchen below hits that cold glass, condenses, and runs down onto the frame and the ceiling line. People blame the seal, but the unit is intact. It is simply a poor thermal performer, and the answer is a warmer unit, which counts as a replacement upgrade rather than anything a roofer can address.

Most failures fall into a handful of patterns. Knowing which one you have tells you whether the whole rooflight is coming out, or whether the problem sits in the roof around it.

The pattern worth noticing is that some of these live in the rooflight and some live in the roof. A misted unit is the rooflight. A ceiling stain after every downpour is very often the roof around it. Sorting one from the other before anyone quotes saves you paying for the wrong work.

This distinction matters because it decides who you call. Replacing a failed rooflight, glass or plastic, seal-blown or thermally spent, is glazing work and it is what we do. Chasing a leak that turns out to be in the flat roof itself is roofing work, and a roofer is the right first call. The two get confused constantly, because a leak near a rooflight looks like the rooflight leaking.

Here is the honest test. If the fault is inside the glass or the frame of the rooflight, the misting, the crazed dome, the cold pane, the worn gasket, then the unit needs replacing and that is a straightforward swap. If water is getting in around the rooflight, at the flashing, the felt or membrane dressing up the sides, or through a kerb that has rotted, then the roof covering is the problem and it needs a roofer to lift and investigate before anyone talks about new glass. Dropping a brand new rooflight onto a tired upstand with failing flashing simply moves the leak along by a season.

Often the two overlap. A dome that cracked years ago has been letting a trickle in ever since, the plywood kerb underneath has softened, and now both the rooflight and the timber it sits on have to be dealt with. In that case the sequence is: sort the structure and the weathering first, form a sound upstand, and then fit the new rooflight onto something worth fitting it to. We survey the whole opening before quoting so nobody is surprised halfway through.

A flat rooflight does not sit flat on the roof. It sits on an upstand, a raised kerb built up above the roof line so that water drains away from the glass rather than towards it. The upstand is usually 150mm or so of insulated timber, and the rooflight is sized to sit over it. This is the part people forget when they think a replacement is a simple like-for-like glass swap. The new unit has to match the existing kerb, or the kerb has to be adapted to the new unit.

Two measurements matter, and they are easy to mix up. The structural aperture is the hole in the roof. The kerb external size is the outside of the timber upstand the rooflight clamps down over. Manufacturers quote flat rooflights either by the aperture they suit or by their own external frame size, so a unit ordered against the wrong reference will not fit. On a replacement we measure the existing upstand rather than guessing from the old glass, because the old rooflight may itself have been the wrong size for the hole.

Standard flat glass rooflights come in a spread of sizes, and picking a stock size keeps both the cost and the lead time down.

• Small squares: 600 by 600, 700 by 700, 800 by 800mm, common over hallways, landings and bathrooms.
• Mid squares: 1000 by 1000 and 1200 by 1200mm, the workhorse size over a kitchen or a single-storey extension.
• Rectangles: 1000 by 1500, 1000 by 2000 and 1200 by 2000mm, used where you want a run of light down a galley kitchen.
• Large fixed panes above 2 metres on a side, which move into structural glass territory and need the opening designed around them.

If your failed rooflight is an odd size, a replacement is the moment to decide whether to keep the awkward dimension or rebuild the kerb to a standard one. Standard is cheaper and quicker. A bespoke unit is worth it when the opening is genuinely fixed by the structure below.

A replacement is a chance to correct whatever the old rooflight got wrong, and on a flat roof that almost always means the glass specification. A flat rooflight faces straight up at the sky, which makes it the most exposed piece of glazing on the whole house. It collects more solar energy per square metre than any pitched or vertical unit, because the summer sun is closest to square-on when it is high overhead. That is a comfort question, and in south Essex it is the question.

Two numbers describe the new glass. The U-value is winter heat loss in watts per square metre per degree, and lower is warmer. Flat glass rooflights run from around 1.5 down to about 0.8 W/m²K depending on the build-up, and a warmer unit is what cures the condensation problem described earlier. The G-value, or solar factor, is the share of the sun’s energy that comes through, from around 0.6 for clear glass down to below 0.3 for solar-control glass. On a flat roof over a room you actually use, the G-value is the number that decides whether the room is pleasant on a July afternoon or unusable.

This is the heart of what we do. Southend faces south across the estuary, with an open horizon and reflected light off the water, so a flat rooflight over a south-facing extension is a solar collector with a sofa underneath it. A blown clear-glass unit replaced with another clear-glass unit clears the misting and leaves the overheating exactly where it was. Replacing it with a solar-control unit takes out more than half the heat while keeping the room bright, because good solar-control glass blocks the invisible near-infrared and lets the daylight through.

• Over a south or west-facing room, specify a G-value around 0.3 or below. This is the single biggest improvement a replacement can deliver.
• Over a north-facing room or a hallway, keep the G-value high, around 0.5 to 0.6, and take all the daylight you can get.
• On anything overhead, the inner pane must be laminated glass under Part K, so a shattered pane holds together rather than falling into the room. This is not optional on a rooflight.
• If the room overheats badly, an opening unit clears the hot air that gathers at ceiling level. We fit electric opening rooflights where a flat roof puts the glass out of arm’s reach.

For the wider case on how a flat roof collects light and why the glass specification comes before the choice of unit, our flat roof skylight installation page walks through the whole approach.

A like-for-like glass swap on a sound upstand is short work. The old rooflight is unclamped and lifted out, the kerb is checked and cleaned, the new unit is bedded onto fresh seals, clamped down and the weathering is dressed back in. On a straightforward flat rooflight this is often a half-day to a day, and the room is watertight the same day.

Where the upstand has suffered, the job grows. A rotten timber kerb is cut out and rebuilt, insulated and made good, the membrane or felt is dressed up the new upstand, and only then does the new rooflight go on. Moving from a plastic dome to a flat glass unit usually means adapting the kerb, because domes and glass rooflights sit on different upstand heights and profiles. None of this is complicated, but it is why an accurate survey beats a phone quote every time.

A new or enlarged rooflight is notifiable under Building Regulations. Part L covers the thermal performance of the glass, and Part K covers the safety glazing overhead. A straight replacement of an existing rooflight in the same opening is treated more lightly than forming a new hole, but the thermal standard still applies to the unit you fit. We make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the work, so the paperwork is handled rather than left for you to chase.

These are industry-typical ranges for guidance, not a quotation. The spread depends on the size, whether it opens, the state of the upstand and how the roof is reached. Treat them as the shape of the market.

The one figure worth spending on rather than saving on is the glass. Upgrading from a clear unit to solar-control glass adds a modest amount to the pane cost and delivers the biggest change you will feel in the room, so it is poor economy to replace a blown south-facing rooflight with the cheapest clear unit and live with the same overheating for another decade. Our costs page sets out how the wider work is priced.

The flat roof stock across this patch is dominated by the rear extension. The 1930s semi belt that runs through Southend, Westcliff, Leigh and out towards Rayleigh has been extended at the back in huge numbers, and those extensions almost always carry a flat or shallow roof over a knocked-through kitchen and dining space. The rooflights over them are now reaching the age where the first generation of sealed units and plastic domes is failing in bulk. Many of them face south into the garden, which is exactly where the solar-control question bites hardest.

Rooflights on a house are generally permitted development where they project no more than 150mm above the roof plane and sit below the ridge, so a straight flat-roof replacement rarely needs planning permission. The exceptions matter: flats and maisonettes, listed buildings, and the conservation areas and Article 4 zones around Leigh, where permitted-development rights for roofing and windows have been removed and a planning application is needed where it normally would not be. Treat all of this as general guidance and check with the local authority before you commit, because the boundary of an Article 4 zone can run down the middle of a street.

If your flat rooflight has misted, yellowed or gone cold, the sensible next step is a survey that measures the upstand, reads the aspect of the roof, and puts a G-value and a U-value against the replacement for your specific room. You can ask us for that specification with both numbers on it, so the new rooflight is chosen for the roof it is going into rather than picked off a shelf to match the old one.