What is the lifespan of a flat rooflight?

A flat rooflight is really three things stacked together: the sealed glass or plastic unit, the frame that holds it, and the kerb or upstand it is bedded onto. They age at very different rates, so the useful answer to how long the thing lasts depends on which part you mean.

The sealed glazing unit is the clock. Two panes of glass are held apart by a spacer bar and sealed around the edge, with the cavity between them filled with argon and kept dry by a strip of desiccant inside the spacer. That edge seal is doing a hard job. It has to stay airtight for decades through every hot afternoon and cold night the roof throws at it. When it eventually breaks down, moisture creeps into the cavity, the desiccant saturates, and you get condensation between the panes that no amount of cleaning will shift. That misting is the classic sign a unit has reached the end of its working life. On a good double-glazed unit that clock runs 15 to 25 years, and the glass at the top of the range tends to be the units with a warm-edge spacer and a well-made secondary seal.

The frame is the slow part. A powder-coated aluminium frame is largely inert and will comfortably outlast the glass it holds, often reaching 30 years and beyond with nothing worse than a tired gasket. When a failed unit comes out, the frame and kerb are frequently still sound, which means the sensible job is a like-for-like replacement of the glazing into the structure that is already there.

The headline figures assume the rooflight was installed properly and sits on a dry, well-drained roof. Several things push a unit towards the shorter end of its range, and most of them are avoidable.

• Standing water. A flat roof is never truly flat; it is laid to a slight fall so water runs off. If the fall is wrong or the outlet blocks, water ponds against the kerb and sits around the bottom edge of the glazing. Constant wet is the fastest way to age an edge seal, so drainage matters more to lifespan than almost anything else.
• Heat cycling. Every day the glass heats up in the sun and cools at night, and the sealed unit expands and contracts with it. The wider that daily swing, the harder the edge seal works. Glazing that runs hot day after day fatigues sooner.
• Poor bedding. A unit that is not sitting square and evenly supported carries uneven load, and stress concentrated at one corner shortens the seal’s life. Careful installation onto a level kerb is doing quiet work for decades afterwards.
• Ventilation of the cavity below. A rooflight over a kitchen or bathroom lives in warm, moist air. If that room is poorly ventilated, the extra humidity gives the seal more to fight against.

On the other side, the things that extend life are unglamorous: correct falls, clear outlets, a level kerb, quality glass with a warm-edge spacer, and keeping the glass and gaskets clean so grit and organic muck are not sitting in the drainage channels.

A flat rooflight is the most exposed piece of glass on the house. It does not face a compass direction; it faces the sky, so it collects the high summer sun almost square on from mid-morning to late afternoon. That matters for lifespan as much as for comfort, because the glass that runs hottest for the longest is the glass whose edge seal works hardest over the years.

Around Southend the problem is sharper than most places. The city faces south across the Thames Estuary with an open horizon and reflected light coming off the water, so a rear extension with a flat roof, which is where most flat rooflights around here end up, gets a long, uninterrupted dose of sun. The 1930s semi belt across Southend, Westcliff and Thorpe Bay was largely built garden-to-the-back, and a great many of those backs face south. Put clear double glazing in a flat roof over one of those kitchens and you get two things at once: a room that overheats in July, and glazing that spends its whole life at the hot end of the heat-cycling range.

Specifying a lower G-value pane is usually sold as a comfort measure, and it is one. It also happens to be kinder to the unit over time, because glass that sheds solar heat before it builds up runs through a gentler daily temperature swing. Orientation, glass specification and glare are what this business is built on, and on a south-facing flat roof they are the same decision. Our solar-control glazing work is aimed squarely at that combination.

Not every flat rooflight is glass. Polycarbonate and other plastic domes are common, cheaper, and lighter, and they age differently. A plastic dome has no sealed edge to mist, but the material itself degrades under ultraviolet light: it yellows, goes brittle and can craze over the years, and the surface dulls. A budget dome might look tired inside a decade, while a better multi-wall sheet with a UV-protective layer holds up longer.

Glass does not yellow or go brittle, and its clarity is unchanged at 20 years right up until the seal fails. So the two materials fail in opposite ways: plastic fades gradually across its whole surface, glass stays perfect and then mists. For a south-facing roof where the glazing sees hard sun for its whole life, the fade resistance of glass is worth having. There is more detail on the plastic option on our polycarbonate rooflight page.

If your flat rooflight is misting between the panes, the sealed unit has reached the end of its life and the glass needs replacing. In most cases the frame and kerb are still sound, so it is a straight swap of the glazing into the existing structure, and that swap is the moment to reconsider the glass specification rather than drop the same unit back in. If the flat roof itself is leaking around the rooflight rather than the glass misting, that is a roofing matter and a roofer is the right person to look at the covering first.

Planning a new flat rooflight, or replacing one that has given up, is worth doing once and properly. Read how we approach the whole job on our flat roof skylight installation page, or tell us the orientation and rough opening size and we will come back with a glass specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the glass.