The short answer
Self-cleaning glass works on a skylight when the roof is steep enough for rain to run off it and the sky does the rest: daylight loosens the dirt and rainwater carries it away. On a low pitch or a flat rooflight the water lingers and dries in marks, so the coating helps less and you may still clean the glass by hand now and then.
How the coating works
Self-cleaning glass has a very thin coating on the outer face of the outer pane. It works in two steps. Daylight, and ultraviolet light in particular, starts a reaction in the coating that loosens and breaks down organic dirt such as pollen, bird droppings and tree sap. The coating also makes water spread out in a thin sheet instead of gathering in beads, and that sheet carries the loosened dirt off as it runs down the glass.
Both steps need something from the roof: light to start the first, and water running over the glass for the second. A steep sloping skylight that sees the sky and takes the rain is the best case. A glass that is shaded all day, or kept dry by an overhang, is the worst.
The coating is fitted to the sealed unit at the factory. It is a feature of the glass, not a product applied to a window afterwards.
Why the pitch of the roof matters
On a steeper pitch, rain runs down the glass and takes dirt with it. On a shallow pitch it runs slowly, and on flat glass it often stays where it lands, then dries and leaves its own marks, drawing the dirt into a ring on the surface. The coating still loosens the dirt, but there is less water moving to remove it.
For a pitched roof window, the coating works in the way it is described. For a flat rooflight, which is set at a slight fall only, we describe it as helping rather than doing the whole job. Our answer on the minimum pitch for a flat rooflight explains the fall such a rooflight sits at.
| Item | Steep roof window | Low-pitch roof window | Flat rooflight |
|---|---|---|---|
| Rain runs off | Quickly | Slowly | Slowly, or stays |
| Dirt carried away | Most of it | Some of it | Less of it |
| Hand cleaning still needed | Seldom | Now and then | More often |
| Worth the extra cost | Often | Sometimes | Where access is difficult |
What it does not do
The coating deals with organic dirt. It does less with mineral deposits, limescale, paint splashes, cement dust from building work or the grey film that settles from heavy traffic, and it cannot shift moss or leaf litter that has gathered along the frame.
A dry spell leaves dirt on the glass until rain returns. In a long summer without rain you may still see the glass go dull, and a wash with clean water will help.
Cleaning the coating also needs care. Abrasive cleaners, scrapers and strong chemicals can damage it. The maker's guidance is a soft cloth or sponge and plenty of clean water, and we pass the specific instructions on with the quote.
When it is a sensible extra
The coating adds cost to the glass line of a quote. It earns that cost most where the glass is hard to reach, such as a rooflight set high on a steep roof, a lantern over a first floor stairwell, or a roof window above a conservatory roof that cannot be walked on.
It also suits a roof under trees, where leaf and sap fall steadily, or a roof near a busy road. Where you can reach the glass from inside, or the rooflight is a small roof window that you can clean from a ladder through a hinged sash, the extra cost may buy you little.
It can sit alongside other glass choices. A unit can carry a self-cleaning outer face and a solar control coating, and our answer on solar control rooflight glass explains that other option. Whether the two are available together depends on the maker and the range, and the quote shows what each suitable option offers.
Choosing at the survey
The survey is where we look at the pitch of the roof, how reachable the glass is, what overhangs it and what drops onto it. From that we say whether the coating is likely to pay its way. If the roof suits, the written quote offers several suitable units from different makers, each priced and described both with and without the coating, so the difference is plain.
- Pitch and shade noted for each rooflight
- Coated and plain glass priced side by side
- Cleaning advice from the maker with the quote
Book a free survey and we will price the options for your roof. Our hub on energy efficient rooflights covers the other glazing choices on a unit.
Related questions
Does self-cleaning glass mean I never wash the skylight?
A rinse with clean water now and then is still sensible, especially after a dry spell or building work nearby. The coating reduces how often you need to clean, but it does not remove the job.
Does the coating work at night?
The first step needs daylight, and it carries on during overcast weather, as ultraviolet light gets through cloud. At night nothing happens until the morning.
Can I clean self-cleaning glass with ordinary window cleaner?
Plenty of clean water and a soft cloth or sponge is the usual advice. Abrasives and harsh chemicals can mark the coating, so follow the maker's own guidance, which we give you with the quote.
Is a coated skylight any darker?
The tint is usually slight, if you notice one at all. On sunny days some people see a faint blue cast looking up through it, and the room remains well lit.