In brief
Self cleaning glass is worth having on a rooflight you cannot reach easily, and less so on one you can wipe from a ladder in a minute. A coating on the outer face breaks down organic dirt in daylight, then rain spreads across the glass and carries it away. It needs rain and some fall on the glass to work, and it reduces how often you clean rather than ending the job.
- A coating on the outside of the glass loosens dirt in daylight, and rain rinses it off
- It works best on glass with a clear fall, so a very flat roof gets less from it
- Bird droppings, sap, cement dust and heavy grime still need a hose or a clean
- The coating has to be on the outer face, so the unit is made and fitted a set way up
- It suits high or awkward rooflights most, where cleaning access is the problem
How the coating works
The coating is a very thin mineral layer applied to the outer pane at the factory. It does two things. In daylight, particularly the ultraviolet part of it, the coating reacts with organic dirt such as pollen, soot and traffic film and breaks it down so it no longer grips the glass.
The second effect comes from water. The coating makes the surface attract water, so rain does not form beads. It spreads into a thin sheet that slides over the glass and carries the loosened dirt with it. Because the water sheets and does not bead, the glass also tends to dry without leaving as many marks.
The coating is on the outside only. It is part of the unit's glazing specification and not a polish, so it cannot be added by treatment once a rooflight is in place. If you want it, it needs to be named in the quote ahead of ordering.
What it needs to work
Two things have to be present: daylight and rain. A rooflight in a shaded spot, under a tree or behind a taller wall gets less of the first. A rooflight under an overhang gets less of the second, because rain never reaches the glass.
Fall matters as well. Rain runs off a steeply pitched roof window easily and takes the dirt with it. On a flat rooflight the glass is often set with only a gentle slope, and in some cases is close to level. Water lingers, dirt gathers at the lower edge and the rinsing is weaker. The coating still helps, but you should not expect a flat rooflight to look as clean as a steep one.
In a long dry spell the dirt breaks down but stays on the glass until it rains. A rinse with a hose, from the ground or from an upper window, gives the coating the water it needs.
What it will not do
- Heavy deposits. Bird droppings, tree sap, leaf stains and cement or plaster dust are not the light organic film the coating is designed for. They need to be softened and washed off.
- The frame and the surroundings. Only the glass has the coating. The frame, the flashing and the kerb still collect dirt and need an occasional wipe.
- The inside. The inner face is not coated, and it gets its own film of dust and cooking residue.
- Replacement of the maker's advice. Cleaning products and tools can damage a coating, so the maker's instructions apply. Avoid abrasive pads, scrapers and strong solvents.
The right expectation is a rooflight that stays clearer between cleans, not one that never needs attention. For the person who has to reach the glass, that is the whole point, since reaching the glass is the hard part.
Where it makes sense and where it does not
The strongest case is a rooflight that is hard to reach. A roof lantern over a double-height space, a flat rooflight on a rear extension over a conservatory-style roof, or a roof window high on a three-storey house all mean ladders, towers or a window cleaner with a pole. Reducing the number of those visits is where self cleaning glass earns its cost.
It also suits coastal and urban air, where a film builds up on the glass. We work across Southampton and the towns within 30 road miles, and many of those roofs are open to salt-laden wind or traffic haze. The coating does not change that weather, but the rinse from rain helps to keep the glass clear.
It is less compelling for a small roof window that you can reach from the loft or a landing, for a sun tunnel with a small dome, or for a rooflight that sits permanently under shade. In those cases a wipe is quick and the coating does not have much to work with.
Walk-on rooflights are a separate case, since people stand on them and the surface takes wear. Ask the maker how the coating is protected on any glass that is walked on.
Combining it with the rest of the specification
The coating is one layer among several. The same outer pane may carry a solar control coating, and the maker will say which combinations they make and how the look and the tint change. Our guide to solar control glass for rooflights explains that side. The heat loss through the unit is set by the inner pane, the gas fill, the spacer and the frame, and we discuss it in our answer on heat loss through skylights.
Some makers apply a slight tint or haze that you can see when you look at the glass from a low angle. It is minor, but the sample is worth looking at before we order. A written quote names the glazing, the coating and the lead time, which the manufacturer sets and the order confirms.
If you are choosing between several upgrades and cannot fit them all in the budget, a warm edge spacer and a good low-emissivity coating improve how the room performs every day, whereas self cleaning glass improves how the glass looks and how easily you can keep it that way. Our page on energy efficient rooflights sets the options side by side, and our note on the warm edge spacer covers one of the quieter ones.
Related questions
- Can self cleaning coating be applied to glass that is already fitted?
- The factory coating cannot be added on the roof. Aftermarket treatments exist, but they are not the same as a coating applied in production, and we would not specify one without the maker's approval for your unit.
- Will the coating wear off?
- Any coating can be damaged by abrasive cleaning, and the maker's guidance on lifespan and care applies to the unit named in your quote. Use a soft cloth, clean water and mild soap when you do wash the glass.
