Self cleaning glass carries a permanent coating on the outer face that does two things: it uses daylight to break down organic dirt, and it makes water spread into a sheet rather than form droplets, so rain carries the loosened dirt away rather than leaving spots. On a rooflight, which is awkward to reach and takes rain directly, it is more useful than on a window.
The two mechanisms
Worth separating, because they work at different times.
The first is photocatalytic: daylight activates the coating, which chemically breaks down organic material sitting on the glass. That happens continuously, in daylight, whether or not it is raining.
The second is hydrophilic: water spreads across the surface instead of beading, so it washes the loosened dirt off in a sheet and dries without spots.
Why a roof suits it
Three reasons, and they all point the same way.
A rooflight is hard to reach, so cleaning it is a job people put off. It takes rain directly and at volume, which is what the coating needs to work. And it takes daylight all day, which is what activates it.
A north facing wall window has none of those advantages.
What it does not do
The limitation worth stating plainly, because the name oversells it.
It does not deal with inorganic dirt. Salt film, mineral deposit from hard water, sand, cement splashes and bird droppings that have dried are not organic material and the coating does nothing about them.
On this coast that is a real qualification.
What it does mean in practice
A longer interval between cleans rather than never cleaning.
Glass with the coating stays visibly clearer for longer and shows fewer water spots. It still needs cleaning eventually, and considerably less often.
How long the coating lasts
The life of the glass, in normal use.
It is fired into the surface during manufacture rather than applied afterwards, so it does not wear off with weather or with ordinary cleaning.
What does damage it is abrasion, which is a real risk on a gritty coastal roof.
How to clean it without damaging it
Gently, and never dry. Rinse first to lift grit, then wash with water and a soft cloth. No abrasive pads, no scrapers, and nothing containing a scouring agent.
Dry wiping a gritty rooflight scours the coating and the glass beneath it, which is the commonest way a good unit is spoiled.
Sealants and the coating
A specific caution during installation. Silicone and some sealants can leave a residue on the coating that stops it working locally, and it does not clean off easily.
Which is a reason installation matters: the coating is only as good as how carefully the unit was handled going in.
Does it work in shade
Less well, and this decides whether it is worth specifying.
The photocatalytic action needs daylight, including on overcast days, but a unit permanently shaded by a mature canopy gets less of it.
On a heavily overshadowed slope the coating is doing less than it would in the open, and that is worth knowing before paying for it.
Where it earns its place
Three cases consistently. A unit that is genuinely hard to reach. A unit under trees, where organic debris is the problem. And a household where nobody is going to be getting a ladder out.
Where it does not
A low bungalow roof somebody can reach in five minutes, and an exposed coastal position where the deposit is salt rather than organic.
On the coast, rinsing regularly does more than the coating does, and it is on the coast that rinsing matters most anyway.
Combining it with other coatings
Entirely possible, and most manufacturers offer it.
Self cleaning sits on the outer face; low emissivity and solar control sit on internal faces. They do not compete for the same surface.
So specifying self cleaning does not mean giving up solar control, and the two are frequently ordered together.
What it costs
A modest addition to the unit price, decided at order.
It cannot be retrofitted, because it is part of the glass rather than something applied to it, so the decision is made once.
How it looks
Very slightly different, and most people never notice.
The coating can give a faint haze or a slight colour cast at some angles, particularly when new. It settles in use and it is not something anybody comments on afterwards.
Whether it helps with condensation
External condensation, occasionally, and not the kind people complain about.
Because water sheets rather than beads, external dew clears more evenly. Condensation on the room side is a different problem entirely.
That is on how do I stop condensation on a rooflight.
How it performs in the first weeks
A detail worth knowing so nobody concludes it does not work.
The coating needs a period of daylight exposure after installation before it is fully active, so a unit fitted in midwinter may not show its benefit until the days lengthen.
It is normal, it is temporary, and it is the commonest reason somebody reports that their self cleaning glass is not self cleaning.
What we recommend instead sometimes
On a low reachable roof, the money is better spent elsewhere.
A better blind, solar control glass on a south facing slope, or simply the habit of rinsing twice a year will do more for most households than the coating will.
What we always specify on the glazing
We specify it where a unit is genuinely hard to reach or sits under trees, and we say plainly that on a coastal roof it does nothing about the deposit that actually matters.
The whole picture is on the energy efficient rooflights page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.