How do you clean a roof lantern?

Why a lantern gets dirty faster than a window

A vertical window is largely self-cleaning because rain runs down it and takes the dust with it. A lantern is pitched at 25 to 40 degrees, so rain runs slower, and the flatter the glass the longer the water sits before it moves. Anything suspended in that water gets left behind as the surface dries.

What lands on it is different too. Vertical glass collects road film. Sloping glass collects what falls out of the air: pollen in May, lime and sycamore honeydew through summer, leaf litter and bird droppings all year, plus the fine grit that comes off any roof covering upstream of it. On a rear extension under a mature tree, the glass will visibly haze within a season.

Twice a year, and choosing the months

Late spring and mid autumn are the two dates worth putting in a calendar. Late spring clears the pollen and the tree drop that has accumulated since March, which matters because that is exactly when you start noticing the glass, with the sun low enough to light every mark on it. Mid autumn clears the leaf debris before winter, when nothing dries and organic matter left in the corners stays wet for months.

A third visit in high summer is worth it under limes and sycamores, where honeydew turns to a sticky film that then collects dust. Left through a full year it becomes genuinely hard to shift.

Reaching the glass safely from outside

Never stand on a lantern. The glass is toughened or laminated and will usually take a person, but the frame junctions and the gaskets will not thank you, and no lantern sold for domestic use is specified as walk-on glass unless it says so. Assume it is not.

The realistic options, in order of preference: a water-fed pole from ground level, which reaches most single-storey rear roofs and needs nobody off the ground; a tower or a properly footed ladder to the flat roof, then a short pole across the lantern from the deck alongside it; or a window-cleaning contractor with pole equipment, which for a two-storey rear addition is the sensible answer rather than the cautious one.

What to use on the outer surface

Pure water through a fed pole is the best result available, because deionised water dries without leaving mineral marks and needs no drying off. Failing that, warm water with a small amount of washing-up liquid, applied with a soft brush or a lambswool applicator, then rinsed thoroughly and drawn off with a rubber blade.

What to keep away from it: any scouring pad or wire wool, which will scratch coated glass permanently; a scraper blade, which takes coatings off in strokes you will see forever afterwards; solvent-based cleaners and bleach, which attack the gaskets and the edge sealant of the sealed unit far faster than they attack the dirt; and a pressure washer, which drives water into the glazing rebates and past details that were designed for rain, not for a jet at 100 bar.

Self-cleaning coatings and how they change the routine

Many lanterns are ordered with a pyrolytic coating on the outer face. It works in two stages: sunlight breaks down organic material on the surface, then rain sheets across the glass rather than beading, carrying the loosened material off. It genuinely reduces how often you need to go up there.

It does not eliminate cleaning, and it changes what you can use. Detergent films and any waxy product will sit on top of the coating and stop it working until they weather off. On coated glass, clean with water alone wherever possible, and expect the coating to take a few weeks of weather to settle back down after any wash.

The underside, which needs different kit

Inside glass gets cooking grease in a kitchen, dust everywhere else, and in both cases it builds slowly enough that people do not notice until the sun catches it. A microfibre head on a telescopic pole, dampened with warm water and a little detergent, then a dry microfibre pass, deals with it from floor level on most lanterns.

In a kitchen, degreasing matters more than polishing. Grease film on the underside is also what makes any condensation on that surface leave marks when it dries. If the room routinely runs damp, the condensation page covers the ventilation side of that, which is the actual cause.

Frames, gaskets and the channels that drain

The frame deserves the same visit as the glass. Powder-coated aluminium wants nothing more than water and mild detergent, and no solvent at all. Gaskets last longer if the grit is washed out of them rather than ground in.

The part people never look at is the drainage. A lantern is designed to let a small amount of water into the glazing rebate and then take it back out again, through channels at the eaves and weep holes at the bottom of each rafter bar. Moss, needles and silt block those. Clearing them is a soft brush and water, no tools, and it takes a minute per bar. A blocked channel is the single most common reason a sound lantern starts showing water at the internal edge.

When cleaning is not the answer

Some marks do not come off because they are not on the surface. Mist or a bloom between the two panes of glass means the sealed unit has failed, and no amount of washing reaches it: the unit is replaced. White staining that returns days after cleaning is usually mineral run-off from an adjacent stone or concrete detail upstream of the glass, and the answer is upstream, not on the lantern.

Fine scratching that shows only in low sun is almost always historic cleaning damage, and it is permanent. That is the argument for getting the method right from the first year. We install and replace rooflights across Oxford and 25 miles around it, so if a unit has failed rather than dirtied, call 01865 704245 and we will tell you which of the two it is before quoting anything.