Cleaning and maintaining a roof lantern safely

A wall window sheds most of what lands on it. Rain runs off, dust blows past at an angle, and gravity keeps the worst of the grime moving downwards and away from the glass. A roof lantern does none of that. Its panes are pitched, usually between about 15 and 25 degrees on the sloping sides, and the flatter that pitch, the longer water sits on the surface before it drains. Sitting water dries into rings. Dust settles rather than sliding off. Pollen, salt carried in off the estuary, and the sooty film from traffic all land on the top face and stay there.

The result builds up in a way you often do not notice from inside, because it happens evenly across the whole roof and dims the light gradually. A lantern that has not been cleaned for two or three years can be passing noticeably less daylight than the day it went in, and the glass looks fine from the sofa because your eye has adjusted to the loss a little at a time. Clean it and the room jumps.

South Essex adds two ingredients most inland areas do not. The first is salt. Anywhere within a mile or two of the seafront picks up a fine saline haze that dries to a faint white bloom on glass and, over years, works at aluminium and its coatings. The second is that the whole point of a lantern here is a bright room facing south across the water, so the glass is baking in direct sun through the middle of the day, which cooks dust and organic matter onto the surface harder than shade ever would. Both are reasons to clean a little more often here than a book written for the Midlands would suggest.

The single most important rule is that you should almost never need to walk on a roof or a lantern to clean it. The overhead glass of a domestic lantern is not designed to be stood on, and a roof surface pitched even slightly is a genuinely dangerous place without proper edge protection and a harness. Falls from height are the thing that hurts people doing this, and the good news is that the tools to avoid it are cheap.

For a single-storey extension, which is where most lanterns in this area sit, a telescopic water-fed pole reaches the glass comfortably from the ground outside. These run from about 3 metres up to 9 metres or more, with a soft brush head on the end and purified water fed up through the pole. Purified water is the point: it dries without leaving the mineral spots that tap water leaves, so you brush, rinse and walk away with no need to reach up and buff the glass dry. That is what lets you clean overhead glass from a standing position on solid ground.

The outer face of the glass takes plain water and a soft brush. Work from the top of the pitch downwards, let the flow carry the loosened dirt off the lower edge, and give it a final flood of clean water. Avoid the following, all of which do damage that is expensive to undo:

• Abrasive pads, wire wool, or scouring cream. They scratch the glass permanently and can strip a surface coating.
• Strong solvents, oven cleaner, or anything strongly alkaline. If a self-clean or easy-clean coating is on the outer face, harsh chemicals wreck it, and it does not come back.
• A pressure washer. The jet can drive water past the seals and the drainage details, force grit under the gaskets, and lift the sealant along the edges.
• Cleaning in full midday sun on a hot day. The glass flashes the water dry before you can rinse, and you get streaks and mineral marks baked straight back on. Early morning or an overcast day is better, which in practice is easy to arrange here.

If your lantern has a self-cleaning coating, and many fitted in the last decade or so do, understand what it actually does. The coating uses daylight and rain to break down and wash away organic dirt over time. It reduces how often you need to clean, it does not remove the job, and it hates abrasives and harsh chemicals more than ordinary glass does. Plain water and a soft brush is exactly right for it. Anything stronger shortens its life.

The inside of the glass is the easy half. It never sees rain or salt, so it collects only household dust, cooking film in a kitchen, and the odd fingerprint from an opening mechanism. A microfibre cloth on an indoor telescopic pole, or a soft cloth and a squirt of ordinary glass cleaner if you can reach, handles it. Vinegar diluted in water works and leaves nothing behind. The frequency the inside needs depends entirely on the room: a lantern over a hob wants wiping several times a year, one over a quiet sitting room barely needs touching.

The exception is condensation running down the inside, which is not a cleaning problem and is dealt with further down, because it is telling you something about ventilation or, occasionally, about the glass unit itself.

Maintenance that depends on remembering never happens. Tie it to the calendar instead. The figures below are sensible intervals for a coastal, south-facing setting like this stretch of Essex; a sheltered lantern well back from the sea can stretch them out, and one right on the seafront may want tightening.

Book the two bigger jobs, the drainage clear and the annual inspection, for spring and autumn. Spring clears the winter’s debris before the bright season when you use the room most, and autumn gets the leaves out before they can build up and freeze in the channels over winter.

The glass gets the attention, but a lantern is a system, and the bits around the glass are what keep water out of your ceiling. They need a look at least once a year.

Drainage channels and weep holes

Most modern lanterns are designed to let a small amount of water in past the outer line and then carry it back out through hidden channels and small weep holes along the bottom of the frame. This is normal and deliberate. It only becomes a problem when those channels silt up with pollen, moss, blown leaves and grit, because then the water they were meant to drain has nowhere to go and can find its way inwards instead. Once a year, clear them out with a soft brush or a gentle flush of water and make sure the weep holes at the low edge are open. On a south-facing extension backing onto a garden, expect more organic debris, especially under overhanging trees.

Gaskets and seals

The rubber gaskets that press against the glass and the sealant lines along the frame are what stop wind-driven rain. Wipe them clean when you do the annual check and run an eye along them. Rubber that is still soft, in place and unbroken is doing its job. Rubber that has gone hard, cracked, shrunk back from a corner or started to lift is not, and that is worth a note in your diary. Small perished sections of external mastic are a maintenance item; widespread failure, or water actually coming through, is a sign the whole glazing line is past its service life and is a matter for a replacement unit rather than a smear of fresh sealant over a tired one.

The frame

Aluminium lantern frames are powder-coated, and the coating is tough but not immortal, particularly with salt in the air. Keep it wiped down with a damp cloth, the same as the glass, and do not take anything abrasive to it. If you see the coating chalking, blistering or lifting, that is weathering rather than dirt, and no amount of cleaning brings it back. A timber lantern, which you see occasionally on period properties in the conservation areas around Leigh Old Town and Clifftown, needs its finish checked yearly and re-treated on the schedule the coating maker specifies, because bare timber overhead is the first step towards rot.

Water on the inside of a lantern worries people, and most of the time it should not. There are two completely different kinds, and telling them apart matters.

The common kind is condensation on the room-side face of the glass, which you can wipe off. This is ordinary physics: the inner glass runs colder than the room air, warm moist air from cooking, showering or drying washing touches it, and the moisture drops out as a film, exactly as it does on a cold drink in summer. It is most likely on a clear winter morning. It is a ventilation story, not a fault. A lantern sits at the highest point of the room, which makes it the perfect place to let warm damp air escape, so an opening pane earns its keep here. On a high ceiling where nobody will use a pole twice a day, an electric opening unit on a humidity or timer control does the venting for you. Trickle ventilation, an extractor in the kitchen or bathroom, and not drying washing directly under the glass all cut it down.

The other kind is misting or droplets sealed between the panes, where you cannot wipe it away. That means the seal around the double-glazed unit has failed and the gas or dry air in the cavity has been replaced by damp outside air. Cleaning will never touch it because the moisture is on the inside of a sealed sandwich. A misted unit has reached the end of its working life, it will not recover, and the answer is a new glazed unit. Replacing a failed pane in an existing lantern is straightforward installation work and it is the right moment to reconsider the glass specification while you are at it.

That last point matters more on a south-facing roof than most people expect. A lantern facing south across the estuary is collecting solar energy through the whole middle of the day, and how much of that arrives as heat in the room below is set by the glass, specifically its G-value. If an old clear unit has failed and you are replacing it anyway, moving to a solar-control specification on a south or west plane turns a summer overheating problem into a bright, comfortable room for very little extra over the cost of like-for-like glass. Keeping the glass clean plays into the same idea from the other direction: solar-control coatings are selective, letting daylight through while turning back the invisible near-infrared that carries the heat, and a film of grime does not stop the heat but it does steal the daylight you paid for. There is more on choosing that glass on our energy-efficient glazing page, and the detail of specifying and fitting the units themselves on the roof lantern installation page.

Almost all routine care is a homeowner job done from the ground with a pole and clean water. A few things are not, and pretending otherwise is how people get hurt or make an expensive mistake.

• Anything above a single storey. A lantern on a two-storey rear addition, or one you can only reach across a fragile or steep roof, is a job for someone with the right access equipment, a tower, a cherry picker or proper roof-edge protection. It is not worth a ladder leaned at the wrong angle. If you do use a ladder for lower work, foot it on firm level ground at roughly a 75 degree angle, one out for every four up, tied at the top, and never lean or reach sideways from it.
• A unit that has misted between the panes. That is a failed glazed unit, not a cleaning task, and it needs a new unit fitted.
• Water actually coming through into the room. An active leak around a lantern is a matter for a roofer to trace and put right at the roof covering and flashings. That is roofing work rather than glazing, and it is the right call to bring one in.
• A frame or seals that have widely perished. When the powder coating is breaking down or the glazing line has failed along its length, cleaning is not the answer, and it is worth getting the lantern looked at with replacement in mind.

Keep the routine simple and it looks after itself: rinse the outer glass every few months from the ground, wipe the inside a few times a year, clear the drainage and check the seals each spring and autumn, and give the whole thing one proper look-over annually. A lantern treated that way stays as bright as the day it went in and lasts its full service life without drama. If yours has misted, or you are weighing up a new unit and want the glass specified for the way your roof faces, tell us which way the roof points and we will talk you through it.