Can a flat rooflight go over a shower?

Steam goes up, and the glass is what it hits

A shower running for eight minutes puts something in the order of a litre of water into the air of a small room. That vapour rises in a column, spreads across the ceiling and looks for the first surface below its dew point.

In a room with a rooflight, that surface is the glass. It is the only part of the ceiling with an outside face, and even a good triple-glazed unit runs cooler than plasterboard sitting under 150mm of insulation. So the glass will mist and the frame will carry droplets. That is physics, not a defect, and the job is to control it rather than to pretend it will not happen.

Choosing glass that stays warmer than the air

The variable you can actually move is the temperature of the inner pane. A unit with a centre-pane U-value around 1.0 W/m2K and a warm-edge spacer keeps its inner surface several degrees higher than an older double-glazed unit with an aluminium spacer bar.

Those few degrees decide whether the glass mists for two minutes or twenty. Over a shower we specify the warmest sensible build-up rather than the cheapest that meets the regulation, because the regulation is written for a bedroom, not for a room that is deliberately filled with hot vapour twice a day. The spacer matters as much as the gas fill: cold edges produce a rim of water before the middle of the pane has clouded at all.

Where the extract fan has to sit

An extract fan in the wrong place fights the rooflight instead of helping it. Vapour rises, so a fan set low on a wall, or in a corner behind the door, pulls in dry air from the landing and leaves the plume undisturbed above the tray.

The fan wants to be at high level and close to the source, within about a metre of the enclosure, drawing from the ceiling void. On a shower room we would rather see a continuous-running unit at a low trickle rate with a boost on the light switch and a fifteen-minute overrun than a bigger intermittent fan that only runs while somebody is in the room. Most of the drying happens after the shower stops.

Electrical zones over a tray

If the unit opens, something has to open it, and above a shower tray that decision is governed rather than optional. The volume directly over the tray up to 2.25m above the floor is treated as a wet zone, and equipment installed in it has to carry an appropriate ingress protection rating and be fed through a residual current device.

In practice, on a typical extension ceiling at 2.4m the unit sits above that zone, and a mains actuator is straightforward. On a lower ceiling, or where the tray is raised, we specify a low-voltage actuator with the transformer remote from the room. A manual pole-operated unit avoids the question entirely and is worth considering.

Clear, obscured or tinted above the enclosure

Clear glass is right in almost every case. There is no overlooking from directly above unless a neighbour’s upper window has a sightline down onto the roof, which is worth checking on the survey rather than assuming.

Obscured glass in a rooflight is a common request and usually a mistake. It cuts the light, it shows water marks far more readily than clear glass, and the privacy it buys is privacy from birds. Where a genuine sightline exists, a body-tinted or solar-control pane handles it better and keeps the view of the sky. Where the shower faces a south-west elevation and glare is the concern, solar control is the right specification regardless.

The reveal finish is what gives up first

The vertical face between the ceiling and the frame takes more punishment in a shower room than anywhere else in the house, and it is where a badly built installation shows itself within two winters.

Ordinary plasterboard on a timber kerb, skimmed and painted with contract emulsion, will soften and stain. We line the reveal in moisture-resistant board, tape and fill the junction with the frame rather than relying on a bead of sealant to hide a gap, and finish in a paint intended for bathrooms. Splaying the reveal outwards by twenty or thirty degrees also throws more light into the room and gives condensate a route back to the frame instead of down the wall.

Keeping the kerb out of the vapour path

The kerb is the part nobody sees and the part that decides whether the ceiling stays dry. Warm wet air from a shower is under slight pressure and will find any unsealed junction in the ceiling and travel up into the roof build-up, where it condenses out of sight.

So the vapour control layer of the roof has to be carried up the inside face of the kerb and taped to the frame, continuously, with no gaps at the corners and no penetrations left open for cables. Downlights cut into a shower ceiling next to an untaped kerb are a reliable way of pushing moisture into a roof. Our upstands, kerbs and warm roof build-ups page sets out how that detail is formed.

What we ask before pricing one of these

Four things, and they take a survey rather than an email. What the ceiling height is over the tray, because that sets whether the unit is inside a wet zone. What extract already exists and where it terminates. Which way the roof falls, because the kerb has to stand clear of water on the outside as well as vapour on the inside. And whether the shower is used daily by a family or twice a week in a guest room, because that changes how hard the ventilation has to work.

The wider question of glazing a wet room, including tiled reveals and units above a bath, is covered on flat rooflights over a bathroom or wet room. We install and replace rooflights across Oxford and up to twenty five miles by road, we have been doing it for fifteen years, and the workmanship carries a ten-year guarantee.