Yes, and a bathroom is one of the better rooms for one. It solves two problems at once: it removes the moisture that a bathroom generates faster than any wall vent will, and it delivers daylight without anybody being able to see in. The specification does need to change, though. A polyurethane sash rather than lacquered timber, an opening arrangement you can actually reach over a bath, and a sensible answer on electrics if the unit is motorised.
The design problem is moisture from inside
Rain on the outside of a bathroom window is the same rain that lands on every other window in the house. What is different is the inside face. A shower puts a large volume of water vapour into a small room in a short time, and that vapour condenses on the coldest surface available. In a room with a roof window, the coldest surface is the glass, and after that it is the bottom rail of the sash where the water collects and sits.
Every specification decision below follows from that. The material has to tolerate standing water, the ventilation has to remove the vapour before it condenses, and the reveal has to be built so that the corner where the frame meets the ceiling is not the cold spot in the room.
Polyurethane rather than lacquered timber
A timber roof window has a laminated pine sash with a lacquer on the inner face. In a bedroom that finish lasts for years. In a bathroom, water running off the glass sits on the bottom rail after every shower, and the lacquer breaks down far sooner. Once it does, the timber greys, the lamination opens and the sash is finished.
A polyurethane unit has a timber core inside a moulded polyurethane skin with no exposed joints and nothing to maintain. It is the right product for a bathroom, a shower room, a utility and usually a kitchen. The cost difference over a timber unit is small and it is recovered comfortably in service life. We specify polyurethane in wet rooms as a matter of course and we will say so even if you asked for timber.
What the opening has to achieve
Building Regulations ask for extract ventilation in a bathroom, and an openable window on its own does not discharge that duty in a room with no external wall opening. Practically, most bathrooms want both: a mechanical extract fan for the shower, and an opening window for purge ventilation.
The roof window is unusually good at the purge part. Warm wet air rises, and an opening at the highest point of the room lets it leave under its own buoyancy while cooler air is drawn in under the door. Ten minutes with the window fully open after a shower shifts more moisture than an hour of a tired fan. The ventilation flap in the head of the sash gives you background airflow with the window locked shut, which is the setting to leave it on overnight in winter. There is more on this on condensation and ventilation with rooflights.
Where the window should sit relative to the shower
Directly over a shower is the position people ask for and it is rarely the best one. The vapour rises straight onto the glass, condenses immediately and drips back down, and the handle is somewhere you can only reach while getting wet.
Better is to put the unit over the general floor area or over the bath, close enough to the shower for the buoyant air to reach it, far enough away that the glass is not the first surface the plume hits. If the window has to go over a bath or a shower enclosure, plan the operation before you plan the position: either an electric operator or a control bar at the head of a centre-pivot sash that you can reach from outside the enclosure.
Motorised units and the electrical zones
An electrically operated roof window in a bathroom is a genuinely good idea, because it lets you purge the room without leaning over a bath. It also means the electrical zoning rules apply.
Zones are defined by distance from the bath and shower. Anything within them needs an appropriate ingress protection rating, and the circuit needs residual current protection. A roof window motor mounted high above a shower is usually in zone 1, and the unit and its controller have to be suitable for that position, with the wiring installed by somebody qualified to certify it. This is notifiable electrical work in a bathroom.
Run the cable while the reveal is open. Adding it afterwards means taking a finished reveal apart. Solar powered operators avoid the wiring entirely and are worth considering where the slope gets reasonable light. More on the options is on opening rooflights electrically.
Glass specification for a wet room
Standard specification for a roof window is a laminated inner pane, which is what you want overhead in any room and particularly above a bath. Laminated glass holds together if it is broken rather than coming down in pieces.
Beyond that, think about the slope orientation. A south facing bathroom with a large unit will get hot, and a solar control coating on the outer pane costs nothing extra in labour if it is decided at the order. A north facing bathroom benefits from the best thermal performance you can get, because the glass is the coldest surface in a room full of vapour, and a warmer inner pane condenses less. Triple glazing is worth its money in this one room more than in most.
Privacy, and why obscure glass is usually unnecessary
A roof window in a pitched slope faces the sky. Unless there is a taller building directly overlooking the roof, nobody can see through it, and obscure glass simply removes the view of clouds and trees that makes the window worth having.
Check the actual sightlines rather than assuming. If a neighbouring upper storey does look down onto that slope, a pleated blind gives you privacy when you want it and clear glass when you do not, which is better than permanently obscured glazing. Obscure glass on a roof window is also harder to source and often extends the lead time.
The reveal, and the corner that goes mouldy
The failure people blame on the window is nearly always the reveal. If the insulation collar is not continuous around the frame, or the vapour control layer is not sealed to the plasterboard, the plaster at the head of the reveal runs colder than the rest of the ceiling and mould appears in that corner within two winters.
Done properly, the collar wraps the frame, the vapour layer is taped to the frame and to the board, and the reveal is splayed so warm room air washes across the plaster instead of stalling in a square corner. In a bathroom, use a moisture resistant board and a paint intended for wet rooms. Those four details are invisible once the beading is on, which is why we guarantee the workmanship for ten years.
What we look at on a bathroom survey
Where the shower is, where the extract terminates, whether there is an existing fan and what it actually moves, the rafter spacing, the pitch, whether you can reach the control from a dry position, and whether the roof build-up is warm or cold. Then a unit is specified against that rather than against a size code you saw online.
We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road. Fifteen years in the trade, a ten-year workmanship guarantee, and VELUX, Fakro, Keylite and Roto units fitted. We handle the Building Control notification, and the roof is never left open overnight.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements