Can a roof window be fitted in a bathroom?

What a bathroom does to the specification

A bathroom generates more moisture, faster, than any other room in a house. A hot shower puts a great deal of water vapour into a small volume of air in a few minutes, that air rises immediately, and the coldest surface it meets is the glass overhead. Everything about the specification follows from those two facts.

So three decisions get made differently here. The sash is specified in a moisture-tolerant material rather than lacquered timber. The glass includes a laminated inner pane, and often an obscure one. And the ventilation strategy is decided as a whole, with the window and the extract fan doing separate jobs rather than one being expected to cover for the other.

The sash finish that survives steam

Every major range offers a sash built around a timber core moulded in one piece of white polyurethane, with no joints in the surface and no coating to maintain. That is the right specification for a bathroom, and it is worth the difference.

A lacquered pine sash in a bathroom works harder than the same sash in a bedroom. The finish takes a wetting and a drying cycle twice a day, and once it dulls or breaks at an edge the timber underneath starts taking on moisture at the exposed grain. The maintenance interval that would be four or five years on a bedroom window becomes shorter here, and it is the interval most likely to be forgotten. Polyurethane removes the question. Service life across the materials is set out at how long a roof window lasts.

Laminated glass above a bath or a shower tray

This is not a preference, it is a requirement, and it is the same principle that governs glass above a stairwell.

Roof glazing is toughened on the outer pane, which is the face that takes hail, branches and foot traffic during work on the roof. Where there is habitable space beneath, and specifically where somebody stands or lies directly under the glass, the inner pane is laminated: two sheets of glass bonded to an interlayer, so if it ever breaks it holds together in the frame rather than falling. A person standing in a bath in bare feet is the exact case the requirement exists for.

Laminated glass has a second benefit here that people notice from the first storm. It cuts rain noise noticeably, and a bathroom on a rear addition roof under a shallow slope is a drumming room without it. There is more on that at are roof windows noisy in rain.

Privacy on a slope, and the glass that answers it

Most roof windows in bathrooms need no privacy treatment at all, because a window in a slope looks at sky and nobody standing at ground level can see through it. That is the whole reason people put them in bathrooms rather than fitting another wall window.

Three situations change that. A taller building behind, which in central Cheltenham and in Gloucester’s tighter terraces is routine. A rear addition roof overlooked by the first floor windows of the main house next door, which is extremely common on Victorian terraces where the additions run parallel a few metres apart. And a steep slope, at 45 degrees or more on Forest of Dean and Cotswold roofs, where the window faces outwards enough to be seen into from further away.

Where any of those apply, the answer is obscure glazing specified in the sealed unit at order, which is permanent and needs nothing operated, or a blackout blind, which is reversible and gives you the choice. Obscure glass costs daylight quality rather than quantity: the light still comes through, it just arrives diffused. Permitted development also imposes obscure glazing on side-facing roof windows in a room formed in the roof, along with a restriction on opening below 1.7 metres above the floor, so on a side slope the decision may already be made for you.

Ventilation, and the two jobs it splits into

A roof window and an extract fan are not alternatives, and specifying them as though they were is the usual mistake.

An extract fan runs continuously or on an overrun and removes humid air at a steady rate while somebody is showering. Its job is to stop moisture accumulating in the fabric of the room. A roof window provides purge ventilation, which means clearing a saturated room fast, and it does that better than any fan: opening the sash for a few minutes after a shower drops the humidity in a small bathroom faster than a fan will in half an hour, because warm wet air leaves upward under its own buoyancy the moment there is a hole at the top of the room.

Purge ventilation to a habitable room means an openable area of at least one twentieth of the floor area, and a modest roof window clears that easily in a bathroom. The vent flap in the head of the unit provides background ventilation between times, which is the setting most often left closed. Both matter, and the fan is still specified to its own rate. Filter and flap detail is at roof window ventilation flaps and filters.

Condensation on the coldest surface in a small room

Water on the room side of the glass in a bathroom is not water getting in. It is the room’s own moisture landing on the coldest surface available, and in a bathroom it will appear on a cold morning whether or not anything is wrong.

Three things control how much. The glass specification, since a current unit at around 1.0 to 1.4 W/m2K runs a warmer inner surface than a 1990s one at 2.8 and condenses far less. The ventilation described above. And the reveal geometry, which is where most of the difference is actually made. A horizontal lining under the window lets warm air rise straight up the face of the glass and keeps the surface above dew point, while a splayed cill traps a pocket of still cold air against the pane and guarantees a wet sill every morning. A vertical lining above the window throws the light deeper into the room at the same time.

Behind the plaster, insulation runs continuous and tight to the frame with no voids, and the vapour control layer is taped to the frame rather than trimmed short of it. A gap there is a cold bridge, and a cold bridge in a bathroom is where black spotting shows up in February. The full reasoning is at rooflight reveals and why splaying them matters.

Building the reveal in a room that gets tiled

The reveal in a bathroom is built the same way as anywhere else and finished differently. Standard plasterboard is a poor choice around a bathroom rooflight; moisture-resistant board is the minimum and tile backer board is used where the reveal will be tiled or where it sits directly above a shower.

Where the reveal is tiled, the tiling has to stop short of the frame by the amount a blind’s side channels need, and that dimension is looked up before the tiler starts rather than after. The junction between the reveal and the frame gets a flexible finish rather than a rigid one, because timber and plaster move at different rates and a hairline crack in that line in a wet room is a route for moisture into the construction.

An opening you cannot reach, in a room full of hard surfaces

Bathrooms put the window in awkward places. Over a bath, over a shower enclosure, or high on a steep slope where the head of the frame is well above two metres. In all three, a manual bar handle is a bad answer, because reaching it means standing on something wet.

The options are a control rod, which hooks into the handle from floor level and is the simple answer on a moderately high window, or an electric unit with a wall switch and a rain sensor. The electric route brings its own rules in a bathroom: the wiring is designed around the zones the electrical regulations set out relative to the bath and shower, the switch position is chosen accordingly, and the work is signed off under the electrical part of the regulations. Actuators live outside the wet zones and the cable is run before the reveal is boarded. That is covered at electric rooflight installation.

Blinds in this room want a coated or foil-backed fabric that survives repeated wetting and drying, on a moisture-tolerant frame, and a solar-powered operation avoids running another cable into a wet room. Supply detail is at do roof windows come with blinds.

Gloucestershire bathrooms, and the ones with no outside wall at all

Two local patterns come up constantly. The first is the loft en suite, formed when a Victorian terrace in Gloucester or a Cheltenham townhouse is converted, where the bathroom takes the smaller half of the loft and a roof window is the only glazing possible. On those, the run of slope and the position of a purlin usually decide the size before anything else does.

The second is the ground floor shower room in a two-storey rear addition, boxed in on both sides by the neighbouring additions, with one small opaque window facing a two metre gap. A roof window in the lean-to slope above transforms how that room reads, and the constraint is the run of slope, often only 1.8 to 2.4 metres gutter to abutment, which caps the length of the unit.

Where a bathroom is fully internal with no roof directly above it, a sun tunnel is the answer instead. A reflective tube runs from a small dome on the roof, down through the loft, to a diffuser in the ceiling, and it delivers real daylight into a windowless room. That is described at sun tunnel installation.

What to send before a bathroom window is priced

Severn Skylights Gloucester fits new roof windows and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road. A bathroom job is notified to Building Control as part of the work, covering the structure, the glazing, the ventilation and any electrical sign-off, with the completion certificate coming back in the homeowner’s name.

Email info@severnskylightsgloucester.co.uk with a photograph of the roof from outside, a photograph of the room showing where the bath or shower sits relative to the slope, the covering if you know it, whether anything overlooks the roof, and the address. Brand ranges and the wet-room sash options are on the VELUX window installation page. An indicative range comes back within a day or two and a fixed price follows the survey, as set out on costs.