Guide · Flat Roof Rooflight Installation
Flat rooflights over a bathroom or wet room
A rooflight over a bathroom is one of the better ideas in domestic design and one of the more demanding installations. The room produces more water vapour per square metre than anywhere else in the house, the ceiling is often the coldest surface in it, and privacy rules out the obvious answer of a window in the wall. Overhead glass solves the light and the privacy at once.
It also puts a cold plane directly above a steam source, which is why the specification matters more here than over a kitchen. This page covers glass choice, ventilation, electrical zoning, the ceiling build-up and what a wet room adds on top. It sits under flat roof rooflight installation.
Why a bathroom is the hardest room to glaze overhead
A shower running for ten minutes releases a substantial volume of water into the air. A bath does it more slowly. Either way the room reaches close to saturation, and warm saturated air rises to the highest available surface.
That surface is now glass. Glass is a better conductor than a plastered and insulated ceiling, so its inner face runs cooler than the surrounding ceiling even on a well specified unit. Warm wet air meets it, cools past its dew point, and condensation forms. This is not a fault, it is thermodynamics, and the design job is to make it happen less and to clear it quickly when it does.
Dew point, and where the glass sits relative to it
The number that governs everything here is the inner pane temperature under load.
At 22 degrees and 70 per cent relative humidity, which a bathroom reaches easily during a shower, the dew point is around 16 degrees. Any surface below that temperature will wet. A single glazed or single skin unit in a Chelmsford January might have an inner surface at 6 or 8 degrees, and it will run with water. A double glazed unit with a warm edge spacer and a soft coat low-E might sit at 15 or 16 degrees and wet only briefly. Triple glazing pushes it to 17 or 18 and, with decent extract running, the glass stays clear.
Glass specification for a wet room
Three parts of the make-up earn their place, and none of them is exotic.
- A low emissivity coating on the cavity face of the inner pane, which is what keeps the room-side surface warm.
- A warm edge spacer rather than an aluminium one, because the perimeter of the pane is the coldest strip and an aluminium bar makes it colder still.
- Argon fill, which is standard on modern units and adds a little more surface temperature for no cost.
Where budget allows, triple glazing over a shower is one of the few places in a house it clearly pays for itself, not in heating cost but in the glass staying clear. The glazing options are worth reading alongside this.
Whether obscure glazing is needed
Usually not, which surprises people. A rooflight in a flat roof faces the sky, and unless the roof is overlooked from a taller building or a neighbouring first floor window at a shallow angle, nobody sees in.
Overlooking is worth checking properly rather than assuming. On a Victorian terrace in central Chelmsford with a back addition, the neighbour’s rear bedroom window may look directly down onto a side return roof. Where it does, options are an obscure or acid etched inner pane, a body tinted unit, or an integrated blind. Etched glass keeps the daylight and removes the image, and it does so permanently without a mechanism to go wrong.
Extract ventilation, and what Part F asks for
The rooflight does not remove moisture. The extract does, and Building Regulations Part F sets the duty.
For a bathroom with or without a bath, intermittent extract is 15 litres per second. A continuous system runs lower but never stops. The critical detail is where the extract sits relative to the rooflight. An extract fan on the far wall from a steam source pulls moist air across the ceiling, and therefore across the glass, on its way out. A fan positioned near the shower, and a rooflight positioned away from it, keeps the wet air off the cold surface.
Where the duct terminates, and why it matters here
An extract duct discharging into a roof void is a genuine problem and it is common in older Essex conversions.
The moist air condenses on the underside of the deck, wets the insulation, and shows up as a stain on a ceiling a metre away from anything obvious. Where a rooflight opening is being formed, the void gets opened up and the duct route becomes visible, which is often the first time anybody has looked. A duct wants to terminate through the roof or through a wall to outside, insulated along its run so it does not condense inside itself, and set at a slight fall back towards the terminal.
An opening rooflight as a purge route
An openable unit over a bathroom does something a fan cannot. It clears a saturated room in a couple of minutes by stack effect, because the warmest wettest air is already at ceiling level and an opening at that level lets it leave under its own buoyancy.
It is a supplement to mechanical extract rather than a substitute, since Part F wants a fan regardless. But on a summer morning it is the difference between a mirror that clears in thirty seconds and one that takes ten minutes. The unit needs to be genuinely reachable or genuinely automatic, because an opener nobody uses contributes nothing.
Electric openers and the bathroom zones
Part P of the Building Regulations governs electrical work in a dwelling, and bathrooms carry defined zones with specific requirements for what may be installed where.
A rooflight actuator in a bathroom ceiling sits outside the immediate zones around a bath or shower in most layouts, but it still needs to be a suitable product and installed by somebody competent to certify it. Low voltage actuators, typically 24 volt, run from a transformer sited outside the room, which simplifies matters considerably. Solar powered openers avoid the mains question entirely and are worth considering where a cable route is awkward. Wiring for any of these is trivial at first fix and disruptive afterwards, so the decision belongs at design stage even if the actuator is fitted later. The options are set out under electric rooflight installation.
Humidity sensors and how they behave in practice
Automatic control sounds ideal and needs a little care.
A humidity sensing actuator opens when relative humidity rises past a threshold, which is exactly what you want after a shower and exactly what you do not want at two in the morning in December when the same threshold is crossed by a cooling room. A rain sensor closes the unit when it detects water, which protects the room but responds to the first drops rather than anticipating them. In practice a switched opener with an override, plus a decent extract fan on an overrun timer, is more predictable than full automation.
The vapour control layer around the opening
Cutting a hole in a bathroom ceiling cuts the vapour control layer, and that layer is doing the most important job in the build-up.
On a warm roof the vapour barrier sits below the insulation and above the plasterboard, keeping moist internal air out of the structure. Where an opening is formed, the layer has to be carried up the inside face of the kerb and sealed to the rooflight frame, continuously, with proper tape rather than optimism. A gap here over a bathroom will put water vapour into the deck year after year. The wider build-up sequence is covered on the main flat roof page, and cold roof arrangements need a different approach again.
Insulating the reveal so it does not run
The reveal between the ceiling and the glass is a common cold spot, and in a bathroom a cold spot becomes a wet spot within one winter.
The kerb is insulated on its inner face, the insulation is continuous from the roof deck up to the underside of the rooflight frame with no gap at the top, and the plasterboard lining is fixed over it. Where insulation stops 40mm short of the frame, that 40mm becomes the coldest line in the room, condensation forms there first, and it shows as a dark line around the top of the reveal. It is invisible while the reveal is being boarded and permanent afterwards.
Finishing materials that survive the environment
Ordinary plasterboard in a bathroom reveal is a short-lived decision. Moisture resistant board is the minimum, and in a wet room where the surface may take direct spray the lining wants tile backer board or a cement based board with a tanking coat.
The paint matters too. A vapour permeable finish lets the wall dry inwards, while heavy vinyl silk traps moisture behind it and lifts. A mould resistant emulsion on a well insulated reveal keeps its colour for years. Where the room is a genuine wet room with a level access shower, the tanking membrane wants carrying up the reveal to a sensible height rather than stopping at the ceiling line.
Splaying the reveal in a small room
Bathrooms are usually the smallest rooms in the house, so the reveal geometry does proportionally more work than anywhere else.
A square reveal in a 200mm roof build-up drops a hard rectangle of light straight down. Splaying the lining outwards towards the room spreads it across the ceiling and makes the opening read as considerably larger than it is. In a two metre by two and a half metre bathroom, a 700 x 700 rooflight with a splayed reveal reads better than a 900 x 900 one with a square shaft. It costs a little more plastering and nothing at all in glass.
Position over a shower, a bath or the floor
Where the unit sits changes both the light and the condensation behaviour, and the two pull in opposite directions.
| Position | What it gives | What to watch |
|---|---|---|
| Directly over the shower | Excellent light where you stand, a striking effect | Steam hits the glass first and hardest |
| Directly over the bath | Sky view while lying down, good daylight | Access for cleaning is awkward |
| Over the centre of the floor | Even light, easy access, less steam contact | Least dramatic of the three |
| Over the basin | Good light for a mirror, practical | Overhead light alone flattens faces at a mirror |
Where the shower position is fixed and dramatic light is wanted, the answer is better glazing and better extract rather than a compromised position.
Blinds, and whether a bathroom wants one
Frequently not. Where privacy is settled by geometry or by etched glass, and where the room is used mostly for short periods, a blind adds a mechanism into a humid environment for little benefit.
Where one is wanted, for a bathroom that doubles as a place to lie in a bath on a bright afternoon, it needs to be specified with the unit rather than added later. Rooflight blinds run in side channels within the frame, because gravity will not hold a blind against a horizontal surface, and that channel is part of the product. In a wet room the operating cords or the electric motor want to be suitable for the humidity.
Frame finishes that stand up to the humidity
The frame sees the same saturated air the glass does, and not every finish is indifferent to it.
Aluminium rooflight frames are polyester powder coated, usually to a marine grade specification on units intended for coastal exposure, and that finish is entirely happy in a bathroom. What matters more is the internal face: a thermally broken frame keeps the room-side aluminium warmer, so it does not become a condensation line of its own around the perimeter of the glass. Untreated or lightly finished timber in a reveal is the surface that suffers, since it absorbs moisture and moves. Where a timber lining is wanted for its appearance, it wants a proper moisture resistant finish on all faces including the back, applied before it is fixed rather than after.
Keeping the glass clear through an Essex winter
Two habits do most of the work, and neither is onerous.
Run the extract during the shower and for the overrun period afterwards rather than switching it off as you leave, because the vapour is still in the room when the door opens. And keep a small amount of background ventilation going through the winter, which is what trickle vents or a continuous low-rate fan are for. The rooflight itself wants washing once or twice a year from outside, and the autumn clear of leaves around the kerb is the same five minute job as on any other flat roof. Where an old single skin unit is misting constantly, no amount of ventilation will fix a failed sealed unit and it wants replacing.
A bathroom rooflight lives or dies on two numbers: the inner pane temperature and the extract rate.
Keep reading
More on flat roof rooflight installation
Flat rooflight falls, drainage and ponding
Read the guide →Walk-on rooflights and where they make sense
Read the guide →Single-ply, felt and GRP roofs around a rooflight
Read the guide →Flat rooflights on a cold roof build-up
Read the guide →How many rooflights a flat roof extension needs
Read the guide →
Get a quote
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