Roof window height from the floor in a sloping ceiling

A roof window is positioned on the slope, but it is used from the floor. The only height that matters to the person in the room is the distance from the finished floor to the bottom of the glass.

Builders tend to set windows from the outside. They count tile courses up from the eaves, chalk a line on the rafters and fit the frame to it. That method works for the weatherproofing, and it is exactly why the inside height can come out wrong. The frame sits on the slope, the ceiling finishes somewhere below it, and the floor finishes below that. Three layers sit between the tile batten and the carpet: the rafter depth, the insulation and lining, and the floor build-up. Each one is small. Together they shift the window several centimetres from where the outside drawing put it.

So the first job at a survey is to establish one datum inside the room. We mark the finished floor level, allowing for the boards, the underlay and the covering that is going down, and work every height from that line. Where the floor is still bare joists, the finished level is written on the plan so nobody measures from the wrong surface. A floor that rises by 20 millimetres after the window is in leaves a handle that is 20 millimetres lower than intended, and a cill that catches the new carpet edge.

• Datum: the finished floor, not the joists or the old boards.
• Lower edge of the glass: the height that decides reach and furniture.
• Upper edge of the frame: the height that decides headroom and how the ceiling meets the glass.

If you cannot comfortably reach the opening control, the window stays shut. A rooflight that never opens is only half the job, because ventilation is the second reason to have one.

Handles sit in different places depending on how the sash is hung. On a centre-pivot window the handle is normally along the top of the sash, so you reach up to it and pull down. On a top-hung window the handle is usually at the bottom, where a seated person can get at it easily. Our page on centre-pivot or top-hung roof windows sets out the differences, and the choice feeds straight back into height: a top handle wants the window lower on the slope, and a bottom handle gives you more freedom to go high.

As a working guide, an adult can usually operate a control at roughly shoulder height without strain, and a fair stretch above that without losing grip. Somewhere around 1,700 to 1,800 millimetres from the floor is a comfortable limit for a handle that has to be pulled or pushed with some force. Above that, people reach on tiptoe, lean on furniture or fetch a pole. A pole is fine for a deliberately high window, but it is poor planning for a bedroom window you want to open at night.

Who uses the room changes the number. A child's room needs a lower handle than a teenager's. A room that will be shared by people of very different heights wants the compromise figure, not the tallest person's. We ask at the survey who sleeps, works or sits under the glass and write the answer on the job sheet. That is a short conversation, and it saves a window that only one person can open.

Electric and solar-operated units take the reach question away, because a remote or wall switch does the lifting. They suit windows that have to sit high for other reasons, and our guide to electric and opening rooflight installation covers how they are wired and controlled.

A window that is 1,180 millimetres long down the rafter does not rise 1,180 millimetres up the wall. The steeper the roof, the taller the glass stands in the room. That single fact is why a height copied from another house so often fails.

The vertical rise of the glass is its length multiplied by the sine of the roof pitch. The horizontal run, how far the glass reaches into the room, uses the cosine. The table below works this through for a window 1,180 millimetres long on the slope, which is a common size. Dimensions are rounded, and the real figures change a little with each manufacturer's frame depth.

Read the table as a cause and effect. At 30 degrees the glass is only about 600 millimetres tall as you see it from the floor, but it stretches more than a metre across the ceiling. At 45 degrees it is 830 millimetres tall and no deeper than it is high. If the glass starts at 900 millimetres at either pitch, the top edge lands at very different heights, and the standing person under it reaches a different part of the sash.

Pitch also controls how far from the wall the handle sits. On a shallow roof the top of the window may be a metre or more into the room, so a handle there is reached by stepping towards the window. On a steep roof, the same handle is closer to the wall. The page on roof pitch and roof windows explains which angles suit which products and where a pitch is too shallow for the standard detail.

People use a sloping ceiling from three postures, and the window has to work for the ones the room is built for. A bedroom, an office and a landing ask different things of the same square of glass.

Standing rooms. A landing, a dressing area or a bathroom is used on your feet. The lower edge of the glass can start at about 900 to 1,100 millimetres, which clears a worktop or a basin and leaves the handle inside easy reach. The glass can run up towards the ridge side of the room without harm.

Seated rooms. A study or a home office puts a person on a chair for hours. Desk height is around 720 to 750 millimetres, and a chair puts the head around 1,100 to 1,300 millimetres above the floor. A window that starts at 1,200 millimetres leaves a seated person looking at plaster, so the lower edge should come down to something nearer 800 to 1,000 millimetres if the desk sits beneath it. If the desk is under a lower, flat part of the ceiling, the window can go higher and still deliver daylight to the work surface. That is a question of where the light lands on the desk, and it matters more than the exact glass height.

Lying rooms. A bedroom is the hardest to get right. Light at the bed head wakes you early, so a window centred over the pillow makes more trouble than one set a little to the side. If the bed sits under the eaves, the lower edge of the glass can start above the top of the headboard, and the handle has to be reachable from the mattress edge or from beside the bed. We mark the bed position on the floor with tape at the survey for this reason. The page on blinds for a nursery roof window shows how the same height logic applies when a blind has to be reached from a cot side.

Blind fixings need room too. A blind rail along the lower edge of the sash needs clear space to run and a hand-hold to pull it. A blind that sits behind a bookcase or bed head cannot be used, so height and the blind are chosen together. Our roof window blinds guide goes through the options.

In a loft conversion the window sits above a short vertical wall, the knee wall, and below the ridge. The space between those two limits is where the height is decided.

Start from the knee wall. Where the knee wall is 1,000 millimetres high, a window that starts at the top of it has a lower edge at about 1,000 millimetres, which is a good height for standing and reasonable for sitting. Where the knee wall is only 400 or 500 millimetres, the glass starting at the wall top puts the sill almost at floor level. Some people like that, particularly for a low window beside a bed. It needs more care with falls, with furniture and with the cleaning of the lower edge, and it raises the guard question discussed below.

At the other end, the ridge sets a ceiling on how high the glass can go. The frame needs a clear strip of roof above it so the flashing and the covering can be dressed cleanly. How close to the ridge you can go depends on the product, the roof covering and the pitch, and the page on how close to the ridge a roof window can go gives the working rules. The upshot is that the lowest usable edge and the highest usable top edge leave you a band. Inside that band you choose by posture and by where the handle falls.

Rafter spacing then trims the choice again. A window between two rafters can only be as wide as the gap allows, and its height on the slope is limited by where the trimming can go. The guide on roof window sizes and rafter spacing helps you test a height against the timbers, and the page about trimming rafters for a roof window explains what has to be cut and supported when the preferred height lands on an existing rafter.

A bedroom in a converted loft has to offer a means of escape. The window may have to do that job, and then its height stops being a matter of taste.

Approved Document B, the fire safety volume of the Building Regulations, sets out what an escape window must offer in a typical house. The opening has to be large enough to climb through, and the bottom of the opening cannot be too high above the floor, or a person cannot step out of it. The published figure for the lowest edge of the clear opening is 1,100 millimetres above the floor, with minimum dimensions for the clear opening itself. A window set to suit a tall user's handle reach alone can easily break that limit. Conversely, a window set low to suit the bed can satisfy it while being too short to meet the size rule. Our answer page on how big a roof window needs to be for a fire escape explains the rule with the working dimensions, and you can read the source documents on the Approved Documents collection at gov.uk.

Whether a particular loft needs an escape window at all depends on the layout, the stairs and the storey arrangement. Those points are checked for the address, and where the work is notifiable we handle the Building Control notification. The outcome has to be settled before the height is fixed. Moving a frame after the tiles are back on is not a small job.

Approved Document K covers protection from falling. A window with a low lower edge may need a guard, a restrictor or a particular type of glazing, depending on how far the drop is outside and what sits beyond the glass. A window that opens onto a steep roof slope is different from one that opens above a balcony. We raise this at the survey rather than after the frame is in.

Height alters what the room hears as well as what it sees of the sky. A window set low on a slope sits closer to the bed, to the desk and to the listener.

The glass is a thin part of the roof, and the lower it sits, the closer your head is to it. A centre-pivot window over a pillow means rain and aircraft reach the ear with very little between them. Set a little higher, or shifted to one side, and the same glass becomes part of the ceiling rather than the nearest surface to your head. That is a modest gain, but it is free, because it costs a different position and not a different product.

The more reliable way to quieten the room is in the build of the window. A laminated acoustic inner pane damps the tap of rain on the outer glass, and well-fitted seals close the gap around the sash. Both of these are written into the quiet spec that comes with every quote, along with what we heard at the survey. Rain on a roof window is different from rain on a flat lantern, and the page about whether roof windows are noisy in the rain takes the point further. In Crawley, where a bedroom under the roof may hear the Gatwick approach on some mornings, the choice is worth making before the frame goes in and not after the first night. The page on acoustic glazing for loft roof windows covers the options.

The final number is found in the room, not on the screen. It takes about half an hour, a tape measure, some masking tape and a chair.

1. Establish the finished floor level and mark it on the knee wall and the gable.
2. Mark the furniture positions on the floor: bed head, desk, wardrobe, door swing.
3. Draw the window outline on the ceiling and knee wall with masking tape, using the actual frame size for the product chosen.
4. Stand, sit and lie down under the tape. Reach for the imagined handle.
5. Check the lower edge against the escape and guarding rules for that room.
6. Test the top edge against the ridge clearance and the rafter positions.
7. Write the height from the finished floor on the drawing, and only then order the frame.

The tape test shows problems that a drawing hides, such as a bed head that sits exactly where the handle would swing, or a desk that is half in the shade of the ceiling. It is easy to move tape. Moving a window costs tiles, flashings and lining. The page about how a roof window is installed in a pitched roof shows the order of work once the number has been fixed, and the guide on roof window linings and making good the reveal explains how the inside finish meets the glass at the height you have chosen.

Sometimes no single height works. A long, low room with a bed at one end and a desk at the other asks two things of the glass, and one window cannot satisfy both.

The simplest answer is two windows at two heights, each matched to its own use: a lower one over the desk, a higher one near the bed. The less obvious answer is a stacked or side-by-side combination that gives one continuous opening with the sash at the height you can reach. Our page about stacked and side-by-side roof window combinations describes the layouts. A combination also helps where the pitch is shallow and a single tall window would run out of slope.

Where the lower edge must be high to clear a bed, or low to suit a seated worker, a larger window is tempting. It adds light, but it also adds heat in summer and the noise of a larger pane in rain. Choosing two smaller windows is usually the calmer specification, because each can be placed at its own height and each can carry its own glazing build. The guide to roof windows for a loft conversion looks at how many windows a typical loft room can carry.

These are the questions that come up when a homeowner stands in the loft with a tape measure. Each answer is a starting point, and the real figure is confirmed in the room.

What is a normal height for the bottom of a roof window?

There is no single figure, because it depends on the pitch, the knee wall and what the room is for. For a standing room, about 900 to 1,100 millimetres from the finished floor is a common band. For a seated desk, a lower edge at 800 to 1,000 millimetres keeps the glass useful at chair height. If the window is an escape route, the lowest edge of the clear opening must be within the Approved Document B limit.

Can I raise or lower the window after the roof is tiled?

Not easily. The frame is fixed to the rafters, the flashing is dressed into the covering and the lining is cut to the opening. Moving the window means stripping the flashing, cutting new rafter trimming and reworking the tiles or slates above and below. Fix the height with the tape test before the frame is ordered, and avoid a second opening.

Does a higher window let in less light?

The glass lets in the same amount of daylight, but it lights a different part of the room. A higher window throws light deeper into the space and onto the far wall and ceiling. A lower window lights the floor and furniture closer to the wall. Neither is wrong. Match the lower or higher position to where you need the brightness.

How do I reach a window that has to sit high?

There are three routes. A pole with a hook will work the handle of a top-hung or centre-pivot window. A motorised unit opens by remote or wall switch. Or the window moves lower and a second one takes the high position. We suggest the route at the survey, once we know who will use the room and how often the window will open.

If you would like a height fixed for your own room, tell us the pitch, the knee wall height and the use of the room through the quote form, or send the photos on WhatsApp. The full service is on our roof window installation in Crawley page.