Roof window escape openings and the rules
An escape roof window has to give a clear openable area of at least 0.33 square metres, with neither the height nor the width of that opening less than 450mm, and the bottom edge of it between 800 and 1100mm above the finished floor. Those three figures are the whole test, and a window either meets them or it does not. Everything else people worry about, the brand, the glass, the colour of the sash, has no bearing on it. The reason this matters so much on a loft conversion is that the escape opening is usually the constraint that fixes the window size and its position in the slope, before daylight, view or symmetry get a vote. General installation detailing sits on the VELUX window installation page. What follows is the escape requirement on its own, and how a Gloucestershire roof tends to argue with it.
What the guidance is actually asking the opening to do
The requirement exists so that somebody in a room in the roof, cut off from the stairs by fire below, can get out through the roof onto a surface where the fire brigade can reach them. That is the purpose, and reading the figures with that purpose in mind makes them easier to remember.
0.33 square metres is roughly the aperture an adult can pass through wearing normal clothes. The 450mm minimum on both dimensions stops that area being made up of a long thin slot that nobody could climb through. And the 800 to 1100mm cill band puts the opening at a height a person can climb to from the floor without a chair, but not so low that a small child can walk into it.
The 0.33 square metre figure, measured on the hole and not the frame
The area is measured on the clear opening once the sash is fully open and held there. It is not the frame size, it is not the glass size, and it is not the structural opening in the roof. The sash, its hinge arms, the handle and any restrictor all take space out of it.
This is why two windows with the same size code can behave differently. A 780 by 1400mm frame sounds generous. Take off the frame section on all four sides and the depth the sash occupies when it stops rotating, and the actual measured aperture can drop well below what the outside dimension suggests. Manufacturers publish a clear openable area for each unit and each opening type, and that published figure is the one that gets checked.
Cill height between 800 and 1100mm, and where it is measured from
The bottom of the openable area, not the bottom of the frame, has to land in that band above the finished floor. On a roof window in a sloping ceiling, the frame is set at an angle, so the bottom of the frame and the bottom of the aperture are at different heights. The steeper the pitch, the smaller that difference becomes.
Finished floor means the floor as it will be when the room is done, including a floating floor, insulation and a carpet. A conversion set out off the joists and then finished with 70mm of build-up has moved every one of those measurements down by 70mm, and a window that cleared 1100mm at first fix can fail at completion.
Hinge position decides whether an opening is usable at all
A centre pivot sash rotates about a bar roughly halfway up. Open it and the lower half of the sash swings into the room while the upper half swings out. What is left is not one clear aperture but two slots, one above the pivot and one below it, with the sash itself blocking the middle. Neither slot on its own gets near 0.33 square metres on a normal size.
That single geometric fact rules a standard centre pivot out as an escape window, no matter how large it is. It is the most common and most expensive mistake on a loft conversion in this county, because the centre pivot is the cheaper unit, it is the one on the shelf, and it looks perfectly adequate until an inspector asks for the escape figure. The two designs are compared in detail on the centre-pivot and top-hung comparison page.
Top hung sashes, and the arc that clears the opening
A top hung window is hinged along its head and swings outward from the bottom like an awning. Push it fully open and the whole frame aperture is clear, because the sash is above and outside it rather than sitting in the middle of it.
Most top hung units in the common ranges will open to somewhere around 45 degrees under a friction stay. That is enough on a larger size, and on a smaller one it is not, because the sash still hangs across the top of the aperture and eats into the measured height. The published clear opening figure accounts for this, and it is why size and opening angle have to be read together.
Escape hinges, and the sash that swings clear to ninety degrees
The purpose made escape units go further. The sash is hung on a hinge that lets it swing out past the normal stop, in some ranges to a full ninety degrees, so the sash stands proud of the roof plane and the aperture underneath it is completely unobstructed. Some are ordinary top hung windows with an escape specification; others are dual function units that pivot for cleaning and then release onto a top hinge for escape.
The release is deliberately simple, usually a lever or a catch at the frame head, because it has to work in the dark by somebody who has never used it. It also has to work from inside without a key, which is the same principle that governs ground floor escape windows.
Which nominal sizes actually clear the requirement
As a rule of thumb, a top hung escape window needs to be at least 780mm wide, and once you are at 660mm wide the height has to be substantial to make the area up. The table below shows the arithmetic that decides it, using nominal frame sizes in centimetres.
| Nominal frame size | Width available at the aperture | Why it does or does not work |
|---|---|---|
| 550 x 980 | Under 450mm once the frame section is deducted | Fails on width before area is even considered |
| 550 x 1180 | Under 450mm | Same. Extra height cannot rescue a narrow sash |
| 660 x 1180 | Around 550mm | Marginal. Depends on the opening angle and the range |
| 780 x 1180 | Around 660mm | Usually clears the area with a top hung or escape sash |
| 780 x 1400 | Around 660mm | Clears comfortably. The common specification on a conversion |
| 940 x 1400 | Around 820mm | Clears with room to spare, and takes the least run of slope for the area |
The figures move a little between manufacturers and between generations of the same range, so the published clear opening for the exact model gets checked against the drawing rather than assumed from the table.
Where the window has to sit in the room, not just in the roof
An escape window has to be reachable. Guidance expects the opening to be positioned so that a person can get to it, which means it cannot be behind a fitted wardrobe, above a fixed bed platform, or in a dormer cheek where the floor in front of it is 400mm wide.
On a room in the roof with knee walls, the floor within reach of the slope is often the part everybody wants to fill with storage. Setting out the escape window early, and keeping a clear floor zone in front of it on the layout, avoids a joinery argument at second fix.
What the opening has to drop you onto
Getting through the window is half of it. The guidance also expects the escape route to lead to a place free from danger, which in practice means a roof slope a person can move across and be seen from, then down to the ground by ladder.
Two situations in this county fail that test regularly. A rear slope over a narrow lightwell or a walled yard, where anybody coming out lands in an enclosed space with no way on. And a valley position between two roofs, where the window opens into a gutter with no route out of it. In both cases the window may meet every dimension and still be the wrong window in the wrong place.
More than one storey above ground changes the answer entirely
Escape windows are the accepted route for a single storey loft conversion over a two storey house, so the escape floor is the second storey. Add another storey, so the new room is on a third storey above ground level, and windows stop being an acceptable escape route on their own. A protected stairway enclosure is then required, running the full height of the house to a final exit, with fire doors and appropriate wall and floor construction.
This catches out the taller Victorian and Regency houses. A three storey Cheltenham terrace or a tall Gloucester town house converting the roof space is very often into protected stairway territory, and the roof window question becomes a daylight question rather than an escape one.
The protected stairway, and the work it brings with it
Where the stairway route applies, the whole enclosure has to be dealt with: self closing fire doors to every habitable room off it, upgraded floor construction between storeys, and a final exit direct to open air. Existing glazed doors and open plan ground floors are usually where the difficulty sits, because an open plan kitchen and hall with the stair rising out of it has nothing to protect.
None of that is roof work, but it decides the roof work, because a conversion that has a compliant stairway does not need an escape specification window and can use whatever unit suits the room. That is worth establishing before the windows are ordered.
Sprinklers, open plan ground floors, and the alternative route
Where a protected stairway cannot reasonably be formed, most commonly because the ground floor has been knocked through, a designer may propose a domestic sprinkler system to the open plan area as an alternative approach. Building Control assess that case on its merits, and the outcome depends on the layout, the water supply and the design.
It is a design decision taken by whoever is producing the conversion drawings, and it changes the window specification. The practical point for a homeowner is to have it settled on paper before the roof is opened, because retrofitting an escape window into a finished conversion means the covering off again and a rebuilt reveal.
Interwar hips at Hucclecote and Longlevens, and the window that will not fit
The interwar and post war semis across Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings are hipped, pitched at 30 to 35 degrees, and covered in concrete interlocking tile. The rear slope narrows towards the hip, and there is usually a purlin across the middle of it.
An escape window wants to be 780 or 940mm wide and around 1180 to 1400mm long. On a hipped rear slope with a purlin at mid height, the clear rectangle is frequently shorter than 1400mm in the run of slope and only two rafter bays wide. The answer is often a wider, shorter unit set below the purlin, since 940 by 1180 makes the area up on width rather than length, and it keeps the cill inside the 800 to 1100mm band on a 32 degree pitch.
Trussed estates, and the width the trusses will not give
At Quedgeley, Abbeydale, Bishops Cleeve, Ashchurch and Yate the roofs are trussed at 600mm centres, and the clear width between two trusses is around 550 to 570mm. The narrow 550mm wide units exist for exactly that gap, and none of them can serve as an escape window, because 550mm nominal leaves under 450mm of clear aperture.
An escape opening on a trussed roof therefore means taking out part of a truss and carrying it on a designed trimmer that spans to the trusses either side. That is an engineer’s calculation and a structural submission to Building Control. Attic trusses, used where the loft was designed as a room from the start, have a clear middle by design and usually take an escape size without any of that. The wider issue of what can and cannot be cut is on the trussed roof page.
Cotswold stone slate and conservation units in an escape position
On a stone slate roof through Painswick, Bibury, Northleach or the wolds villages, the conservation style unit is usually what a conservation officer will accept. Those units come in a narrower range of sizes than a standard roof window, and their opening mechanisms vary between makers, so the clear openable area needs checking against the escape figures at specification stage rather than at the end.
Where the room genuinely needs an escape opening and the acceptable unit cannot deliver one, the conversation moves to a different position on the roof, a different route out of the building, or a pre application discussion with the conservation officer about a larger opening on a hidden slope. Getting that discussion started early is the difference between a workable conversion and a stalled one. The conservation rooflight page covers the sizes and finishes available.
Vale rear additions, low pitches, and the cill height that drifts
A Gloucester or Tewkesbury rear addition roof at 22 to 25 degrees creates a specific problem for the cill band. On a shallow pitch, a given length of window occupies far more vertical distance than the same window on a 45 degree slope, so the bottom of the aperture drops quickly as the unit gets longer.
Set a 1400mm long window on a 22 degree pitch and the vertical drop from head to cill is over 500mm. Position the head high enough for headroom and the cill can fall below 800mm; drop the head to lift the cill and the window ends up too low in the slope to see out of properly. The usual resolution is a shorter, wider unit, which is also the resolution the run of slope wants on a narrow rear addition. Cill height and the view are covered further on the cill height page.
What the inspector checks, and what gets recorded
On the completion visit the escape window is opened, the aperture is measured in both directions, the area is calculated, and the height from the finished floor to the bottom of the aperture is measured. The release mechanism is operated to confirm it works from inside without a key and without excessive force. The route out is looked at.
Getting this settled before anything is ordered is straightforward. At survey the pitch is taken off a digital level, the run of clear slope is measured between the eaves, the purlin and the ridge, the finished floor level is established from the conversion drawings rather than the existing joists, and the escape figures are worked backwards from there to a size code. The Building Control notification is submitted and closed out as part of the job, and the completion certificate comes back in the homeowner’s name.
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