What an escape window is for
A roof window fire escape, or escape window, is a roof window sized and placed so that a person trapped in a room by fire can open it, climb out and wait at the eaves to be reached from the ground. It is a specific item in Approved Document B, the fire safety guidance that sits behind the Building Regulations, and it comes with measurements that decide which roof window you can use and exactly where it goes on the slope.
The idea is simple. In a house, a fire on the ground floor fills the hall and stair with smoke first. Anyone upstairs then has two ways out: down a stair that has been built to hold back fire and smoke for long enough, or through a window. The guidance treats these as alternatives in some parts of a house and as a pair in others, and the answer depends mainly on how high the floor of the room is above the ground outside.
A roof window can do this job well, but only if it is chosen for it. A standard centre pivot window turns on a hinge across its middle, so the open sash leaves two narrow gaps rather than one clear hole, and the top gap is out of reach. An escape roof window opens from the side or from the top with the sash swinging well clear, giving a single, large, unobstructed opening. Our guide to types of roof window covers how the opening types differ in daily use; this page is about the fire rules that sit on top of that choice.
It is also worth being clear about what an escape window is not. It is not a way of making a room safe on its own, and it is not something a homeowner can decide is adequate by eye. It is one part of a fire strategy for the whole house that Building Control agrees before work starts, alongside smoke alarms, doors and the stair.
When the guidance calls for one
The trigger is height: rooms on a floor up to 4.5 metres above ground level normally need either an escape window or direct access to a protected route out, while floors above 4.5 metres rely on a protected stair instead, because a window at that height is no longer a sensible way down.
That 4.5 metre figure is measured from the ground outside to the floor of the room, not to the window. In a typical two storey house the first floor sits roughly 2.5 to 3 metres up, so first floor bedrooms fall inside it. A loft room added above a two storey house sits higher, usually above 4.5 metres, and that changes the approach completely.
- Room in the roof of a bungalow or chalet bungalow. The loft floor is usually well under 4.5 metres above the garden, so each habitable room there generally needs an escape window or access to a protected route to the exit. This is where roof window escape rules come into play most directly.
- Loft conversion on a two storey house. The new floor is usually above 4.5 metres. Current guidance looks to a protected stair running from the loft down to the front door, with fire doors onto it, rather than to windows in the roof. Some layouts and some alternative approaches still include an escape window, and it is Building Control that decides.
- Inner rooms. A room whose only way out is through another room (a loft bedroom reached through a study, for example) is treated as an inner room, and the guidance generally expects it to have an escape window.
- Open plan layouts. Where a ground floor has been opened up so the stair rises out of a living space, the stair is no longer protected. Separate provisions then apply to the floors above, and escape windows can be part of them.
Across Cambridge this splits the housing neatly. The chalet bungalows and single storey homes in the villages around the city, and the dormer bungalows along many village roads, are the homes where a pitched roof skylight fire escape is a live design question. The two storey Victorian and Edwardian terraces of Romsey, Petersfield and the streets off Mill Road, when they gain a loft room, usually rely on the protected stair, with roof windows chosen for light and air instead. Our guide to roof windows for loft conversions looks at those terrace lofts in more detail.
The size of the opening
The guidance sets a minimum unobstructed openable area of 0.33 square metres, with the clear opening at least 450mm high and at least 450mm wide. All three conditions have to be met at once.
The word that matters is unobstructed. The figure is measured across the gap a person can actually climb through with the sash fully open, not across the outside of the frame and not across the glass. A roof window frame is thick, and the sash, handle and hinges all take a bite out of the hole. A window whose frame measures 780mm by 1,180mm offers a clear opening noticeably smaller than those numbers, and the manufacturer publishes that clear opening figure for each escape model. That is the number Building Control will check.
The two minimum sides and the minimum area also interact. A gap of 450mm by 450mm gives only about 0.2 square metres, which falls short. To reach 0.33 square metres with a width of 450mm, the height needs to be around 735mm. In practice, escape roof windows are made in widths and heights that clear the minimum comfortably, and the narrowest frame widths made for fitting between closely spaced rafters usually fall short on width alone.
| Measure | Guidance figure | Measured where |
|---|---|---|
| Clear opening area | At least 0.33 m² | Across the open gap |
| Clear width | At least 450mm | Between the frame sides |
| Clear height | At least 450mm | Along the slope |
| Bottom of opening | 600mm to 1,100mm | Above the finished floor |
| Eaves to bottom of opening | Up to 1,700mm | Along the roof slope |
The last two lines of that table are what make a roof window different from an ordinary wall window, and they are where the real design work lies. They are set out in the next two sections. As with any figure from the guidance, the current edition of Approved Document B and your Building Control body have the final word, so treat these as the working numbers to plan around rather than a substitute for that check.
Sill height inside the room
The bottom of the openable area should sit no more than 1,100mm above the floor, so that someone can get a knee onto it and climb out without a step, and roof window escape guidance sets it at no less than about 600mm so it is not so low that it becomes a hazard in daily use.
On a sloping ceiling this is harder to achieve than it sounds. The height of the bottom of the window above the floor depends on three things: the height of the knee wall (the short vertical wall where the slope meets the floor), the pitch of the roof, and how far up the slope the window starts. On a steep roof the slope climbs quickly, so a small move up the rafters makes a large change in sill height. On a shallow roof you have to go a long way up the slope to gain the same height, and that pushes the window further from the eaves.
That 1,100mm limit is also close to the height that gives a good view out of a roof window when you are standing, which is a helpful coincidence. A sill in this band lets you see the garden and the street, not just sky. Our guide to positioning a roof window in a room goes through head height and view in general; for an escape window, the sill position is set by the fire guidance first and the view follows.
Low sills bring a second set of rules into play. Approved Document K deals with protection from falling, and it looks for guarding where a window sill is below 800mm in an upper floor. An escape window needs to be easy to open and climb through, while a low opening needs to stop a child falling out. Building Control balances the two for each room, and the usual answers are a sill at the upper end of the escape band, a restrictor that releases easily from inside, or both. There is more on restrictors in our answer on how to make a roof window child safe.
Distance from the eaves
Once through the window, the person escaping waits on the roof at the eaves for a ladder, so the window has to be close enough to the gutter line for them to get there safely. Guidance for escape roof windows puts the bottom of the opening no more than 1,700mm from the eaves, measured along the slope outside.
This is the measurement that catches people out, because it is taken on the outside of the roof and it pulls against the sill height inside. The eaves are at the bottom of the slope; the room floor is some distance above the wall plate; the knee wall and the pitch then set how far up the slope the 600mm to 1,100mm sill band falls. On a steep roof with a short knee wall, both measurements fit easily. On a shallow roof with a tall knee wall, the sill band can sit more than 1,700mm up the slope, and then no roof window position meets both.
When that happens, there are a few ways through, and each has a cost in the design:
- Lower the floor or the knee wall, if the structure allows, so the sill band moves down the slope.
- Use a taller escape window so the bottom of its opening reaches down towards the eaves while the sill inside stays within 1,100mm, sometimes with a raised step or platform inside that Building Control accepts.
- Replace the escape window with a dormer, whose vertical window can meet the sill rule without depending on the slope.
- Change the fire strategy so the room is served by a protected route to the exit rather than by a window.
The eaves themselves need a second look. The ground below should be somewhere a ladder can stand: a garden or a path, not a glass roof over a kitchen extension, a conservatory or a steep drop into a basement light well. If the escape route leads into an enclosed back garden with no way out except back through the house, the guidance expects that garden to be deep enough to be clear of the building. On a Cambridge terrace with a single storey rear extension, this can rule out the rear slope and point to the front, where the eaves face the street.
How the slope and the light fit in
An escape window is also a real roof window that will light the room every day for decades, so the choice of slope and the glass should be planned with the same care as the fire rules, because it is the part you will notice most.
Because it sits low on the slope, near the eaves, an escape window lights the floor and the lower half of the room strongly and throws less light across the ceiling. It is excellent for a view and for a sense of openness, and less effective at lighting the back of a deep room. Pairing it with a second window set higher on the same slope, or on the opposite slope, spreads the light far better. Our guide to roof windows for loft bedrooms covers that pairing in a bedroom, where early sun and blackout matter.
The fire rules decide which slope has usable eaves; they do not care about the sun. If the only workable slope faces south or west, the escape window will take full summer sun for several hours, and on the flat land around Cambridge there is little to shade a roof. Solar control glass and an external awning blind make a large difference there, and neither gets in the way of opening the window fully. An internal blind should be one that moves with the sash or clears the opening completely, so it cannot block the way out.
North slopes are the opposite: steady, soft, glare free light, which suits a study or a studio. If the choice of slope is open, this is exactly the question a light plan answers: where the sun patch falls through the day and the year, and which glass keeps the room comfortable.
Size needs the same balance. Escape windows are by nature on the larger side, and a large opening in a small bedroom can bring more heat and glare than the room wants. Choosing the smallest escape model that clears the opening figures, and pairing it with the right glass, usually gives a better room than going bigger for the sake of it.
Structure and the party wall zone
An escape window is wide, and the rafters in older Cambridge roofs are set close together, so fitting one usually means cutting one or more rafters and trimming around the opening. Its position is also limited by fire rules at the sides of the roof as well as at the bottom.
Rafter spacing in Victorian and Edwardian roofs varies, but it is rarely wide enough to take a full escape width between two rafters. The cut rafters are carried on new timbers across the top and bottom of the opening, sized to take the load back to the uncut rafters either side. This is normal work and Building Control checks it as part of the same notification, but it does mean the width of the escape window affects the structure. Our guide to trimming rafters for a roof window explains the method.
At the sides, a separate fire rule applies. On a terrace or a semi, the party wall between you and your neighbour runs up into the roof, and Part B guidance keeps rooflights out of a zone of 1,500mm either side of it, so that a fire cannot jump across the roof from one house to the next. On a narrow terrace, with the party walls on both sides, that zone takes a real bite out of the usable slope. Combine it with the eaves limit at the bottom and the sill band inside, and the escape window has a fairly small area of roof where it can go. Mapping that area is one of the first things we do at a survey.
Roof windows near a boundary also need an external fire rating suitable for their distance from it. For the escape windows made by the main manufacturers this is covered by their published test data, but it is part of what gets checked.
Building Control and replacing older windows
A new roof window is notifiable building work, and an escape window adds the fire strategy to what Building Control checks. We handle the Building Control notification where the work is notifiable, so the escape window, the structure and the rest of the installation are signed off together.
For a new loft room, the escape window is normally part of a larger application covering the conversion as a whole: the stair, the doors, the smoke alarms, the floor and the insulation. The roof windows are drawn on that application, and Building Control will want to see the escape model, its clear opening figures, its sill height and its distance from the eaves. For a roof window added to an existing room in the roof of a bungalow, the application is smaller, but the same measurements apply. Our answer on whether roof windows need building regulations covers the wider process.
Replacing an older roof window raises its own point. If the existing window was installed as the escape window for the room, its replacement needs to keep that role: at least the same clear opening, the same or better sill and eaves position, and an opening type that still clears the gap. Replacing an escape window with a centre pivot model of the same frame size would quietly remove the room's way out. Older loft rooms sometimes have roof windows that were never large enough for escape at all, and a replacement is a good moment to upgrade to one that is, if the opening can be widened.
If you are planning a room in the roof or replacing an older window in one, you can book a light survey and we will measure the knee wall, the pitch and the eaves, find the zone where an escape window can go, and set it out in the written light plan with the quote. The roof window installation page covers the rest of the service.
Questions people ask
Do I need an escape window in my loft conversion?
It depends on the height of the loft floor and the fire strategy for the house. In a bungalow room in the roof, where the floor is under 4.5 metres above ground, an escape window is usually part of the answer. In a two storey house gaining a loft room, a protected stair is normally the main route out, and Building Control confirms whether a window is also needed.
Can a centre pivot roof window be an escape window?
Generally not. The sash turns on its middle and leaves two narrow gaps instead of one clear opening, so it cannot provide the unobstructed area the guidance asks for. Escape roof windows open from the side or from the top, swinging the sash clear.
Can an escape window have a restrictor or a lock?
Yes, with care. Opening stays and restrictors are allowed if they release easily from inside, and a lock should not need a separate key to open it. The aim is a window a child cannot open by accident but an adult can open in the dark without searching for anything.
What if my roof is too shallow to meet the eaves distance?
Then the answer is a taller escape window, a change to the floor or knee wall, a dormer instead of a roof window, or a different fire strategy for the room. We work that out at the survey, before anything is ordered.
