Entry 12 · Windows & Glazing
Escape windows: what a bedroom window has to do
What a bedroom escape window must achieve under Approved Document B, how the clear opening is measured, and what blocks it in a real fire.
A bedroom escape window exists so that a person can get out of a smoke-filled room and a firefighter wearing kit can, if needed, get in. In England the practical benchmark comes from Approved Document B, the government guidance that supports the Building Regulations: a habitable room on an upper floor generally needs an escape window if it has no other route to a final exit. The window must open far enough, wide enough and low enough in the wall for a person to climb through, and it must stay clear of anything that stops it doing that job at two in the morning.
This article sets out what the opening has to give, how it is measured on site, and the everyday obstacles that defeat a window which looked correct when the fitter left. The measurement rules are national; the habits that keep the route usable are household ones, and publications such as home fire safety checks cover the nightly and seasonal routines that keep exits real rather than theoretical.
01 What does an escape window have to do in a bedroom?
Approved Document B gives a clear functional test. Where a storey is between 3.5 m and 4.5 m above ground level, every habitable room, including bedrooms, needs either a door to a protected escape route or an escape window. Below 3.5 m the expectation in most dwellings is that any inner room, typically a bedroom off a landing, has an escape window as its way out.
The window has to serve two people at once, in effect. First, the occupant: an adult, perhaps woken from sleep, possibly in smoke, who has to operate the handle and get their body through the frame. Second, the fire service: crews arriving with breathing apparatus often prefer to use an existing opening rather than break glazing, and a window sized for escape is also usable for access and rescue. A window that only tilts, or that opens to a gap the width of a hand, fails both purposes even if it technically opens.
There is also a geometric condition. The window must be positioned so that a person can reach safety once outside: not over a deep lightwell, not onto a fragile roof without a place to stand. If the drop from the window to ground is more than about 4.5 m, an escape window alone is not accepted, and the design needs a protected stair or other route instead. The requirement is about the whole journey from the bed to open air, not the frame alone.
02 How is an escape opening measured?
The numbers to remember are 0.33 m and 0.45 m, and the geometry behind them. Approved Document B asks for a clear opening area of at least 0.33 square metres, with no dimension of that clear opening smaller than 0.45 m. The clear opening is the free space a body can actually pass through once the sash is fully open, measured at the tightest point, not the size of the glass or the outer frame.
Measuring it properly takes two people and a tape. Open the window to its maximum, then measure the smallest clear width and the smallest clear height, taking account of any sash, friction stay, restrictor or handle that intrudes into the hole. Multiply the two smallest figures: the result must reach 0.33 square metres. A common error is to quote the frame size, say 450 mm by 900 mm, and assume it passes, when a restrictor arm or a trickle vent housing cuts the usable gap below the minimum. On a typical sash window the meeting rail eats a large slice of the height, which is why many older sashes fail the test even though the frame is generous.
Position matters as much as size. The cill should be between 800 mm and 1100 mm above floor level, low enough to climb over without a chair, high enough that a child cannot fall out in normal use. The opening portion of the window must also be reachable and operable without a key or a tool under panic conditions: a locked handle that needs a key kept in another room defeats the requirement in practice, even where the hardware is legal on paper.
Homeowners replacing windows should have the escape provision confirmed in writing before signing, and keep the FENSA certificate afterwards, because that document is the record that the new installation preserved or provided the required opening. A survey that never measures the clear gap is a warning sign.
03 What blocks a window that is meant to be opened in a fire?
Most escape windows fail not in the showroom but in the room. The recurring obstacles fall into four groups.
Hardware and restrictors. Friction stays that limit the sash to 100 mm for child safety, locks with removed keys, and painted-shut frames are the most common faults in English bedrooms, particularly in older properties where a previous occupant sealed everything for draughts. A restrictor can be acceptable only if it releases quickly without tools; anything that needs a screwdriver is not an escape arrangement.
Furniture and storage. Beds pushed against the window, a wardrobe under the cill, a chest of drawers used as a bedside table in front of the glass: each turns a compliant opening into a wall. The check is simple. Stand where the bed is and try to reach the handle with one arm and climb through a cleared path; if a person has to move furniture first, the window is not an exit.
Glazing and secondary panes. Secondary glazing panels screwed into place, heavy curtains on a closed track, insect mesh fixed with fasteners, and window film applied over the original glass all add seconds or make opening impossible. In homes with listed or conservation-area glazing, magnetic secondary panels that lift out in one movement are the type to look for, and the escape opening should be tested with the panels in place, not removed for the occasion.
Outside the frame. External bars, fixed grilles, window boxes, dense shrubs and lattice all block the exit or the drop. Security bars are a genuine tension between burglary fear and fire safety, and the accepted answer is bars that release from the inside without a key, hinged or releasable, kept oiled and tested. The landing zone below matters too: a clear path away from the building, not a drop into a basement well.
04 How households keep the route honest
A window that passed inspection in 2010 may not pass tonight. Paint layers, a new lock, a donated sofa or a grown shrub can each close an opening that was once fine. The practical routine is an annual check: open every bedroom window fully, measure the gap if there is doubt, confirm the cill height still suits the room, clear the floor in front, and walk the outside of the house to see what now sits under each opening.
Keys belong on the handle or adjacent, out of reach of small children but findable in the dark. Everyone who sleeps in the room, including guests and teenagers, should know which window opens and how the stays release. Landlords have specific duties here: under the Housing Safety and Fitness standard, and the smoke alarm requirements in the Smoke and Carbon Monoxide Alarm (England) Regulations 2022, rental bedrooms must provide a usable escape route, and blocked or locked-out escape windows are a recognised hazard category.
The building rules set the geometry; the household maintains the reality. A clear 0.45 m by 0.73 m gap, a handle that turns without a key, a floor free of furniture and a drop that ends somewhere safe: those four conditions together are what turns a pane of glass into a way out.
A thermal barrier stops heat crossing a wall, but a bedroom window has a second duty that has nothing to do with insulation. It must give a way out of a bedroom: an opening large enough for a person to pass through in a fire, measured clear of the frame and any sash that swings into it. The usual blockers are a restrictor left engaged, a key kept out of reach, or furniture set in front of the reveal. The opening is measured as clear width and clear height, not as the glass size.