Question answered
What size must an escape roof window be?
The short answer
An escape roof window needs an opening area of at least 0.33 square metres, with a height and a width of at least 450 mm each. The lower edge of the part that opens must sit no higher than 1100 mm above the floor, and the sash has to stay open on its own. Approved Document B applies these figures to upper storeys up to 4.5 m above ground.
An escape window has to be big enough to climb through and low enough to reach. Approved Document B sets the numbers, and they decide which roof window you can order.
The four numbers in paragraph 2.10
Approved Document B, the fire safety volume of the Building Regulations, says what an escape window must provide. The area is at least 0.33 m². The height and the width are at least 450 mm each. The bottom of the part that opens sits at most 1100 mm up from the floor. Finally, once opened, the sash has to stay where it is put.
Notice that 450 mm by 450 mm gives only about 0.2 m², so the area figure is the one that sets the size. A window 450 mm wide would need to be about 733 mm high to reach 0.33 m², and one 600 mm wide needs roughly 550 mm. Measure what actually opens, with the sash held at the angle where it stays, and not the size printed on the box. A pivot bar, a blind or a thick frame all eat into the clear gap, so a window that looks big enough on paper can fall short once it is open.
Why the 4.5 m line matters
Paragraph 2.2 deals with upper storeys up to 4.5 m above ground level. In a house with a single stair, every habitable room on such a storey, apart from a kitchen, needs either an escape window or an external door, or a direct way out to a protected stairway, meaning a stair walled off from the rooms around it so that it stays usable in a fire.
Storeys higher than 4.5 m follow separate provisions, so the window sizes above are not the whole story for a tall house. A loft room over a two-storey house generally needs a protected stair, not merely a larger roof window. Settle that question first in any loft conversion, before choosing the window, because it can shape the layout of the floor below. Building control confirms what your design needs, and the choice between a roof window and a kerb-mounted unit is looked at in rooflight or roof window.
Sill height is where roof windows struggle
A roof window sits in a sloping surface, so the height of its lower edge above the floor depends on where it is placed up the slope and how tall the room's knee wall is. The 1100 mm limit is measured to the bottom of the part that opens, not the top of the frame. A window set high in a steep roof can fail on this point even when the glass is generously sized.
The usual remedy is to move the window lower down the slope, to choose a taller unit whose bottom edge reaches near the floor, or, on a larger project, to combine a roof window with a wall window. The range of sizes on sale is wide, so the decision is mainly about position. On a planned loft room we mark each opening against the intended floor level during the survey, before any rafter is cut, which keeps the conversation about the escape route with building control straightforward.
Practical points before you order
Look at how the window opens. A centre-pivot unit turns on its middle axis, so the sash sits across part of the opening, whereas a top-hung or projecting design swings out and up. A friction hinge or a restraint that holds the sash at a safe angle helps with the rule about staying open unaided. A motorised window is a separate choice, discussed in electric and solar roof windows, and if it doubles as an escape route you should ask how it behaves when the power fails.
Remember too that a roof window also has to satisfy the energy limits in Part L, which is met by a roof window rated at 1.4 W/m²K or in Band B overall. A bigger opening is therefore not a free choice, because larger glass loses more heat. The effect of frame size on the total is described in whether a skylight or a window is cheaper.
Side slopes and what comes after
If you rely on permitted development, a window on a side slope must have obscure glass, and it may not open at all unless its moving part is over 1.7 m above the floor. An escape window needs to open and sit low, so the two requirements pull against each other. Where a side slope is your only option, talk to the planning team before you order anything.
Once the window is fitted, its flashing and seals decide whether it stays dry. The signs of trouble are listed on our page about a leaking roof window. For the whole range of questions about roof glazing, start at the skylights and roof windows hub. A site visit lets us check the actual floor height and rafter positions, which is far more reliable than working from a drawing.
Escape windows in a loft room: two checks
Does a landing or bathroom roof window need to meet escape sizes?
No. The escape rule concerns habitable rooms, such as bedrooms and living rooms, on upper storeys, and a kitchen is left out. A bathroom or landing roof window is chosen for light, ventilation and fit, though building control can confirm where any escape route has to be.
Can I check the clear opening myself before ordering?
Yes. Open the existing window to the angle it holds, measure the unobstructed height and width between sash and frame, and multiply the two. If the result is close to 0.33 m², allow for the pivot bar and any blind, and ask building control to confirm before you buy.
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