Can a roof window be an escape window?

What the escape opening is actually for

The opening is not there so you can climb out and walk down the roof. It exists so that somebody trapped in the room can get to fresh air and be reached from outside, and so a firefighter has a route in. That is why the rules care about clear dimensions and about how high the opening sits, and why they say nothing about what happens once you are through it.

The requirement applies to habitable rooms in a loft conversion where the stair is not protected all the way to a final exit, and to inner rooms in some layouts. Where the stair enclosure is fully protected with fire doors and the correct construction, an escape window may not be required at all. That is a design decision for the person handling the conversion, and it is worth settling early because it changes which unit gets ordered.

The three numbers that define the opening

Approved Document B gives a clear openable area of at least 0.33 square metres, with a clear height of at least 450mm and a clear width of at least 450mm. All three have to be satisfied together. An opening 450mm wide by 450mm high is only 0.2 square metres, so meeting the two minimums does not meet the area.

The word that does the work is clear. It means the unobstructed gap you could pass through with the sash held open, measured between the frame and the sash, not the size of the glass and not the size of the frame. Manufacturers publish this figure for each size code and hinge type, and it is the only number worth checking on a specification.

How high the bottom of the opening may sit

The bottom of the clear opening must be no more than 1100mm above the floor of the room. That is the constraint that catches people out, because on a shallow pitch a window positioned for a good view can easily put the sash bottom higher than that.

In practice designers keep the bottom of the opening at or above roughly 800mm as well, so it is not a fall hazard for a child in a loft bedroom. That is not a maximum imposed by the guidance, it is sensible design, and it leaves a working band of about 300mm within which the cill has to land. On a 30 degree roof that band is a matter of a few hundred millimetres of position up the slope, so the setting out has to be done properly at survey.

Why the hinge decides whether the unit qualifies

A centre-pivot roof window rotates about an axis partway up the sash. When it is open, the top half of the sash has swung down into the room and it sits directly across the opening. The clear gap you could actually pass through is the space between the bottom of the rotated sash and the head of the frame, and on most centre-pivot units that gap is nowhere near 450mm high.

This is why a standard centre-pivot window, however large, is usually not an escape window. The glass is enormous, the opening is not.

Top-hung and escape hinged units

A top-hung unit swings the whole sash outward from a hinge at the head, leaving the full frame aperture clear below it. That is the geometry an escape opening needs. Most manufacturers also make a dedicated escape version, where the sash swings up through a much larger angle, often to around 45 degrees or beyond, held on a strengthened arm.

Some units are sold as combination products: they behave as a centre-pivot for everyday use and cleaning, with a secondary escape hinge that releases the sash to swing fully from the head when it is needed. Those are worth the extra where the window is the escape route and also the one you open every morning.

What the roof pitch does to the clear opening

The steeper the roof, the further a top-hung sash has to travel before it clears the opening, and on very steep pitches gravity works against the sash staying up. On shallow pitches the sash opens easily but the run of the window up the slope is long, which pushes the cill height problem back at you.

Published clear opening figures assume the unit is operated properly, and some manufacturers state a pitch range within which the escape function is valid. Check the range against your actual pitch rather than your estimate of it. A roof that looks like 45 degrees is often 38.

Where the window on its own is not enough

An escape window does not remove the need for interlinked smoke alarms on every storey, for fire doors where the design calls for them, or for the correct construction to the stair enclosure. If the conversion is being signed off, all of it is looked at together, and a compliant window in a non-compliant stairwell does not pass.

Nor does it help if the room below the opening is arranged so that nobody could reach the window. A wardrobe under the escape opening defeats it. So does a unit positioned in an alcove that a bed has to occupy.

Getting it signed off, and who tells Building Control

Fitting a roof window into a loft conversion is notifiable work, and so is fitting one into any habitable room where the opening is new. The inspector will want to see the unit’s published clear opening area against the room, and the cill height measured from the finished floor rather than the joists.

When we fit the window we notify Building Control as part of the job, and we set the unit out on the roof to land the cill in the right band before anything is cut. If somebody else is running the conversion, tell us at the enquiry that the window is the designated escape opening, because it changes the unit we specify and often the size. The wider set of rules is covered on building regulations for rooflights, and the loft context on rooflights in loft conversions.

We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road. Fifteen years in the trade, a ten-year workmanship guarantee, and VELUX, Fakro, Keylite and Roto units fitted. The roof is never left open overnight.