Can a roof window be an escape window?

Which rooms actually need one

The requirement is not attached to roof windows, it is attached to rooms. Broadly, a habitable room in a new loft conversion at first floor level needs either an escape opening or a protected escape stairway, and in practice on a two-storey house being converted to three, the answer is usually the protected stairway with escape windows as a supporting measure. On a single-storey extension or a bedroom formed at first floor in a two-storey house, an escape opening in the room itself is the common route.

Where an inner room, a room you can only reach by passing through another room, is created in a loft, the escape opening becomes more important again. And where a bedroom is more than one storey above the ground outside, an escape window on its own stops being an acceptable answer, because nobody is expected to drop that far. Which route applies to your house is a Building Control conversation rather than a product decision, and it is settled before a window is ordered.

The three numbers, in one paragraph

An escape opening needs a clear openable area of at least 0.33 square metres, with a minimum of 450mm measured in both height and width, and the bottom of the openable area between 800 and 1100mm above the floor of the room. Those numbers are measured on the hole you would actually climb through, not on the frame you ordered and not on the glass. That distinction is where most of the trouble comes from, and it is worked through properly on the escape openings page.

Why a fixed unit cannot serve, whatever its size

This sounds obvious written down and it is still one of the more expensive mistakes made on loft conversions. A fixed roof window has no openable area at all, so its clear opening is zero regardless of how much glass it carries. A three metre run of fixed glazing in a loft bedroom satisfies the daylight requirement completely and satisfies the escape requirement not at all.

It goes wrong because fixed units are the least expensive in the range and the specification often gets trimmed late, after the layout has been agreed and before anybody re-checks which window was carrying the escape duty. If a room has one window and that window is the escape route, it opens. Where a room has several, one of them is nominated and the rest can be whatever the daylight and the budget want.

The hinge, and the sash that has to get out of the way

A sash sitting in the middle of the opening blocks the very hole you are trying to climb through, which is why hinge arrangement decides this as much as size does.

A standard centre-pivot sash rotates about its middle, so at any normal opening angle half the sash is inside the room and half is outside, and the clear gap is well under half the frame. Manufacturers answer this with an escape hinge, sometimes described as a top hung escape function, which lets the sash swing right up and clear to around 90 degrees so the whole aperture is available. Top hung units open on a hinge at the head and clear the opening naturally, which is why they turn up so often in escape positions. The full comparison of the two designs is at centre-pivot and top hung roof windows compared.

Where the window sits in the room, not just in the roof

The 800 to 1100mm band is measured from the floor of the room to the bottom of the openable area, and on a pitched roof that is a different thing from the bottom of the frame. On a shallow slope the sash sits a long way up the roof for a modest gain in height above the floor, and on a steep slope it climbs fast.

That is why the position is set out from inside before anyone marks the roof outside. Push the window 200mm up the slope for a tidier tile course and the cill can drift above 1100mm, at which point the opening is technically out of the band and somebody has to argue it at inspection. Push it down for a lower cill and it can drop under 800mm, which the guidance also rules out because the opening becomes a hazard in ordinary use.

There is a second question the drawing has to answer, which is what the opening leads to. A window that opens onto a roof valley, a parapet gutter or the ridge line of a lower extension is a different proposition from one that opens onto a clear slope with a drop to a garden. That geometry is covered on the escape openings page.

Gloucestershire roofs that make the size hard to reach

Three local situations regularly collide with the escape requirement, and all three are better identified at survey than at inspection.

The trussed estates at Quedgeley, Abbeydale, Bishops Cleeve and Ashchurch give a clear 550 to 570mm between trusses, which is enough for the 450mm minimum width on paper but leaves the whole burden on the height of the opening, and the narrow units are not all long enough to make the area up. Anything wider requires a designed trimmer with the affected truss carried on a beam.

The interwar hips at Hucclecote, Longlevens, Warden Hill and Charlton Kings run out of slope. A purlin across the middle and a binder in the ceiling frequently leave around 1.2 metres of vertical run, and once the flashing laps are taken off that, a long escape sash simply does not fit in the clear zone.

The Severn Vale rear additions in Gloucester and Tewkesbury have the opposite problem. The lean-to pitch is often 22 to 30 degrees, and on a shallow pitch a window has to sit low on the slope to bring the openable cill down within the band, which then runs into the head lap the flashing kit needs above it.

On Cotswold stone slate above Painswick and Bibury, a conservation rooflight is frequently the right unit visually, and the conservation ranges include opening versions built to serve as escape openings. That gets confirmed model by model rather than assumed, because not every conservation pattern does.

Building Control, and what gets checked on the day

An escape window is notifiable work and the notification is handled as part of the job here, with the inspections booked and the completion certificate coming back to the homeowner in their name. Keep it: it is the paperwork a buyer’s solicitor asks for, and finding it missing at the point of sale is a much worse problem than dealing with it now.

At inspection the officer opens the window, measures the clear aperture with the sash held where it actually sits, and measures the cill height from the finished floor. Both measurements are taken on the installed unit, so the drawing being right is necessary and not sufficient. Photographs of the opening at each stage go to the homeowner with the completion paperwork.

How this gets settled before anything is ordered

At survey the rafter or truss centres, the depth, the pitch off a digital level, the position of any purlin, binder or strut, and the finished floor level are all recorded, and the escape arithmetic is done on that page rather than in the brochure. The output is a specific size code with a specific hinge option, and the cill position marked in the room.

Severn Skylights Gloucester fits new roof windows and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road. Model-by-model detail on which sizes clear the requirement is on the VELUX window installation page. To start, email info@severnskylightsgloucester.co.uk with a photograph of the roof, a photograph of the room, the floor to slope dimensions if you have them and the address, and an indicative range comes back within a day or two.