What an escape window has to do
An escape window is an opening a person can climb through if fire blocks the stairs, so the test is the clear gap the open sash leaves, not the size of the glass.
Approved Document B, the fire safety guidance that sits behind the Building Regulations, sets out what an escape window should achieve in a dwelling. The opening needs an unobstructed openable area of at least 0.33 m², and that area has to be at least 450 mm high and at least 450 mm wide. A gap 450 mm by 450 mm on its own is only about 0.2 m², so a unit that just scrapes both dimensions will not reach the area figure. In practice the clear opening needs to be comfortably larger in at least one direction.
The guidance also cares about where the opening sits. For a window in a roof slope, the bottom of the openable area should be within easy reach from inside the room, and the opening should let someone climb out and wait somewhere they can be seen and reached, usually towards the eaves. Whether a particular arrangement is acceptable in your house is a decision for Building Control on the project, and we treat their view as the final word rather than our reading of the diagrams.
Why conservation rooflights make this harder
A conservation skylight is built to look small, slim and flat on a historic roof, while an escape window needs a generous clear opening, and those two aims pull against each other.
Officers in Cambridge conservation areas generally favour modest units, set flush, with a slim steel frame and a vertical proportion that echoes the old cast-iron rooflights on terraces around Mill Road and Romsey. Many standard conservation sizes are narrow because they were drawn to sit between rafters and to look discreet from the street. A narrow unit can fail the 450 mm width on the clear opening even when the outer frame looks wide enough, because the frame, the upstand and the hinge all take a share of the width.
Some features of a conservation rooflight also affect the gap. A central glazing bar is normally part of the sash, so it moves with the glass and does not block the opening, but a fixed bar or a stay that projects across the opening would. The stay itself matters too: a top hung sash has to open far enough, and stay open safely, for someone to get through, and any restrictor has to release easily from inside without a key. You can read how the bars are made in conservation rooflight glazing bars, and how the standard sizes are judged on a historic slope in conservation rooflight sizes and proportions.
Which conservation units can meet the guidance
The route to an escape-size conservation rooflight is usually one of the larger sizes in a range, with a sash type the maker states is suitable for escape, checked against the real clear opening.
| Option | What to check |
|---|---|
| Side hung sash | Clear width once the sash swings open |
| Top hung sash | Opening angle, stay release, clear height |
| Larger standard size | Area of at least 0.33 m² in the clear |
| Two smaller units | Each must meet the guidance on its own |
Product details vary by maker, by size and between versions of a range, so we do not assume any unit qualifies from its outer dimensions. Before a rooflight goes on the light plan for an escape role, we check the manufacturer's stated clear opening and whether they describe that size and sash as suitable for escape, then set it against the room and the guidance. Two small rooflights side by side do not add up to one escape window: each opening is judged on its own.
Position on the slope and the planning balance
An escape rooflight has to sit low enough to reach from the room and close enough to the eaves to be useful, and that position is also the one the conservation officer will look at hardest.
On a terraced roof, the low part of the slope near the eaves is often the most visible from the street. Officers tend to prefer rooflights on rear slopes, clear of the ridge and valleys, and in modest numbers, which is covered in what conservation officers look for in a rooflight. If the only room that needs an escape window faces the street, the conversation becomes a trade-off between a larger unit placed low on the front slope and a different layout, such as moving the bedroom to the rear. Where a front or rear choice exists, front or rear slope placement sets out the light on each side.
The light matters here too. A larger unit near the eaves throws a lot of daylight onto the floor close to the wall, and on a south-facing slope it can bring noticeable summer heat into a small bedroom. Glass choice and a blind help, and how we plan the light explains how we work out where the sun patch will land through the year. In a listed building the size and position also form part of the listed building consent application, so escape needs should be raised early, as set out in listed building consent for rooflights.
When an escape window is the right answer at all
Escape windows are one tool in the fire safety design of a home, and in some layouts the guidance relies on a protected stair instead, so the first question is what the whole scheme needs.
Approved Document B treats rooms differently depending on their height above ground and how the stairs are arranged. In a loft conversion that creates a floor well above ground level, the guidance generally looks to a protected route down through the house rather than relying on climbing out of the roof. An escape window may still form part of the scheme, but it rarely replaces the stair arrangement on its own. Because this depends on the existing house and the new layout, the fire strategy is agreed with Building Control on the project, and we handle the Building Control notification where the rooflight work is notifiable. We then fit the rooflight to the size, position and opening the agreed scheme calls for.
Questions people also ask
Can a top hung conservation rooflight work as an escape window?
It can, if the sash opens far enough to leave a clear gap of at least 0.33 m² that is at least 450 mm high and 450 mm wide, and the stay releases easily from inside. Many top hung sashes open only to a ventilation angle, so the maker's details decide it.
Does an escape conservation rooflight need planning permission?
The escape role does not change the planning position. A rooflight in a conservation area may be permitted development unless an Article 4 direction removes it, and listed buildings need consent. See Article 4 directions and rooflights and our conservation rooflight installation page.
Can an existing conservation rooflight be replaced with an escape size?
Often, although a larger unit means a wider or taller opening in the roof, which may need extra trimming of rafters and, on a sensitive roof, fresh planning or listed building consent. We confirm the structure and the consent position at the survey.
Will an escape-size rooflight make the room too bright or hot?
A larger unit does let in more light and, on a south or west slope, more summer heat. Solar control glass and a fitted blind keep it comfortable, and the written light plan shows how the room will feel before anything is ordered. Book a light survey to see it for your roof.
