Escape openings and the rules that govern them

The numbers

An escape window must give an unobstructed clear opening of at least 0.33 square metres, with a minimum of 450mm in both height and width, and the bottom of that opening between about 800mm and 1100mm above the floor.

Every one of those refers to the clear opening once the sash is fully open, not the frame size. That is why an escape window is a larger unit than the figures suggest.

Why top hung

The sash has to swing entirely clear so a person can get through. A centre pivot window leaves the sash across the middle of the opening.

Escape units are top hung and open wider than a standard top hung window, typically to 45 degrees or more, and the hinge and stay are designed for it.

It cannot be created afterwards

This is the part that catches people. Changing a handle, removing a restrictor or adjusting a hinge on a standard window does not make it an escape window.

Where a conversion needed one and a standard unit was fitted, the remedy is a new, usually larger, window and frequently an altered opening. It is discovered at completion, which is the worst possible moment.

When it applies

Generally to habitable rooms in a loft conversion, and to inner rooms where escape depends on the window. It forms part of the Building Regulations approval, so it is settled at design stage rather than at ordering.

What we do

Ask whether escape is required before quoting, every time, including where the conversion is being carried out by somebody else. It changes the unit, the opening and the price.

Why a roof window is frequently doing two jobs

In a habitable room in a loft, Building Regulations require a means of escape in case of fire. In the great majority of loft conversions that duty falls on a roof window, because there is no other opening in the room and no other route out.

That turns a decision people expect to make on daylight and appearance into a compliance decision with hard numbers attached, and it rules out designs that would otherwise be the obvious choice.

It is settled at design stage. Reversing it after the unit has arrived is expensive; reversing it after the roof is closed up and the plastering is done is considerably worse.

What the requirement actually asks

Three things, and a window can fail on any one of them while passing the other two.

A minimum clear openable area. Measured as the genuinely unobstructed aperture, not the size of the window and not the size of the glass. Anything sitting in the opening when it is open reduces it.

Minimum opening dimensions. Both a minimum width and a minimum height, so that the aperture is not a long thin slot that meets the area figure while being impossible to climb through.

A maximum height of the bottom of the opening above the finished floor. So that a person can reach it. There is also a practical expectation that it can be reached from within the room without climbing over furniture or a fixed obstruction.

All three are checked against the specific model rather than against a general assumption, and manufacturers publish the clear opening figures for each unit in each size.

Why the opening type decides it

This is where most people are caught out, and it is a design question rather than a size question.

A centre pivot window rotates about a horizontal axis roughly halfway up the sash. When it is open, the sash sits across the middle of the aperture. That obstruction frequently takes the clear openable area below the requirement however large the window is, and a bigger centre pivot window does not fix it, because the sash scales with the frame.

Centre pivot is popular for a good reason: the sash rotates fully so the outer face turns inward and can be cleaned from inside the room. On a high level window that is the most useful feature a roof window can have. It is simply not, in most cases, an escape window.

A top hung window is hinged at the head and swings outward from the bottom. Nothing sits in the aperture, so the full opening is available. This is what gets specified for escape duty.

Some manufacturers make a specific escape hinge that allows the sash to swing further clear than a standard top hung unit, and where the numbers are marginal that is worth asking about by name rather than assuming a top hung unit will do.

Dual action units open top hung for use and pivot for cleaning. They cost more and on a high level window with an escape duty they remove the trade off entirely, which is frequently money well spent.

The height trap

The other common failure, and it comes from a good instinct.

Setting a window high in the slope gives more sky, more even light across the room, better privacy and less low angle sun. In a loft room with a knee wall it also throws light further back into the space. All of those are real reasons to want a window higher.

But the escape requirement sets a maximum height for the bottom of the opening above the finished floor, and a window positioned for the view can put the aperture out of reach for escape purposes.

The floor level matters here too, and it is easy to forget that a loft conversion floor is built up above the existing ceiling joists. A position measured against the old joist level is not the position it will occupy against the new floor.

When the requirement does not apply

Where an alternative means of escape exists, or where the conversion has a protected stairway meeting the relevant standard, the duty may be satisfied elsewhere. The roof window is then free to be chosen purely for daylight, and centre pivot becomes a perfectly good option again.

Which situation you are in is established with whoever is handling Building Control, at design stage, and it changes what you should buy. It is worth asking the question explicitly rather than assuming, because the two answers lead to different windows.

It also applies to rooms rather than to buildings. A loft used as storage is not a habitable room; a loft used as a bedroom or an office is. Converting a loft without notification and then calling it storage is a familiar arrangement and it surfaces at sale.

What else is going on at the same time

A loft conversion is notifiable under Building Regulations whether or not planning permission is needed, and the escape provision is one of several things being assessed. The others that touch the rooflight are worth knowing.

Part L, energy. The rooflights form part of the thermal calculation for the converted space.

Part F, ventilation. An opening rooflight contributes to the purge ventilation provision, calculated on free open area, which is another figure where a top hung unit and a centre pivot of the same size are not equivalent.

Part A, structure. Forming or widening the opening. In a traditional cut roof that is ordinary carpentry with a designed trimmer. In a trussed rafter roof, cutting any truss member needs an engineer, and a trussed loft is not convertible at all without replacing the roof structure.

The practical sequence

Establish whether this window carries the escape duty. If it does, that fixes the opening type and constrains the position.

Then check the specific model’s clear opening figures against the requirement, in writing, before ordering. Not the nominal size: the published clear opening.

Then choose position within what is left, balancing daylight against the maximum sill height.

Then, and only then, consider the things people usually start with: glazing specification, blinds, and whether it should be motorised.

A note on motorised escape windows

An escape window has to be openable by hand, without power and without a tool, by somebody in a hurry.

Motorised units are entirely compatible with that where they have a proper manual release, and it is worth confirming for the specific model rather than assuming. Find out at handover how the override works, demonstrated rather than described, and satisfy yourself that a person could operate it in the dark.

Where a window is both an escape route and above easy reach, that is a case for taking care over the specification rather than a reason to avoid a motor.

What has to be satisfied

An escape window has to provide an unobstructed clear opening meeting a minimum area and a minimum dimension in both directions, with the bottom of that opening within a workable height above the floor.

All three conditions apply together. A window can be generously sized and still fail on one of them.

The clear opening trap

This is where roof windows differ from wall windows and where most of the confusion comes from.

On a centre pivot roof window the sash rotates about an axis near its middle. When it is open, half the sash occupies the opening. The unobstructed gap a person could pass through is substantially smaller than the glass area.

That is why a large centre pivot window can fail an escape requirement that a smaller top hung one passes, and why escape windows on roofs are top hung or purpose made escape models that swing the sash fully clear.

Sill height on a raked surface

On a wall, sill height is one number. On a roof it depends on the pitch and on where the window sits along the slope, because the slope rises as you go up it.

The same window in the same position on a shallow roof and a steep roof will have its sill at very different heights above the floor.

A window set high on a shallow roof can have a sill well above the workable range even though it is a large window and looks entirely suitable.

This has to be worked out on a section drawing rather than estimated.

Whether one is needed at all

That depends on the escape strategy for the whole building, not on the room in isolation.

Many loft conversions rely instead on a protected stairway providing a route to a final exit, in which case an escape window is not required. Others depend on the window.

It is a building control question settled at design stage, and it constrains the window model, size, operation and position simultaneously.

Restrictors and the tension with child safety

A restrictor that limits the opening is desirable where children are concerned and unacceptable on an escape window unless it can be released from inside without a key or a tool.

Resolve it through the type of restrictor rather than by choosing one requirement over the other.

The order to do things in

Establish the escape strategy first, then choose the window. Choosing the window first and trying to make it comply is the expensive order, and it is the one people take.