The short answer
The opening distance depends on the type. A roof window usually opens its sash to a set angle or, on a centre-pivot, swings it right round on its axis. A flat rooflight with a motor or gas strut lifts its hinged lid a limited amount, measured in millimetres at the front edge. Neither opens like a door, and the figure in the brochure is the one that counts, because it varies by maker and by size.
What matters in practice is the clear gap you get for air and, in a loft room, whether the window can serve as an escape route. Those are two different questions with two different answers.
Roof windows: pivot and top-hung
The two common roof window actions open in very different ways.
- Centre-pivot
- The sash turns on a horizontal axis near the middle of the frame. Push the bottom out and the top comes in, and the opening is a gap at the bottom and another at the top, each smaller than the full window. Most can be turned right over for cleaning the outer pane from inside. For ventilation, you can usually hold the sash at a small angle with a catch.
- Top-hung
- The sash is hinged at the top and swings out from the bottom, like a loft hatch, so the opening is one large gap at the lower edge. Because the bottom is lifted outward, you get a bigger clear opening for the same size window, and a person can reach out or climb through more easily. Some top-hung types limit the angle to hold the sash against the wind.
In VELUX terms, the centre-pivot codes are GGL and GGU, and the top-hung are GPL and GPU. The exact opening angle is set by the manufacturer, so we give the figure for the model you are quoted rather than a general one.
Flat rooflights and lantern vents
A flat rooflight that opens is a hinged glass lid on a kerb, lifted by a chain actuator, a screw drive or a gas strut. It opens to a fixed limit, and the gap is at the front edge. The limit is more modest than a roof window, usually a few hundred millimetres at most, set by the length of the actuator and the weight of the glass. A lantern may have one or two opening vents in its sides or roof, each with its own stop.
Smaller travel is not a fault. It is a design choice that keeps the lid stable in wind and stops it hitting the kerb or a neighbouring unit. For a kitchen or a stairwell where the aim is to let warm air rise out, a few hundred millimetres of clear gap along the full width does the job. Our page on opening rooflights for ventilation and cooling explains how much air that moves.
Opening size and escape from a loft room
If the rooflight is in a loft conversion bedroom, the opening has a second job. Under Approved Document B, a roof window can serve as an escape window when it meets minimum size and position rules for the clear opening and the height of its bottom edge above the floor. The sash has to open far enough, and be held open, to give that clear gap. A centre-pivot that only opens a few degrees at the bottom may not qualify where a top-hung would.
This is one reason we look at the room before choosing the action. It is not a point to assume. The exact dimensions depend on the building and are checked with Building Control, which we notify for the work. Where the window is meant as an escape route, we confirm the clear opening for the model on the quote.
What limits the opening
Several things hold a rooflight back from opening further.
- The stops or restrictors on the hinges, which keep the sash from flying out in a gust.
- The length of the actuator on a motorised unit, which sets the travel.
- The height of the kerb or the depth of the lining, which can foul a sash.
- Insect screens, blinds and shutters, which sit in the same space and can reduce the opening.
- A planning condition on a side-facing window, which calls for obscure glass and a fixed sash unless the part that opens sits higher than 1.7m above the floor.
The lining matters too. A deep, splayed reveal around a roof window gives the sash room to swing and keeps the finished edge clear of it. A tight straight lining can leave the open sash close to the plaster. That is worth resolving on paper, which is what the finish schedule is for.
Opening angle against the gap you can feel
An angle on a data sheet is not the same as the airflow in the room. A roof window opened to a modest angle on a steep pitch gives a tall gap at the bottom, while the same angle on a shallow pitch gives a much smaller one. The pitch of the roof, the width of the frame and the depth of the lining all change what the opening distance means once the window is fitted.
Ventilation also depends on having air able to leave as well as enter. A single opening rooflight works best when another opening elsewhere in the house, such as a ground-floor window, lets air pass through. Two rooflights at different heights, or one rooflight and a low window, set up the stack effect that carries warm air up and out. That is a more useful aim than chasing the widest possible angle on one unit.
Under Part F, an opening rooflight can contribute purge ventilation, which is the fast air change you use for cooking smells or a hot afternoon. The size of opening needed depends on the room, so it is worked out for the actual space, not assumed from the window alone.
Asking for the figure you need
Before you order, ask three things: the maximum opening angle or travel, the clear opening you will actually get for your size, and whether it is held at that position in wind. If a loft room is involved, ask whether the model can be used as an escape window and who confirms it.
For a wider look at powered and hand-operated units, see electric or manual opening rooflights, and for the whole range, our electric opening rooflights page. To have the opening checked against your own roof and ceiling, ask for a quote and we will measure it at the survey.