Three ways a rooflight moves
The words open and opening cover very different motions, and the distance depends on which one you have.
- Pivot. The sash turns on a horizontal axis near the middle of the frame. The top edge comes into the room and the bottom edge swings out. On many models it rotates far enough to bring the outer pane inside for cleaning.
- Top-hung. The sash is hinged at the top and the bottom edge swings out and up. Some are held on a friction stay at a chosen angle.
- Hinged vent on a kerb. A flat rooflight or lantern vent is hinged along one edge and lifts on an arm, a gas strut or an electric actuator.
Each has a travel limit, and the limit is part of the design. Without one, wind could take the sash further than its hinges allow.
A fourth case is a sun tunnel or a fixed lantern, which has no opening at all. Those are chosen for light and the maker does not give them a stroke.
What stroke means
Stroke is the distance an actuator or stay moves the sash, measured along its travel. A longer stroke lifts the sash higher at its free edge. It sets the size of the gap you get for air.
Stroke figures are written in millimetres on the maker's data sheet, along with the opening angle of the sash. Both depend on the model, so we read them from the data sheet for the unit on your quote and not from a general rule.
The same hinged rooflight opens through a different gap depending on its actuator.
The guide to electric and manual rooflights compares the two control routes.
Pitch and opening
The opening angle is measured from the frame and is set by the hinge, the stay or the actuator. The pitch changes how that opening sits in the room: on a shallow slope the open sash lies flatter, and on a steep one it stands nearer upright, which alters the headroom under it and the view out. Every roof window has a rated pitch range from the maker, and the survey measures your slope to confirm that the model works at that angle.
On a flat roof the kerb height matters in the same way. A hinged vent lifts away from the kerb, and the finished roof covering and any insulation upstand around it have to leave the lid free to travel.
Check clearance on the inside too. A sash that swings in, as a pivot does at the top, needs room on the room side, so a bed head or a shelf under the window is noted at the survey.
Opening for air
Opening rooflights help to ventilate a room, because warm air rises and leaves through the highest point. A roof window or lantern vent sits at that point, which is why they suit loft rooms, landings and stairwells that trap heat.
A small opening still moves air. Many roof windows have a ventilation flap at the head of the sash that lets air through while the sash stays closed and locked. That is a background setting for air, separate from opening the sash.
A wider opening is better when the aim is to pull a large volume of air through a space, such as a kitchen or a stairwell with a lantern at the top. The distance between the lowest opening in the house and the rooflight gives the stack effect more to work with.
Opening in the evening and closing in the morning is a common pattern for loft bedrooms. In summer it lets the heat that has built up in the roof space leave overnight, and an electric unit with a timer or remote makes this easy to do every day.
Opening for a way out
Rooms in the roof of a bungalow or chalet bungalow can use a suitably sized roof window as an escape window, provided it opens wide enough. Under Approved Document B the clear opening must be 0.33 m² or more, with neither its height nor its width under 450 mm. The maker's data sheet gives the clear opening size, and Building Control checks that figure against the room.
A storey more than 4.5 m above the ground is not served by an escape window at all. On a two-storey house, a loft room therefore needs a protected stair for escape, and a bigger opening on the roof window does not change that. The guide to rooflights and Building Regulations explains the route for a loft conversion.
A wide opening for escape and a narrow one for night ventilation are different jobs. A window that serves both has a catch for each position, which is another detail on the data sheet.
Checking before you order
The survey sheet records the measured pitch, the kerb or frame size, and the clearance inside the room or above the ceiling line. With the data sheet in hand, these show how far the unit will open on your roof, and we check that the sash clears curtains, blinds and furniture inside.
If you have an existing fixed unit and want one that opens, the answer on swapping fixed for opening covers the questions that arise. Our range of opening and electric units sets out the models we fit.
faq
opening questions
Can I stop the sash part way?
On a friction stay or an electric actuator, yes. A pivot window holds at the angles its catch and friction allow.
Does the window open wide enough to clean the outside?
A pivot roof window turns far enough to bring the outer pane inside. Other types are cleaned from the roof or by reaching through the opening, depending on the model.
Does a bigger opening let in rain?
A rain sensor on an electric unit closes it when water falls. On a manual unit you close it yourself.
Is the opening angle the same on every slope?
Measured from the frame, the angle is set by the hinge or actuator. The pitch changes how the open sash sits in the room, so the survey reading is checked against the data sheet.