How far does an electric rooflight open?

An electric rooflight opens because a small motor pushes the glass panel away from the frame. That motor is called an actuator, and almost everything sold in south Essex uses one of two designs.

The first is a chain actuator. A folded steel chain lives coiled inside a slim case along one edge of the frame, and the motor drives it out rigid, like a bicycle chain that has been taught to stand up straight. Chain actuators are compact and quiet, and their reach, the engineer’s word is stroke, is usually 200mm, 300mm, or up to around 380mm. That reach is the gap you get at the edge of the panel furthest from the hinge.

The second is a spindle or rack actuator. This is a rigid arm on a threaded rod or a toothed rack, and it can be both stronger and longer than a chain. Strokes of 300mm to 600mm are common, and because the arm is stiff along its whole length it can shift a large, heavy pane or a laminated safety unit without flexing. If a rooflight needs to open a long way, or the glass is big enough that a chain would struggle, this is the mechanism doing the work.

So when a brochure says a rooflight opens “up to 400mm”, it is quoting the actuator, not a property of the glass. The same frame with a longer actuator opens further, and the same actuator on a wider panel opens the same distance at the edge but sweeps a larger triangle of air.

How far a rooflight opens and how much air it moves are two different questions. A hinged panel does not slide upwards evenly. It pivots along one edge, so the opening is a thin wedge: nothing at the hinge, widening to the full stroke at the far side, with two triangular gaps down the sides.

The figure that matters for airflow is the geometric free area, which is the smallest cross-section the air has to pass through. On a panel that opens to 300mm, the free area is a good deal less than the flat area of the glass, because most of the perimeter is opening by less than the maximum. Doubling the stroke does more than double the free area, which is why a unit that reaches 400mm ventilates far better than one stuck at 200mm, not just twice as well.

Position helps too. Warm air collects at the highest point of a room, so a rooflight set into the ceiling vents the hottest layer first. Open one at the top of a room and cooler air is drawn in through a door or a low window to replace it. That stack effect is the reason an opening rooflight earns its keep on a still day, when there is no wind to push air through and a wall vent does almost nothing.

The style of rooflight sets the range you can expect.

• Flat glass rooflights. These sit on a flat or near-flat roof and hinge along one edge. A chain actuator on this style typically gives 200mm to 400mm at the leading edge, which is plenty for ventilation on a kitchen or a bathroom.
• Top-hung electric roof windows. On a pitched roof, an electric roof window hinged at the top swings its lower edge outwards. These tend to open to roughly 40 to 60 degrees of sash angle rather than a fixed millimetre figure, which on a standard window is a wide, clear gap.
• Centre-pivot roof windows. These rotate on a central axle. Electric versions usually open the top of the sash outwards by a modest amount for ventilation, since the glass has to clear the frame both sides.
• Roof lanterns and large flat units. An opening pane in a lantern is normally driven by a spindle actuator with a longer stroke, and it is specified for the ventilation the room needs rather than to a standard figure.

Whatever the style, an electric unit almost always carries a rain sensor that closes it on its own when the first drops land, and many also close in high wind. That means you can leave a rooflight cracked open on a warm day without watching the sky, which is part of why people fit the electric version in the first place.

Three situations call for more than a ventilation gap. Roof access rooflights open wide, sometimes to 90 degrees or on a sliding frame, so someone can step out onto a flat roof or terrace. Escape rooflights, used as a means of escape from a loft room, have to give a clear openable area large enough to climb through, and the rules on minimum height, width and cill position are set by Building Regulations, so that is a specification to confirm with the local authority before ordering, not to guess at.

The third is heat, and this is where the estuary comes in. Southend faces south across the Thames, so a rear extension or loft room here often takes direct sun on the glass through the whole warm half of the year. An opening rooflight lets the hottest air escape at the ceiling, and it is a genuine part of the answer. It is not the whole answer. Opening the glass does nothing about the energy pouring in through it while it is shut, which is why we specify the glass first and the opening second. On a south-facing roof the pane wants a low G-value so it blocks solar heat before it arrives, and the electric opener then clears whatever warmth still builds up. Pairing the two is the point of our electric skylight installation work, and it sits alongside the solar-control glazing we fit on hot rooms. On a large flat roof skylight installation we often set one opening unit for ventilation next to a fixed low G-value pane, rather than asking one opening panel to do everything.

Work out what you want the rooflight to do before you shop for a stroke figure. If it is background ventilation for a kitchen or bathroom, a chain actuator giving 200mm to 400mm is usually right and a longer one adds cost for little gain. If it is a hot south-facing room, put the effort into the glass and treat the opening as the second stage. If anyone will use it to get onto the roof, or it counts as an escape route, the opening size stops being a comfort choice and becomes a specification you confirm against the regulations.

Tell us the room, which way the roof faces and the rough size of the opening, and we will come back with the actuator stroke, the ventilation you will get and the right glass to sit behind it. Ring 01702 898232 or request a quote, and we will look at the whole room rather than just the size of the gap.