Can a flat rooflight be opened?

The whole pane is the sash on a flat unit

A roof window in a pitched roof is two frames: an outer one fixed into the rafters, and a sash carrying the glass that pivots inside it. A flat rooflight is built the other way round. The outer frame sits on the kerb, and the sealed unit itself, bonded into a slim aluminium carrier around its edge, lifts as one piece.

There is no second frame visible from the room and no bar across the aperture, which is why an opening flat rooflight is indistinguishable from a fixed one until it moves. It also means the moving part is the heaviest part of the whole product. Every other decision on this page follows from that.

Weight is what limits manual opening

Glass in a made-to-size flat unit runs around 35 to 45 kilograms per square metre double glazed, and nearer 55 to 60 triple glazed, before the carrier frame. A 1000 by 1500mm double glazed lid is therefore about 60 kilograms of dead weight acting straight down onto its own gasket.

Lifting that by hand, from below, at arm’s length, through a winder or a pole, is unpleasant on the first day and abandoned by the second. Manual operation on a kerb-mounted unit is realistic up to roughly 800 by 800mm. Above that the answer is an electric actuator, which is not a preference but a consequence of the mass. Powered options are set out on the electric rooflight installation page.

What a 300mm stroke is actually worth in ventilation

Take a square metre pane hinged on one edge and pushed open 300mm. The gap is a rectangle across the far edge, 1000 by 300mm, plus a triangle down each side of half that again. The geometric opening comes to roughly 0.6 square metres, and the useful figure is a little under that once the frame section is taken off.

Building Regulations ask for purge ventilation of at least one twentieth of the floor area of a habitable room. Six tenths of a square metre therefore covers a room of about twelve square metres on its own. A parallel opening unit of the same size lifted 150mm gives roughly the same number from a quarter of the travel, which is why it suits a unit sitting in an exposed position.

Gaskets under compression, and a lid that rain lies on

On a vertical window gravity is neutral and rain runs off the joint within seconds. On a flat rooflight the rain lies directly on the line where the moving pane meets the frame, and it can lie there for hours.

So the seal works by compression rather than by shelter. The actuator pulls the pane down onto a continuous EPDM gasket and holds it there under load, and behind that gasket is a drained rebate with weep holes at the low corners, so any water that crosses the first line has a route back out onto the glass. Two things follow. The actuator has to hold that compression against wind uplift, not just close the gap. And a stone or a leaf stem sitting on the gasket is the difference between a dry morning and a wet one, which is the whole reason the gasket gets wiped when the glass is cleaned.

Leaving it open, and what decides when it closes again

An electrically operated unit is normally supplied with a rain sensor: a small conductive pad on the frame that bridges when it gets wet and drives the motor shut in ten to fifteen seconds. It reacts to the first few drops rather than to a downpour, and it will also react to heavy dew, to a hose and to a bird.

Wind is the other governor. An open lid is a sail with a long lever arm, and manufacturers publish a wind speed above which the unit should be closed. That matters more on the exposed estates around Ashchurch and Bishops Cleeve, and on the higher ground above Coleford and Mitcheldean, than it does on a sheltered terrace in the middle of Gloucester. Obstruction detection stops the drive if something fouls the pane on the way down, which is worth having where the unit sits above a table.

The kerb under an opening unit does more work

An opening unit puts concentrated loads into the kerb at the hinge line and at the actuator brackets, and it does it repeatedly rather than once. A kerb that would carry a fixed unit’s dead weight without complaint can flex when a motor drives sixty kilograms of glass up against a gust.

The top of the kerb is therefore built level and square within a few millimetres, in structural timber rather than a light batten frame, with the fixings landing in solid material and not in insulation board. Out of square by 5mm across a 1.2 metre pane and the gasket compresses unevenly along one edge, which shows up as an actuator that labours at one end and a thin line of water in the rebate at the other.

Rear extensions across the vale, and the reason people ask

The question almost always arrives attached to a single-storey rear extension: a kitchen and dining room across the back of a Victorian terrace in Gloucester, or the back of an interwar semi in Longlevens, Hucclecote or Charlton Kings, with a flat roof over it and one or two rooflights facing straight up. By four in the afternoon in July that room is the hottest in the house, and the hot air is sitting against the ceiling with nowhere to go.

An opening unit at the high point of the room is the most effective thing available, because it takes the warmest air out at the top and pulls cooler air in through the back door at floor level. Behind a Cheltenham parapet the lid is invisible from the street, which keeps the conservation conversation simple. Where the flat roof doubles as a terrace above a room, the walk-on and access question is covered on the walk-on rooflight installation page.

Deciding this before the covering comes off is what matters, because the frame, the kerb and the cable route are all different. Send photographs of the roof and the room to info@severnskylightsgloucester.co.uk with a rough size and a note of where the room gets hot, and the reply covers which configuration fits before anything is priced. The costs page explains how the range and then the fixed figure are arrived at.