Rooflights and skylights for the brick homes of Bentley

How an opening rooflight works

An electric opening rooflight lifts its sash on a motor. A button on the wall or on a handset sends the pane up to a set angle. Some models are run from the mains, and others take power from a small solar cell on the frame. The hard-to-reach ceiling is the case it suits best, since nobody has to climb to a pole or a winder.

Rain and closing

A rain-sensing model carries a small plate on the frame that registers moisture and shuts the sash by itself. Many electric units carry one. A sensor that is badly placed, or covered by leaves and grit, may not react, so how reliably electric skylights close in rain depends on where the plate sits and whether it is kept clear.

Where the unit goes

Over a hob, a unit close to the cooker clears steam fastest, though grease and heat are better kept away from the glass seals. Over a dining table, a unit in the centre of the room draws air through the doors. The position is a balance between the light you want and the air you want to move.

How much opening is enough

A unit that lifts a few centimetres lets steam out slowly, while one that tilts wide clears a kitchen in minutes. The travel of the sash is listed for each model, and we compare it with the size of your room before one is picked. A big kitchen asks for more opening than a utility room, and a second unit at the far end can help air move through.

Wiring and the ceiling

A motor needs a cable, and the cable has to reach the switch. In a flat roof the cable runs up through the joists from the ceiling. If the room is being rebuilt, it is simplest to bring it in before the plasterboard goes on. If not, the route is planned round the existing ceiling.

When the roof is already covered

An extension that was roofed years ago and is now getting its opening is the usual case. The covering is cut back at the kerb line and lifted, the kerb is built, and the covering is dressed up it. Where the roof is old and tired, it is sensible to renew the covering round the opening while it is open, since that is the one stretch of roof that has to be disturbed.

The kerb and the roof

The flat roof around the opening is raised on a timber kerb, and the covering is lapped up it. Felt, rubber and fibreglass all take the kerb a little differently, so the covering on your extension governs the detail.

The number to know

The U-value gives the heat lost through each square metre for every degree of difference between inside and out, so a lower figure is better. For a new or replacement rooflight in an existing home, The Building Regulations (Part L) cap it at 2.2 W/m²K. A roof window faces the tighter 1.4 W/m²K, or Window Energy Rating B.

What gets a unit there

Two panes with a gas between them, a low-emissivity coating on one pane, an edge spacer that conducts little heat plus a thermally broken frame all play a part. A third pane lowers the figure again, at the cost of weight. Each feature shuts one route by which warmth leaves the room.

The sunny side of the question

A south-facing flat roof in summer takes a good deal of sun. A solar-control layer in the glass holds back the sun's heat, and a blind beneath the frame holds back more. You can have glass that still passes daylight while keeping the room cooler. Glass and blinds each cut the heat differently, and rooms that run hot under rooflights are usually fixed by combining the two.

Condensation at the frame

Warm, damp kitchen air meets cold glass and leaves water on it. A frame with a thermal break stays warmer on its inside face, which keeps that water down. Ventilation finishes the job, and that brings the opening unit back into the picture.

Insulation round the kerb

The timber kerb is insulated, because a bare one would leak warmth round the edge of the glass. The insulation sits inside the upstand and is hidden by the lining of the opening.

Reading the data sheet

Each unit has a data sheet printed with its U-value. The sheet should give the figure for the whole unit and not only the glass, and we read it before a unit is chosen. A figure for the glass alone flatters a unit whose frame leaks heat.

Older extensions

Extensions built a few decades ago often have little insulation above the ceiling. A new rooflight is a chance to improve the strip of roof around the opening, and the kerb insulation closes the gap that an old single-glazed unit left wide open.

Houses and extensions

A rooflight in the pitched roof of a house normally falls within permitted development. A roof lantern on a flat-roofed extension is a separate matter, since it stands well above the roof surface. Put the question to the council, or read the permission that built the extension, which may already cover a lantern.

Flats

A flat is left out of the permitted development allowance for roof alterations, so an application is the normal route; the council advises on it. Where a house has been divided, the parts count as flats.

Listed and protected properties

Listing brings a consent process of its own for any new rooflight, and a protected street may be covered by a direction removing permitted development. A short enquiry to the council settles whether either covers your address.

What to gather

The council needs to see a definite unit. Our written quote is drawn up for one room and one named unit, with its size. We notify Building Control wherever the job requires it. The same details go with any application.

If an application is needed

A householder application for a rooflight is a short form with a location plan and a drawing. The unit, its size and its position on the roof are the main items. We can supply the unit details, and the roof plan is drawn from the measurements taken on the day.

Replacing a unit already there

Swapping a rooflight for another in the same opening is a lighter job than cutting a new hole. The same two questions about listing and flats are still worth asking before work begins.

Where the cable and the pipe run

Before any opening is cut in a flat roof, we check what runs through the joists below: a cable to a light, a pipe or a duct from the cooker hood. The opening can often shift a few centimetres to miss them, and where it cannot, the service is moved by the person qualified to do so.

Neighbouring boundaries

A flat roof extension often runs close to a boundary wall. A rooflight in the roof does not change the footprint, though a lantern's height may be seen from next door. Talking to the neighbour first costs little and avoids surprises.