VELUX window installation in SouthamptonMatched to your roof

Every VELUX window carries a code, and the code is the quickest way to say which window you mean. The width comes from the letter and the height from the digits. A builder who is sent a code can look it up in the maker's table and read off the frame size, the size of the opening to be cut in the roof, and the flashing kit that goes with it.

Two windows can look alike on a drawing and still sit in different codes. A change of one step in the width alters the gap needed between the rafters, and a change of one step in the height moves the head and the sill in the roof slope. That is why we quote the full code, not just an approximate size in centimetres.

Why the code is not enough on its own

The code names the window. It does not tell you where the window sits in the slope, how many rafters have to be cut or trimmed, or whether the flashing is the right one for the covering. Those depend on the roof, so the survey measures them. We check the rafter spacing and the pitch, work out which rafters the opening will interrupt, and say whether trimmers are needed on the head and the sill.

A narrow window can fit between two rafters without any cutting at all. A wider one takes out a rafter, and the load it carried is picked up by trimmers above and below. Your builder or structural engineer decides how that is done. We supply the size and position, and they draw the framing around it.

The sash of a roof window can open in more than one way, and the choice changes how the room works. A centre pivot window turns about a central axis. You push the top of the sash outwards and the bottom swings in, which lets you reach the outside pane from indoors for cleaning. A top hung sash hinges at the head and swings out from the bottom, which gives more clear height at the sill and a wider view out when it is open.

Both types sit in the same roof and look much the same from the street. The difference shows up inside: where the handle is, how far you have to reach, and how the blind hangs when the sash is open. In a loft bedroom with a low sill, a top hung window can be easier to live with. Over a staircase, where nobody will be standing close to the sash, a centre pivot is perfectly good.

Electric and solar versions

A window that is out of reach can be opened by a motor. Electric versions run from a mains supply through a controller, and solar versions carry a small panel and battery that remove the need for a cable. The solar version suits a roof where running a cable would mean lifting plasterboard that is already finished.

Which one suits depends on the stage of the build. If the walls are still bare, a cable for an electric window is a small job at first fix. If the ceilings are on, a solar window may save a morning of cutting out. We would rather settle this at the survey than discover it when the electrician has already left site. Our page on electric and opening rooflights covers how the wiring is planned.

Both versions can be fitted with a rain sensor so the window closes when it starts to rain, and both can be worked from a wall switch or a remote. We name the control in the quote so the homeowner knows what will be on the wall.

The flashing is the part of a roof window installation that nobody sees afterwards and everybody depends on. It is a set of aluminium pieces, supplied by the maker for the window code and for the type of covering, that run round the frame and lap over the tiles or slates. Water is carried down the roof slope over the top of the flashing, not through the gap between window and covering. When the kit is the wrong one for the roof, the laps do not work, so choosing it correctly is the first job at the survey.

Coverings fall into a few families. Plain clay or concrete tiles have a shallow profile and take one flashing type. Interlocking and profiled tiles are deeper and need a kit that follows the shape of the tile. Slate is thin and flat, and needs its own kit again. Each is made for the window code in question, which is why we record the covering as well as the size.

Standard and recessed fitting

A standard fitting places the window so the frame sits proud of the roof surface, with the flashing running up its sides. A recessed fitting sinks the window lower, so the glass sits closer to the plane of the tiles and the window is less obvious from the street. Recessed fitting needs a deeper trim at the head and a flashing kit made for that depth, so it is chosen before the order, not on the day.

The choice matters on a conservation street or any roof where the planning officer has a view on how the window looks. A recessed window reads as part of the roof rather than a feature on it. It also affects the room: the more the frame is recessed, the deeper the reveal inside, and the more the plasterer has to do around it.

Fitting into a roof that is already weathertight is a different job from fitting into a roof that is being built. When a loft is being converted, the roofer may be laying battens and membrane anyway, and the window frame goes in as the covering is finished around it. When a window goes into an existing roof, the fitters strip back to form the opening, set the frame, dress the flashing and re-lay the tiles over it. We tell the homeowner which of those two jobs theirs is.

Where a window is replacing an old one in the same opening, the flashing is renewed with the window. A new frame inside an old flashing is a join we would rather not make. Our guide on replacing an old VELUX window covers what changes when the opening already exists.

A roof window without a blind lets the sun in at five in the morning and turns the room into a greenhouse in July. Blinds are made for each size code, sit inside the reveal, and are fixed to the window frame itself. Some are fitted as the window goes in, and the fixings are hidden by the plaster lining.

That is the argument for ordering the blind with the window. The cords or electric supply can be run while the reveal is open, and the plasterer finishes round a completed installation. If a blind is added after the room is decorated, the fitter has to work against the finished surface, and the fixings sit where the paint has already dried.

The types to choose between

A blackout blind shuts out the light almost completely and suits a bedroom. A roller or pleated blind softens glare and keeps some daylight, and suits a landing, a study or a kitchen. A venetian style blind gives adjustment between the two. There is also an external awning blind that stops the sun before it reaches the glass, and that has to be planned with the window, because its fixings go through the roof covering.

Southampton has a mixed roofscape. There are slate-roofed Edwardian and Victorian terraces and semis, interwar houses in clay or concrete tile, and post-war housing roofed in concrete tile. Each takes a roof window a little differently, and the flashing kit follows the covering.

On an Edwardian house with a slate roof, the back slope is where the windows go, because it faces the garden and is out of sight from the road. Slate is thin and takes a dedicated flashing kit. Old slate can be brittle where it is lifted for the opening, so the fitters work from a scaffold or roof ladder, and the quote allows for matching slates to be sourced.