What Size Flat Rooflight Do I Need?

A flat skylight is usually bought by catalogue size, and that is the wrong place to begin. Stand in the room and ask which part of it feels dim. In a kitchen extension it is often the back of the room, away from the glazed doors. In a hall or landing it is the middle of the floor. Put the glass over that dim zone and it will do more than a larger unit pushed to one edge.

Think about what you want from the opening, too. A unit that only needs to light a corridor can be modest. A kitchen island, a reading chair or a bathroom mirror asks for glass that sits above it, and that decides the length as much as the width. If the unit is meant to open for air, it needs room around it for the sash to rise clear of the kerb, which the fixed or opening comparison goes through.

Ceiling height matters as well. Under a low ceiling the light reaches the floor in a wide, soft spread, so a smaller unit can do the work. Under a tall, vaulted ceiling the same unit casts a tighter beam, and a larger one may be justified. The walls count too: pale walls bounce daylight further into the room, and dark ones swallow it, which can be the real reason a room still feels gloomy.

The room sets the wish and the roof sets the limit. On a flat roof the unit sits in an opening cut through the deck, and the timbers either side carry the weight of everything around it. A skylight that lands between two existing joists is the simplest case. A wider one means cutting a joist and framing the opening with doubled timbers, which is routine but must be done properly, and widening an opening is classed as building work in its own right.

The direction the joists run gives a natural rhythm. Glass that follows the span is simpler to frame than glass that crosses several runs. At the survey we find the timbers, note where any services pass through, and mark the opening on the ceiling so you can judge it from below before it is cut.

Two measurements get confused. The overall size is the outside of the unit, including its frame. The clear glass size is what you will look through. The gap between them depends on the frame, and a sleek, slim-framed unit loses less than a chunky one. The kerb adds to the outside again, as the rooflight kerb guide explains.

VELUX roof windows are named by a short code, such as MK04, which reads width from the letter and height from the numbers. Flat-roof rooflights from other leading makers are often made to the millimetre instead, which gives more freedom when the opening has to be a particular shape.

More glass lets in more light, and it also lets in more heat on a bright afternoon and loses more at night. A south-facing extension with a large unit overhead can warm up quickly, and a large pane needs a good glazing specification to keep the room comfortable. The flat rooflight glass guide covers the choices, including solar control coatings and a laminated inner pane.

Size also affects the price, though not in a straight line, as the broader ranges on our costs page show. Two smaller units over two different zones can serve a long room better than one large one, and they leave the joists less disturbed.

Consider what you will see looking up, too. Glass directly over a seat gives a pleasant sky view, while glass over a doorway mainly lights the floor. It helps to picture where you will be standing, sitting and looking, and the order in which the room is used through the day, before the size is fixed.

We bring the options to you rather than guessing from photographs. At the free survey we measure the room, check the roof construction and talk through where you sit, cook and wash. The written quote lists the size, the kerb height and the opening choices side by side. If you are planning wider work on the same roof, the flat roof rooflight installation page gives the full picture.