In brief
The right size follows the room, not a catalogue figure. It is set by how much light the kitchen needs and where, by the joists that hold the roof up, and by what sits under the glass, such as an island or a dining table. Because the opening is built into the roof structure, the size is fixed on the drawing before the joists go in, not chosen afterwards.
- Start with where the light is wanted: over the island, the table or the cooking area
- Check the joist layout, because the opening is trimmed between them
- Match the unit to the room's proportions so it looks planned
- Consider glare, heat and privacy for the way the kitchen is used
- Fix the size on the drawing before the roof deck is built
Begin with the room and where the light should fall
A kitchen extension usually has windows or doors on one side, which leaves the middle of the room with the least light. A rooflight can put it back, but only if it lands in the right place. The first decision is where in the room you want brightness: over a worktop you stand at, over a dining table, above an island or along the back wall where the extension meets the old house.
Standing in the existing room and marking where the island, table and units will go helps more than any measurement. A rooflight centred on a table makes the space feel arranged around it. One that falls between the table and the cooker often lights nothing in particular. Size comes after position, because the position tells you how much of the room needs the daylight.
How the joists decide the opening
The rooflight goes between joists, and the opening is made by trimming. If the width of the unit fits neatly between two joists, the structure stays simple. If it is wider, one or more joists are cut and trimmers take the weight around the opening, which is more work and needs the carpenter to design it. The joist spacing in your roof therefore suggests natural widths.
In a new extension, the joists are not yet laid when the drawing is made, so the rooflight can be designed first and the joists set out to suit it. That is the biggest benefit of choosing early. The dimensions for the chosen unit go to your builder or architect on paper, and the joists are positioned around it.
Where the roof is already built, the size is limited by what the existing structure can take, and the process is described in adding a rooflight after the roof is done.
It is also sensible to ask your builder about any beam, steel or services that run across the roof. A pipe, flue or extractor duct in the way can move a rooflight by a surprising distance.
Proportion, number of units and the look from inside
In a long narrow kitchen, a single long rooflight can run along the room and lead the eye towards the garden. In a squarer room, a unit centred over the main activity may look better than a band. Two smaller units set apart can suit a room with a beam or a steel across it, because each sits in its own bay and the structure does not cut across the glass.
The edges matter as much as the glass. A deep reveal makes the unit look recessed, a flush plaster detail makes it look thin, and a frameless glass unit shows very little frame on the roof. Those choices are discussed in the guide to frameless rooflights. They can change how big a given size looks, so it helps to decide the finish at the same time as the dimensions.
Glare, warmth and privacy
A large rooflight over a south-facing kitchen brings in a lot of sun. Glazing with a solar control coating reduces how much of that heat enters, and an opening unit lets warm air leave the room. Blinds can be added to a rooflight to shade the room in the afternoon. A rooflight that opens lets warm, moist air escape from a kitchen, which suits the cooking area, although it is not a substitute for an extractor.
Think about the view too. A neighbouring first-floor window overlooking the roof may suggest a smaller or differently placed unit, or glass that is more private. At night, a large rooflight can also act as a black panel that reflects the kitchen lighting, so lights in the reveal or dimmable fittings are worth planning for early.
Checking the opening against the rules
For an existing house in England, no Building Regulations rule sets a daylight requirement, so the size is a design choice, not a minimum. The regulations do set limits on the heat lost through the glazing, and the unit has to meet them. In permission terms, rooflights often fall under permitted development, and the planning guide explains how to check your own case. We look at the position for your address and report back, and the decision is the council's.
Walk-on units, opening units and units with electric vents all add requirements for structure and cabling, and each is better known before the joists are laid.
- The width and length of the opening, and the kerb size that goes with it
- The glass specification and whether the unit opens
- Where the joists are trimmed, and who designs that
- When the sizes reach the builder, ahead of the roof deck
Putting a size into the quote
We survey the room or work from your plans, talk through the position and the options, and send a written quote covering the unit, its glass, the delivery time and the sizes your builder needs. Delivery time rests with the maker and is checked at the order, which is why the size is worth settling early. The price depends on size among other things, as the cost of a flat rooflight explains, and our flat roof rooflights page covers the products themselves.
