How many rooflights do I need? One large, or two smaller

A rooflight lights the floor beneath it and a fan of wall around that. The count is about covering the dark parts, so walk the room first and note where daylight gives up.

Every skylight and rooflight installation in Crawley we plan begins the same way. Stand in the room on a grey morning with the lights off. The corner where you cannot read a label is the part a rooflight has to reach. In a square room, one unit set roughly over the middle of the floor reaches every corner. In a long narrow room, such as a galley kitchen or a loft under a simple pitch, one unit lights its own stretch and leaves the far end dim.

Think in zones rather than square metres. A kitchen might have a cooking zone, a dining zone and a doorway. A loft bedroom might have a bed, a dressing area and a stair. Each zone that matters after dark or on a dull afternoon wants some daylight of its own. If one unit can cover two zones, one is enough. If they sit at opposite ends, it is not.

Wall windows count too. A room that already has a large window on one side needs the rooflight on the far side, not the same side, and that changes how many you need. Our guide on where to position a skylight in a room shows how placement shifts the answer.

Both can work in the same room. The choice comes down to the light you want, the structure overhead and how many joints you are willing to have in the roof.

1. One large unit gives a generous pool of light and a clean ceiling. It suits a room with a single focal point, like a dining table or a kitchen island. The opening is bigger, so it may cut through more timber and need more support.
2. Two smaller units spread light across a long room and let you aim each at a zone. They can sit either side of a rafter with less timber to cut. There are two sets of flashings and two sets of seals, which is a little more to fit and a little more to maintain.
3. A row of three or more suits a long pitch, a corridor or a rear extension with a wide flat roof. The units repeat, so spacing and alignment matter as much as the glass.

Neither route is wrong. A large lantern over an extension is one unit and gives a strong focal point, which is covered in roof lantern installation. A long flat roof might take two or three separate panes, as set out under flat roof rooflight installation.

The timbers in the roof decide how wide a unit can be before the structure needs altering. This is frequently the real reason a room ends up with two units, not one.

In a typical pitched roof, rafters sit at intervals of roughly 400 to 600 mm. Roof windows are made in standard widths so that one fits between a pair of rafters with no cutting. A wider unit means cutting one or more rafters and fitting trimmers to carry the load. That is a structural change, and whether it needs an engineer depends on the roof, which we come back to in does a rooflight need a structural engineer.

Two units side by side, each fitting its own gap, can be the simpler build than a single one twice the width. A row of three units, each between its own pair of rafters, may need no structural work at all. On a flat roof the joists run one way, so the same logic applies across the span instead.

So before settling on a number, the survey looks at the rafter pitch, the span and any purlin, because these decide what can be done cleanly. Our guide to sizing a rooflight for the room sets out how each unit is sized once you know the count.

More glass gives more light, and also more heat in summer, more heat loss in winter and more surface for sound to pass through.

A south-facing room with three large units can turn into a greenhouse by midday. Solar control glass helps, and so does opening or blinds, but a smaller count is the simplest remedy. An electric opening unit, described under electric and opening rooflight installation, lets warm air out from the highest point in the room.

Noise scales with glass area too. Every extra unit is another pane for rain to strike and another place for an aircraft or a train to be heard. In Crawley, with Gatwick close by and the Brighton main line cutting through, a loft bedroom deserves a careful count and a careful build. A laminated acoustic inner pane and a firmly fitted unit keep the quiet. The reasoning sits in the quiet spec that comes with every quote, and in rooflights under the Gatwick flight paths.

The other side is winter. Modern double or triple glazed units with warm-edge spacers hold heat far better than an old single pane, so a replacement count does not have to mean a colder room. Building Regulations for rooflights explains the thermal standard the new work must meet.

Put the room through four checks in order. If the answer is still unclear after the fourth, a survey will settle it.

First, list the zones that need daylight. Second, see how many of them one unit can reach from above. Third, check the rafter or joist gaps to see what fits without structural change. Fourth, look at the aspect, and trim the count if the room faces south. What is left is a count of one, two or occasionally three units. If you already have a rooflight and want another, compare how the existing one performs before you add more glass.

A survey does this on site, with a tape, a torch and a good look at the loft timbers, and the result goes into the written quote. You can get a quote and we will give a count and explain it.

Is there a rule for rooflight area against floor area?

Designers use rules of thumb, but no single ratio suits every room. Aspect, room depth, wall windows and the glass specification all move the answer. We advise on the real room rather than a formula.

Can I start with one and add a second later?

Yes, and it is a sensible way to judge the light before committing. Plan the second opening at the start, so the ceiling, the rafter gaps and the wiring for any electric unit allow for it. The how to choose a rooflight guide covers the order of decisions.

Do two rooflights cost twice as much as one?

Not necessarily, since the larger single unit may need structural work. Costs depend on size, glass, access and the roof, and they are set out on our costs page.

Does a second skylight make the room noisier?

Not if both are specified for the room. The glass build and seals decide the noise, so two well built units can be quieter than one thin, tired pane.