How to choose a rooflight: four questions, in the right order

The pitch, the structure and the age of your roof rule some products out before taste comes into it. Settle the family first and the choosing gets easier.

Stand in the garden and look up. Is the roof flat, pitched, or a mix of the two? That single observation splits the field in three.

A flat roof, such as a kitchen extension or a rear addition, takes a flat rooflight or a roof lantern. Both sit on a raised kerb so that water runs away around them. A flat rooflight is a low, nearly level pane of glass. A lantern is a glazed roof in miniature, with sloping sides and a ridge. Our page on flat roof rooflight installation covers the first, and roof lantern installation covers the second.

A pitched roof, such as a loft conversion or a dormer cheek, takes a roof window. It lies in the plane of the slope, flush with the tiles or slates, and it is framed into the rafters. The rafters set the widths you can use without cutting structure, so the spacing matters early. See roof window installation for how that works on a typical home.

A dark hallway or landing with no loft above it is a different case again. A sun tunnel is a narrow reflective tube that borrows daylight from the roof and delivers it to the ceiling, and it suits places where a full window cannot reach.

Next, think about what lies under the covering. A 1950s or 1960s house in one of Crawley's original neighbourhoods often has a roof built to a standard pattern, with a known rafter spacing. A newer loft conversion may already have steel at the ridge. A tile-hung cottage in the Weald may have old, uneven timbers that do not follow any pattern. You do not need to know which you have. The survey will find out, but your guess tells you how much structural work to expect.

One more point belongs here. If you are unsure whether a rooflight and a roof window are really different things, our note on the difference between a rooflight and a roof window sorts out the vocabulary in a few paragraphs.

Size follows the light you want, and the light you want follows how the room is used. A bedroom, a kitchen and a home office ask very different things of the same piece of sky.

Think about the times of day you use the room. A kitchen where the family eats breakfast and supper wants good light at the two ends of the day. A home office wants steady, even light without glare on the screen. A bedroom wants enough light to dress by and little enough that the room stays restful at first light.

Then consider where the roof faces. A south-facing slope delivers the most light and the most heat. A north-facing slope is gentler and more constant. East and west faces give you morning or evening sun respectively. None of these is wrong. They simply call for different glass. A south-facing rooflight over a kitchen may want a solar control coating, while the same unit on a north face would want none.

A common mistake is to think only about area. Two openings of the same size can feel quite different from below. A deep roof light well, where the opening through the roof build is long and square, narrows the beam of light. A splayed well, where the sides flare out towards the ceiling, spreads it across the room. The shape of the well is part of the choice, and it is cheap to get right at the design stage and awkward to change once the plaster is on.

We go into sizing in detail in our guide on how to size a rooflight for the room, and into placement in where to position a skylight in a room. Read both before you commit to a number of openings. For now, hold one rule in your head: ask how the room is used before you ask how big the glass should be.

Every rooflight lets in light. Some can also let out heat and stale air. Whether yours should open depends on the room, and the answer is often yes for kitchens and bathrooms and often no for a stair landing.

Warm air rises and collects under the roof. In a loft bedroom on a hot night, that air has nowhere to go unless something opens. An opening roof window lets you pull fresh air through the room, and in summer it is the difference between a room you can sleep in and one you cannot. A fixed rooflight over the same room gives you the light but leaves the heat trapped.

There are three ways a unit can open.

1. By hand, with a handle or a pole, suited to a window within reach.
2. By an electric motor, controlled from a wall switch or a remote, suited to a rooflight set high over a stairwell or an extension where you cannot reach it.
3. With a rain sensor, so the motor closes the unit by itself when a shower arrives and you are out of the house.

Our page on electric and opening rooflight installation explains the range. A motor adds a cable run, a power supply and a control position, and each of those is easier to plan before the ceiling is closed than after.

Fixed units have their own strengths. They have fewer moving parts, a slimmer frame and a simpler seal, and for a large pane over a kitchen island they are often the right answer. Often the best plan is a mix: a fixed pane where you want light, and one opening unit placed where it can do some work.

Think, too, about ventilation for the room as a whole. A kitchen needs extraction at the hob, and a rooflight does not replace it. A bathroom needs a way to clear steam, and an opening unit can help, though the fan should still do the main job. Approved Document F sets the ventilation requirements for new work, and we build the units into the plan so that the whole room meets them.

The fourth question is the one buyers tend to skip. A roof is usually the thinnest layer between a room and the weather, so glass in the roof becomes a way for sound to reach you.

Sound arrives from three directions in this part of Sussex. Aircraft pass over Crawley and the villages around it on the Gatwick routes, and a loft bedroom hears the sky more clearly than any other room in the house. Road and rail noise rises from the A23, the M23 and the Brighton main line. And rain lands on glass, which is the commonest noise of all, because it affects every flat roof in every town we cover.

The useful fact is that the noise can be planned for. The glazing build is the main tool. A unit made with a laminated acoustic inner pane behaves differently from one made with two sheets of plain toughened glass. Glass thickness, the gap between the panes and the quality of the seals all change what comes through. So does the upstand, the raised edge around a flat rooflight, which can be built solid and well insulated so that it does not ring like the side of a drum.

You do not need to become an acoustician. You need to tell the installer what the room hears. We listen at the survey, note what we hear, and write the glazing build into the quiet spec that comes with every quote, so you can read what was chosen and why. We never quote test numbers for it, because the honest description of a quiet rooflight is the list of what it is made from.

If this is your main worry, there is detail in acoustic laminated glass for rooflights explained. For now, treat noise as a fourth design question that belongs alongside roof, light and air, rather than something to fix later. It costs very little to decide on the drawing board and a great deal to regret on the ceiling.

Once the four questions have narrowed the field, the components need choosing. Glass, frame and coating each do a separate job, and the names are easier than they look.

The glass is nearly always a sealed double-glazed unit, and in some designs a triple. Two panes with a sealed gap between them hold heat in far better than a single sheet. The gap is usually filled with an inert gas, and a thin coating on one pane reflects heat back into the room. Triple glazing adds a third pane and a little more insulation, at the cost of weight and thickness, and it does not automatically make a room quieter, because sound passes through panes of equal thickness in a similar way. A mixed build, with different thicknesses, tends to do more for noise than an extra layer.

The inner pane matters twice over. Laminated glass holds two sheets together with a clear interlayer, so if it cracks, the fragments stay in place. That is a safety matter for glass overhead, and it also helps with sound. The outer pane is usually toughened, which makes it strong against hail, branches and the occasional stray ball.

Frames come in aluminium, timber, PVC or a painted wood core with a metal outer. Aluminium is slim, strong and does not need repainting, which is why lanterns are usually made in it. Timber suits a period home where the look of the frame is part of the character. PVC is common on roof windows and needs no upkeep.

Coatings are the last choice. A self-cleaning coating helps with dust on a pane you cannot reach. A solar control coating cuts heat gain on a south face. A neutral coating keeps the glass looking clear, which matters if the rooflight will be seen from below in a pale, bright room. Each one slightly changes the colour of the light, so ask to see a sample.

With the four answers in hand, the field narrows quickly. This table shows how typical situations map to a starting choice. It is a place to begin, and the survey will test it.

The table has gaps, because real roofs are not standard. A period cottage may need a slimline conservation unit. A very large opening may need engineering. Your installer should say when your case falls off the map, and explain why.

A rooflight is building work, so it carries rules. They do not stop you choosing what you want, but they set limits that are worth knowing while you are still deciding.

The first is thermal performance. Part L of the Building Regulations sets limits on how much heat a new rooflight may lose and, for an extension, on the total area of glazing in proportion to the floor. A large lantern on a small extension can use up that allowance, so the glazing area is worth checking early. Our overview of Building Regulations for rooflights covers the main parts.

The second is safety. Part K governs glass in places where people may fall against it or be struck by it, and overhead glazing has its own expectations. Part B covers fire, and it matters more when a rooflight sits close to a boundary. Approved Document F, mentioned above, covers ventilation.

The third is planning. A rooflight in an ordinary house can fit within permitted development, but that is checked for the address, not assumed. A roof that faces the street, a conservation area or a listed building can change the answer. The Planning Portal at planningportal.co.uk explains the national position, and we check the detail for your property before anything is ordered.

We install to current Building Regulations and handle the Building Control notification where the work is notifiable, so the paperwork does not land on you. That is one less thing to hold in your head while you are choosing glass.

When two quotes arrive, the headline figure tells you very little. What tells you something is what each one says about the four questions. A quote that only lists a product code has answered none of them.

Look for the size of the opening, described in the same terms in every quote, so that you compare like with like. Look for the glazing build, with the pane thicknesses and the coating named, because a unit that looks identical can be made very differently inside. Look for how the upstand or the flashing will be finished, and for what happens to the inside: plastering, the light well and decoration. Each of these is a place where two similar prices hide different work.

Ask who handles the Building Control notification, and where the guarantee is written down. Ours is a ten-year workmanship guarantee, described on the guarantee page. Ask, too, whether the quote says what the room hears. That is the piece we add to every quote.

A good quote reads like a description of your room. If it could be pasted into any house in the county, it was not written for yours.

These are the points that come up once the four questions have been worked through. Each one turns on your own roof, but the reasoning carries from house to house.

Do I need to decide the product before the survey?

No. A rough idea of the room, its use and the noise you notice is enough. The survey turns those answers into a specification, and it is better to arrive with questions than with a fixed order.

Is a bigger rooflight always better?

Not always. More glass means more light, more heat in summer, more heat loss in winter and more surface for rain to strike. A well-sized unit in the right place usually beats a large one in the wrong place, and the regulations cap the total area in an extension.

Should the rooflight open?

If the room gets warm, or has steam, or is a loft you sleep in, yes, at least one unit should. Over a stairwell or a high extension, an electric motor makes opening practical. For a pane that is there for light alone, fixed is perfectly good.

How do I know the quiet choice is right for my room?

Listen to the room on a wet morning and again when the flight paths are busy or the traffic is heavy, then tell us what you hear. We write that into the quiet spec with the glazing build we propose, so you can judge the reasoning before you agree to anything.

When the four answers are written down, get a quote. We will survey the roof, listen to the room and set out the product, the glass and the finish in writing. For the wider picture of what we fit across Crawley and the towns around it, start from our skylight and rooflight installation page.