What size flat rooflight should I get?

Daylight is the thing you are actually buying, so the size that matters first is the glazed area measured against the floor of the room beneath it. The 15 to 20 per cent figure is the range that gives a room that reads as bright without tipping into glare or heat problems. Below about 10 per cent the room still feels like it needs the lights on at midday. Push much past 25 per cent and you are into territory where solar gain and heat loss both start to demand a better glass specification to stay comfortable.

Run the sum for your own room. A 16 square metre kitchen at 18 per cent wants a little under 3 square metres of glass, which a 1.2 by 1.2 metre unit plus a smaller second one covers neatly, or a single 1.5 by 2 metre rooflight if the structure allows one that wide. A small 9 square metre utility or hallway rarely needs more than a 900 by 900 mm unit to feel transformed. The deeper the room runs away from its windows, the more the roof glazing is doing, so a galley kitchen with one external wall justifies proportionally more than a square room with glazing on two sides.

Height helps too. A flat rooflight throws light straight down and spreads it across the room, which is why a single well placed unit often does more than the same area split into a scattered handful. Put it over where people stand rather than over a corner, and let the daylight fall into the middle of the space.

A flat roof is a grid of joists, and every rooflight sits in a hole cut through that grid. Cut through one joist and you can trim around it with doubled timbers on either side and headers top and bottom, which is routine. Cut through two or three and the structure has to be designed properly, often with a steel or a structural timber to pick up the loads that the removed joists were carrying. That is the real reason a 1.2 metre opening is easy and a 2.5 metre one is a bigger job: joists in a domestic flat roof are commonly spaced at 400 or 600 mm centres, so the wider the unit, the more of them it interrupts.

Two other dimensions come with the opening. The unit sits on an upstand, a raised kerb built up off the roof deck so the glass sits proud of any standing water, usually 150 mm or so. Whatever your finished internal opening, the roof hole and the upstand are a little larger again, and the ceiling reveal below is often splayed to throw the light wider. Measure the room you have, not the rooflight in the brochure, because the glazed dimension and the structural dimension are not the same number.

Rooflight manufacturers make a ladder of stock sizes, and staying on a rung of that ladder saves money and time. Common flat rooflight sizes step through 600 by 600, 800 by 800, 1000 by 1000, 1000 by 1500, 1200 by 1200 and 1500 by 2000 mm, with plenty of rectangular options between. A stock unit is cheaper per square metre, quicker to get hold of, and easier to replace years later than a one-off.

Made to measure earns its cost in three situations: when the structural bay gives you an awkward width that no standard unit flatters, when you want a single large expanse of glass rather than several units, and when the room genuinely calls for a walk-on or an unusually long run. If your sums land close to a standard size, round to it. The daylight difference between a 1150 and a 1200 mm unit is nothing you will ever notice, and the price and lead-time difference between stock and bespoke is not. Our skylight installation costs page sets out where the standard and bespoke ranges sit.

A flat rooflight faces the sky rather than a compass point, so it collects the high summer sun almost square on regardless of which way the house is turned. That makes size and glass specification a single question rather than two. Doubling the glazed area of a flat rooflight roughly doubles the solar energy coming into the room on a bright day, and in Southend that day is a common one, because the city faces south across the Thames Estuary with an open horizon and reflected light off the water adding to the total.

The practical rule: the larger you go, the harder the glass has to work. A modest 900 mm unit over a north-lit utility can take standard clear double glazing and the winter warmth is welcome. A 2 metre expanse over a south-facing kitchen extension wants solar-control glazing at a low G-value, or you have built a greenhouse over the one room that already makes its own heat from cooking. Sizing up is a fine ambition, but budget for the glass to match it. That trade-off is the whole basis of our energy-efficient skylight installation work, and it is worth settling the glass specification and the size in the same conversation rather than one after the other.

The same glazed area splits several ways, and the split changes both the look and the ventilation. A single large rooflight gives a clean, open ceiling and the most uninterrupted daylight, but it commits you to the widest structural opening and, if it opens, to one large heavy leaf. Two or three smaller units spread the light more evenly across a long room, interrupt fewer joists each, and let you make some fixed and one opening.

Opening matters more than people expect on a flat roof. Hot air collects at the highest point of a room, which on a single-storey extension is the ceiling, so an opening rooflight vents the heat exactly where it gathers. On a wider glazed area we often pair a large fixed unit for the daylight with one opening unit for the ventilation, rather than pay for a large opening leaf that is heavier and dearer than it needs to be. If a roof lantern rather than a flat rooflight suits the room, the sizing logic is the same, and our roof lantern installation page covers that form.

To settle a size, measure the floor area of the room, take 15 to 20 per cent of it as your target glazed area, then look at where your joists run and which way the roof faces the sun. That gives you a size, a rough structural picture and an early steer on the glass. Round your figure to the nearest standard rooflight where you sensibly can.

If you would rather we sized it for you, tell us the room dimensions and roughly which way the extension faces and we will come back with a unit size, a glass specification and a price. See our flat roof skylight installation service for how the whole job goes together, ring 01702 898232, or request a quote and we will work back from the room rather than the catalogue.