What Size Flat Rooflight Do I Need?

A common starting point is a number seen somewhere else: a metre square, a long strip, a pair side by side. A better start is the room underneath. A kitchen with a single small window needs a different amount of daylight from a landing that has none, and the size that suits one will overwhelm or underserve the other.

Stand in the room at the time of day you use it most and note where the dark part is. A rooflight lights the floor below it and the walls around it. Put over a worktop it gives task light, and over the middle of a room it spreads light more evenly. The position of the dark area suggests where the opening should sit, and the roof then says how big it can be there.

No Building Regulations rule sets a minimum glazed area for an existing house, so size is a matter of what you want the room to feel like and what the structure allows. If you are considering several units instead of one large one, the flat roof rooflight installation page shows the unit types.

A flat roof is carried on joists, timbers laid side by side across the room. A rooflight opening sits between them. Where the opening is no wider than the gap between two joists, no joist is cut. Where it is wider, one or more joists are cut and the load is carried around the opening by trimmers (short timbers fixed across the gap to hold the cut ends).

That is structural work, so it needs Building Regulations approval and sometimes a calculation from an engineer. It is not a reason to avoid a large rooflight. It is a reason to know early whether the unit you want falls inside a single bay or not.

As an example, on a roof with joists at 400 mm centres, the clear gap between two joists is roughly 400 mm less the thickness of one joist. A rooflight 600 mm wide would span more than one bay, and a trimmed opening would be needed. We measure the real centres on your roof, because they vary between houses and between extensions on the same house.

Makers list flat rooflights by a nominal size, and each size has more than one dimension attached to it. The structural opening is the hole in the roof deck. The kerb outside size is the footprint of the upstand. The visible glass, often called the daylight size, is smaller than both because the frame and the kerb take up room.

Two units that look equal in a catalogue can let in different amounts of light because their frames differ in width. Compare the daylight size, not the headline size. Bespoke sizes are available from several makers, so a roof with unusual joist centres need not force a compromise.

The kerb is covered separately in our guide to rooflight upstand height, which explains why its height is set before the unit is ordered.

Moving up a size adds more than glass. A larger unit weighs more, and the kerb and deck below must carry it. A bigger opening may cross a joist, as above. A larger area of glass also loses and gains more heat, so the glazing specification deserves attention on bigger units. Building Regulations set 2.2 W/m²K for a new rooflight, and a large unit over a living area can be specified to beat it.

• A larger unit needs a longer span of kerb, which may need extra support under it.
• The plasterboard opening and light well inside grow with it, and so does the making good.
• Fixed units can be large. Opening units are limited by the size the maker's mechanism is rated for, so the largest opening size in a range is smaller than the largest fixed one in some makers' lists.
• Walk-on and other specialist units have their own size ranges, which differ from standard ones.

Where the opening matters more than the area, several smaller units set in a line can give an even spread of light and avoid cutting joists. The difference between a pair and a single large unit is mostly in the joists and the cost of the structural work.

At the survey we measure the joists, the span, the room below and the existing opening if there is one. We note what the roof covering is and how deep the insulation sits, since the kerb has to rise above both. The measured sheet is handed to you, and the quote names a unit, its structural opening and its kerb outside size.

Replacing a rooflight with a bigger one has additional steps because the opening is enlarged. The page on replacing a rooflight with a bigger one follows that case through.

Size is easier to change on paper than on the roof. If you are undecided between two sizes, we can quote both, and the measured details show what each one involves.