Size the glazed area at 15 to 20 per cent of the floor area of the room below. A 4.0 x 3.0m extension is 12m² of floor, so aim for 1.8 to 2.4m² of glass. Leave at least 500mm of flat roof between the upstand and any parapet, wall or roof edge.
That percentage is a daylight rule, not a Building Regulations figure. It comes from the way a flat rooflight behaves: it faces straight up, so it collects light from the whole sky dome rather than from one compass direction, and it drops that light almost vertically into the room. Bright directly beneath, falling off faster than people expect towards the walls. A pitched roof lantern of the same glazed area throws light further sideways. On a flat roof you buy area, and you buy placement.
Where the percentage comes from, and when to move off it
Below about 12 per cent a flat rooflight reads as a bright hole in a dim ceiling. The contrast makes the rest of the room look darker than it did before the work, which is a genuinely common complaint on undersized units. Above about 22 per cent you are into solar gain territory: a south or west-facing flat unit over 2.5m² without solar control glass will make a kitchen unusable on a July afternoon, because a flat ceiling gives hot air nowhere to go.
Push towards the top of the range when:
- The room has no other window, or one small window on a shaded elevation. Side-return kitchens in central Ipswich terraces are the classic case, where the return is often boxed in by the neighbour’s wall on one side.
- The ceiling is high, over about 2.6m, so the light spreads before it lands.
- The extension faces north, which loses you the solar gain problem entirely.
- The finishes are dark. Dark floors and units eat daylight.
Pull towards the bottom when the room already has good glazed doors, when the aspect is south or west and you are not specifying solar control glass, or when the roof deck is small enough that a large opening would leave no working margin at the perimeter.
The perimeter margin, and why it is a hard number
The 500mm margin between the upstand and any parapet, upstand wall or roof edge is not a comfort figure. Three things need that space.
The covering has to be dressed on both vertical faces. An EPDM or single-ply detail turned up the rooflight upstand and up an adjacent parapet needs room for a roller, a hand, and a preformed corner piece. Squeeze that to 200mm and the installer works blind in a trench, which is where folded corners and starved adhesive come from.
Water has to get past. The rooflight upstand is a dam sitting in a drainage plane. Water arriving from the high side has to run around it to an outlet, and a 200mm channel between an upstand and a parapet silts up with moss and leaf litter within two winters. Ipswich has a lot of mature street trees, and the estate roofs at Kesgrave and Rushmere St Andrew sit under plenty of them.
Somebody has to get round it later. Cleaning, gutter clearing, and eventually the next unit going in.
Measure the deck, not the room. The usable roof area is the extension footprint minus the wall thickness, minus 500mm all round. On a 3.0m wide side return with a parapet one side, that leaves roughly 1.6 to 1.8m of width to play with, which caps the unit width long before daylight calculations do.
A sizing table by extension footprint
| Extension footprint | Floor area | Target glazed area at 15 to 20 per cent | Typical arrangement |
|---|---|---|---|
| 2.5 x 2.5m utility or WC | 6.3m² | 0.9 to 1.3m² | One unit, 1.0 x 1.0m |
| 3.0 x 3.0m small kitchen extension | 9.0m² | 1.4 to 1.8m² | One unit, 1.5 x 1.0m |
| 4.0 x 3.0m side return | 12.0m² | 1.8 to 2.4m² | Two units 1.0 x 1.0m, or one 2.0 x 1.0m |
| 5.0 x 3.5m rear extension | 17.5m² | 2.6 to 3.5m² | Two units 1.2 x 1.2m, or one 2.5 x 1.2m |
| 6.0 x 4.0m open-plan kitchen diner | 24.0m² | 3.6 to 4.8m² | Two units 1.5 x 1.2m, or one 2.5 x 1.5m plus a smaller unit over the dining end |
Glazed area means the clear aperture, not the outside frame dimension. A unit sold as 1.0 x 1.0m typically gives about 0.85m² of glass once the frame is deducted, so round up rather than down.
Worked example: a 4.0 x 3.0m side return in Ipswich
Floor area 12m². Target 1.8 to 2.4m² of glass. The room is a kitchen, aspect roughly west, ceiling 2.4m flat plasterboard, one set of doors at the far end. The roof is 4.0 x 3.0m externally, so after 500mm all round the usable band is 3.0 x 2.0m.
A single 2.0 x 1.0m unit fits, gives around 1.8m² of glass, and lands at the bottom of the target. It also spans two thirds of the roof width and forces all drainage down one side. Two 1.0 x 1.0m units give roughly 1.7m² between them, leave a clear 600mm drainage channel down the centre, and light both ends of a long narrow room rather than one bright patch in the middle.
We would fit the pair, and on a west aspect specify solar control glass at a g-value of 0.30 to 0.35 with light transmission above 60 per cent.
When two units beat one
Two smaller rooflights cost more than one larger one of equivalent area. The hub page prices two 1.2 x 1.2m units at £4,300 to £5,700 against £3,300 to £4,400 for a single 2.0 x 1.0m. You pay that premium for four reasons, and on narrow roofs all four apply at once.
- Drainage. Two units leave a channel between them. One wide unit leaves a slot each side, and a slot narrower than 500mm will pond. See why flat rooflights pond water for what that costs you later.
- Structure. Two 1.2m openings interrupt fewer joists each and usually stay inside standard doubled trimmers. One 2.4m opening triggers an engineer’s calculation, which is the right call but adds cost and time.
- Even light. Two pools of daylight down a long room beat one, particularly where the ceiling is under 2.5m.
- Handling. A 2.5 x 1.5m sealed glass unit is a crane or a six-person lift. Two 1.2m units go up a tower.
The case for one large unit is aesthetic and it is a real case. A single clean rectangle of sky over a dining table reads better than two squares, and if the roof is wide enough to keep the margins, take it.
Practical limits that override the percentage
Permitted development for roof alterations generally allows a projection of no more than 150mm above the existing roof plane, which a flush unit on a 150mm upstand sits right at. That constrains height, not area. What constrains area on a real job is usually joist span, roof width after margins, and whether the outlet can take the extra runoff from a large impermeable surface. A single 63mm outlet on a small roof that has just gained 3m² of glass can be marginal in a summer downpour.
Bigger units also change the specification rather than just the price. Above about 1.5m in either direction an engineer’s calculation is needed, and on a retrofit into an existing roof it is needed regardless of size. Above roughly 1.2m across we specify solar control glass as standard, and the inner pane is laminated on every unit we fit.
Full detail on unit types and glass build-ups sits on flat glass rooflight installation, and if you are weighing a single opener against a fixed pair, fixed versus opening flat rooflights covers that decision. Everything about the kerb the unit sits on is on the parent page, flat roof skylight installation. If price is what you are actually sizing against, what a 1m rooflight costs fitted is the cleanest single figure to start from, and replacing multiple rooflights covers the two-unit arithmetic.
Get the size confirmed on site
Send us the extension dimensions, the ceiling height, which way it faces and what is already glazed, and we will come and set the opening out on the roof. We check joist direction and size, the fall on all four sides, and where the outlet is, then give you one written fixed price with the glass specification and U-value on it.
Request a fixed quote for a flat roof rooflight in Ipswich, Suffolk or north Essex. Every installation carries a 10-year workmanship guarantee.
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