How many rooflights can you put in a flat roof?

Three limits decide the number, and they rarely agree

Homeowners tend to arrive with a plan drawn from the room: four units in a grid, evenly spaced, one over each seating area. The roof almost never agrees with that plan, and it disagrees for reasons that have nothing to do with each other.

Structure is about load paths through timber. Drainage is about where water goes when three obstacles are put in its way. Area is about heat, both losing it in January and gaining it in July. A layout can satisfy any two of the three and fail the third completely, which is why the count is settled on paper before the covering is touched rather than adjusted on the roof.

Every opening trims joists, and two openings can share one too many

A flat roof deck is joisted, usually at 400 or 600mm centres. Cutting an opening means cutting one or more joists and fixing doubled trimmers across the head and the foot of the hole. The load from each cut joist transfers into those trimmers and then sideways into the joists either side, which now carry more than they were sized for.

Over one opening that is routine. The trap is two openings placed close together across the width of the roof, because the joist between them ends up receiving trimmer loads from both sides at once. That joist is then carrying its own share, half of the first opening and half of the second. It usually needs doubling or upgrading to a deeper section, and where three openings sit in a row across a 3.6 metre span the answer is normally a designed member and a calculation submitted with the Building Control application.

The finished roof that has to be left between two kerbs

Kerbs cannot be crowded. The waterproofing has to be dressed up the side of one kerb, run across the deck, and dressed up the side of the next, and somebody has to be able to weld, torch or roller that junction properly with two upstands close to their knuckles.

The working minimum between two adjacent kerbs is 300mm of finished roof, and 400mm is a much better number. Below 300mm the membrane detail becomes a compromise at exactly the point where the roof is most vulnerable, because it sits in the channel between two dams. Two kerbs at 250mm apart also read as a mistake from the garden, where the eye reliably picks up an uneven gap.

A kerb is a dam, and the water from upslope has to get round it

This is the limit people never anticipate. A flat roof falls at around 1:40 towards an outlet, and every kerb standing on it blocks that path. Water arriving from the area above the kerb splits, runs down both sides, and rejoins below it.

Put two kerbs side by side across the fall and the water from the whole upper area has to pass through the gap between them plus the two edge margins. Put three across a narrow roof and the gaps become channels carrying more water than the falls were designed to move, which is how standing water ends up sitting permanently against an upstand. The fixes are real: stagger the units so they are not all on one contour, run a tapered diverter behind the upslope face of each kerb, or add a second outlet. All of them are cheap while the deck is being built and expensive afterwards.

Glazed area against the floor below

Part L of the Building Regulations sets a limit on glazed area in a new extension, taken as 25 per cent of the extension’s floor area, counting windows, doors and rooflights together. A 3.5 by 5 metre rear extension has 17.5 square metres of floor, so about 4.4 square metres of glazing in total.

Bifold doors across the back wall will already have eaten most of that. Two rooflights at 1000 by 1500mm add 3 square metres, and the sum stops working. The limit can be traded away by improving performance elsewhere in the extension, which is a calculation an energy assessor runs rather than a rule of thumb, but the number has to be faced early. It is common for the glazing budget, not the roof, to be what caps the count at two.

Summer gain, and why more glass overhead is not free

Horizontal glass takes the sun at its strongest angle, in the middle of the day, in the middle of summer. A square metre of unshaded rooflight can admit several hundred watts at midday in June, and unlike a south-facing wall window there is no time of day when the sun moves off it.

Doubling the number of units doubles that. On a new dwelling the overheating requirement makes it explicit, but the physics applies to an extension on a 1930s semi just the same. Solar control glass brings the g-value down from around 0.5 to roughly 0.25 to 0.3 and is the single most useful specification change where the roof faces south or west. The numbers are on the energy efficient rooflights page. Sometimes the better answer is three smaller units with solar control glass rather than two large ones without.

Setting them out for the room rather than for the roof

Once the count is fixed, position is the part worth arguing about. Two units are almost always better placed asymmetrically than centred as a pair: one over the working area of a kitchen and one over the table lights the room the way it is used, where two in the middle light the ceiling.

Daylight from a rooflight spreads at roughly the angle of the reveal, so a unit 300mm above the ceiling throws a cone rather than a column, and two cones that just overlap at head height give an even wash. Keep at least 600mm between a kerb and any abutment wall, because a flashing into brickwork and a kerb upstand fighting for the same 200mm is a detail nobody wants to build. Where several units go in together the whole thing is done under one scaffold, one deck sequence and one Building Control notification, which is materially cheaper than adding a second unit next year.

Counting the units on a Gloucestershire rear extension

The stock in this county pushes the answer in particular directions. A Victorian rear addition in Tredworth, Barton or Tewkesbury is one room wide, so the flat roof over it is often 1.8 metres across and takes exactly one unit with a sensible margin either side. A wider modern extension on the estates at Quedgeley, Abbeydale or Bishops Cleeve has the span for three, and there the drainage question becomes the binding one, particularly on roofs laid to a bare minimum fall.

Behind the Cheltenham parapets the count is usually one, because the lead flat is small, the falls run to an outlet shared with next door, and adding a second kerb changes the catchment on somebody else’s roof. Flat-roofed extensions behind Cotswold stone in Painswick or Northleach are limited by what the conservation officer will accept on a visible elevation rather than by structure. And in the Severn Vale, where homeowners have watched water arrive in 2007 and since, the willingness to put three separate holes in a flat roof is understandably lower, which is a reasonable instinct answered by the perimeter detailing rather than by argument.

The way to get a real number is a photograph of the roof from above or from an upstairs window, a photograph of the room, the outside dimensions of the extension and where the outlet is. Send those to info@severnskylightsgloucester.co.uk and the reply gives a count and a layout before anything is measured on site. The costs page covers how that turns into a range and then a fixed price.