Multiple flat rooflights: laying out a row along one roof

Every flat roof is a set of parallel timbers, and the direction they run sets the layout. A rooflight placed to suit the joists is cheap to frame and stays stiff. A rooflight placed against them needs extra steelwork or doubled timbers.

On a typical single storey extension the joists span the short way, from the house wall to the front wall, at 400 or 600 millimetres between centres. They are commonly 150 to 200 millimetres deep on a modest span, and deeper on a wide kitchen. A rooflight that is narrower than the gap between two joists can often sit in that gap with only short trimmers across it. A rooflight that is wider has to interrupt one or more joists, and the cut ends must be carried on a doubled trimmer back to uncut joists on either side.

That is the first argument for a row. Three narrow units set in line with the joist spacing interrupt very little timber. One wide unit of the same total glass area may cut three joists at once. The ends of a cut joist carry load into the trimmer, and the trimmer hangs from the joists beside it, so every extra cut multiplies the load on the neighbours. Our separate note on why the joists need trimming for a flat rooflight explains the framing in detail. Here the point is simpler: measure the joist centres first, write them on the drawing, and let them set the widths of the openings.

Work from the existing structure where it is visible. In a loft or an open ceiling you can see the joists. Under a finished plasterboard ceiling you cannot, so we locate them from the roof surface and confirm them from inside before anything is marked out.

Between two openings there is a strip of roof that has to do real work. It carries the load across the gap, holds the insulation and the membrane, and has to drain. Make it too narrow and all three jobs get harder.

For a roof with upstands on each opening, a working gap of around 300 to 600 millimetres between the outer faces of neighbouring kerbs is comfortable. Below about 300 millimetres the membrane has to be dressed up two upstands that nearly touch, the detail becomes fussy, and there is nowhere for a hand or a brush to reach. Much above 600 millimetres the pair reads as two separate units rather than a row, and the light pattern inside splits into patches with dull bands between.

The gap also decides the structure. If it equals a whole number of joist spacings, each kerb can be framed off its own pair of trimmers and the timber between is continuous. If it falls awkwardly, we move the openings a little until it lands on a joist. A shift of 50 millimetres costs nothing on site and saves a lot of cutting.

The same thinking applies to the edges. A row should stand clear of the parapet, the wall abutment and the gutter edge by the margin we would leave round a single unit. Our guide to flat roof rooflight upstands covers the height and finish of each kerb, which sets how much membrane each opening needs round it.

A flat roof is never level. It is laid with a designed fall so that rain moves to an outlet, and every kerb on the roof is a small dam across that movement. With several kerbs, the water has to get past all of them.

Think about a fall of 1:40 laid to one side, which is a common design. Water runs down the slope in lines. A rooflight set across the slope catches the lines above it, and they divide round its ends. A second rooflight set directly below the first, in the same line of fall, receives the water that just left the first, and then has to divide it again. Stack a third below that and the lowest unit sees the run-off from the whole row above it. The lowest kerb then holds back more water than its neighbours, and leaves are carried to it.

So the layout rule follows from the direction of the fall:

• Set a row of units side by side along the roof, across the fall, so each one meets its own stream of water.
• Where the slope is long and two rows are unavoidable, offset the second row so its units sit below the gaps in the first, not below the glass.
• Keep every gap wide enough that water leaves between the kerbs freely and does not stall in the corners.
• Leave the outlet and its run clear of any kerb, with the membrane dressed so nothing blocks it.

Design the falls at 1:40 so the finished roof still drains at 1:80 or better once built and settled. Our page on falls, drainage and why flat rooflights are never dead flat goes through the numbers, and the answer on why water sits on a rooflight explains what a standing pond on the glass tells you about the layout above it.

From below, the row meets the room's own geometry: the ceiling lights, the kitchen island, the dining table, the bifold doors. A row that ignores these looks accidental. A row that works with them looks intended.

The simplest alignment is to the furniture. Centre each opening over a zone of the room and give it a job. One above the cooking run, one above the island, one above the dining end. Daylight lands where you stand and sit, not on a blank stretch of floor between them.

The ceiling below also carries downlights, and they sit in the same void as the joists. A downlight needs a hole in the plasterboard and clear space above it for the fitting and its fire hood. If a trimmer runs where a downlight was planned, the light moves, and a moved downlight breaks the pattern of the whole ceiling. Draw the electrical layout and the rooflight layout on one plan before either is fixed. We mark the trimmers, the openings and the light positions on the same sheet, so the electrician works to the same lines the carpenter does.

Inside the opening there is also a lining. The reveal rises from the ceiling up to the glass, and on a row it forms a run of matching boxes. The spacing between them should repeat from one to the next, because the eye reads an uneven row at once. Our note on lining the reveal under a flat rooflight covers the depth and the splay.

A row is one answer to a wide roof, but it is not the only one. The alternative is a single long unit, and the choice depends on the span and the ceiling.

A single long rooflight gives an unbroken band of light, and it suits a narrow kitchen where the view from the door runs the length of the room. It also means a long kerb lying across the fall, and a long trimmer carrying the joists it interrupts. A row of shorter units spreads the structural load and spreads the run-off. We talk through that trade in the guide to long and narrow flat rooflights.

If the choice is close, the roof usually decides. A wide extension with a long span tends to suit a row. A short one, with a single cooking wall and a big bifold at one end, may want one generous unit, placed where the light is needed.

Adding units adds glass, and glass loses heat faster than an insulated roof. Building Regulations put a limit on how much of a new extension can be glazed without further calculation.

As a guide, Approved Document L allows the combined area of windows, roof windows, rooflights and doors in an extension to reach around a quarter of the extension's floor area before you have to offset the excess elsewhere or use a fuller calculation. That total includes the bifold doors and the side windows, not just the roof. A big sliding door and three rooflights can reach the limit sooner than you would expect.

So the row is sized against the whole glazed budget. If the doors take up the bulk of it, the rooflights shrink or fall in number. If the wall is mostly solid, the roof can carry more glass. We add up the figures at the survey and show them on the quote, so there is no surprise when Building Control asks. We handle the notification where the work is notifiable, and we install to current Building Regulations. The wider rules are in our guide to Building Regulations for rooflights, and the thermal side of choosing glass is in our energy efficient rooflights service.

Glass choice matters here too. A better-performing unit lets you keep a larger area within the allowance. The answer on what glass a flat rooflight should have describes the options.

More glass in the roof brings more surface for rain to land on. Each unit is a drum skin, and a row of them gives the roof a larger voice than a single pane.

Flat glass responds to rain with a sharp tap that a thin roof membrane absorbs far better. The effect depends on the pane thickness, on the make-up of the glass, on the seals around the frame and on what the upstand is made of. A laminated acoustic inner pane damps the tap. A well-seated seal stops the frame ringing. A properly insulated upstand adds mass round the opening. None of this needs to be dramatic, and all of it is chosen for the room, not applied the same way everywhere.

Three units over a bedroom or a home office in a Crawley extension want a different build from three units over a utility room. In the bedroom, the rain and, near the flight paths, the early aircraft are what you will hear. In the utility room, nobody listens. We listen at the survey and write the result down in the quiet spec that comes with every quote. The page on quieter glazing for a flat roof rooflight over a bedroom explains the build in plain terms, and the quiet spec shows what the written note contains.

Good layout is decided before anyone climbs onto the roof. On site, the work is about holding the dimensions you agreed.

We mark the openings from the inside first, using the drawn positions and the located joists, and check them against the ceiling lights. Then the marks go up to the roof and are checked again against the falls and the outlet. Only then do the openings get cut. The kerbs are built to match and are set level across the row, so the glass sits at one height even though the roof falls beneath it. An uneven row shows from the garden, and from the first floor next door.

Then the membrane goes up each upstand and round each corner, and the units are fixed. Our walk-through of how a flat roof rooflight is installed follows the same sequence for a single unit. The whole row can be completed while the roof is open, with the ceiling below made good afterwards.

Because every unit is the same size and spacing, a row also simplifies later life. One cleaning routine, one kind of blind, one set of spare parts. The guide to how to clean a flat rooflight applies to each of them equally.

Bring the extension drawing if you have one, or just the room dimensions. We will draw the row against your joists and your lights, and set the glazing for the room. You can get a quote here, or message us on WhatsApp. The overview of the whole subject is on our flat roof rooflight installation page, and the costs page explains what moves the price.

These are the points that come up once the plan shows more than one opening.

Can I add a second rooflight later?

Yes, if the first is framed with that in mind. The joists and falls are the same, so a second unit can be added in line later. The membrane is opened, a new kerb is built and the two are matched in height and spacing. Planning both positions now, even if only one is built, saves a clash when the second arrives.

Do all the units have to be the same size?

No, but matching looks calmer, and it makes the framing and the ordering simpler. Different sizes suit different zones, a larger one over the dining table and smaller ones over the worktop, provided the gaps and the alignment are kept consistent.

How close can a rooflight sit to the party wall?

That depends on the wall and on the upstand height, and on any agreement with the neighbour. The answer on how close to a party wall a flat rooflight can go sets out the usual margins, and the row is planned from that edge inwards.

Will a row of flat rooflights make the room too hot?

Sun through a large glazed area warms a room, particularly on a south-facing roof. Solar control glass, a blind or a smaller area all help. The glass is chosen at the survey with the orientation of your roof in mind, and a blind can be fitted to each unit.