How many rooflights a flat roof extension needs

How light behaves

A rooflight lights the area under it and roughly a metre past its edge. Beyond that the level falls away quickly.

A typical rear extension is long and relatively narrow. One unit in the centre produces a bright middle and two dim ends. Two units spaced along the length produce two overlapping pools that cover the room.

The total glass area can be identical. The distribution is what changes, and distribution is what you actually experience.

The structural argument

Two smaller openings often need no steel where one large one would. That is frequently the largest single saving available on the job, and it is why two units are sometimes cheaper as well as better.

The practical arguments

Smaller panes are lighter, easier to lift into position, and do not need mechanical handling or side access for a machine. If a unit ever fails, replacing one of two is cheaper than replacing one large one.

When one large unit is right

In a square room, where a single central unit covers it. Where the room is the architectural centrepiece and the expanse of sky is the point. And where the structure already has a single large opening.

A working rule

Rooms up to about four metres long: one unit, centred on the activity rather than the room. Four to seven metres: two units. Beyond seven: three, or two plus a lantern over the main space.

Ask for both options on the quotation. It costs nothing to price and the comparison is often surprising.

Start with the floor area, not the roof

The useful rule of thumb is glazed area of around 15 to 20 percent of floor area. On a twenty square metre extension that is three to four square metres of glazing, which might be two units of 1000 by 1500 or three of 800 by 1200.

Below about 10 percent a room reads as adequately lit rather than bright. Above 25 percent you are into overheating territory on any south or west facing extension, and you are paying for solar gain and heat loss rather than for useful daylight.

That range is a starting point rather than a rule. Three things shift it.

What shifts it upward

A deep plan. An extension that runs a long way back from the existing house has a dark zone in the middle, where neither the garden doors nor the original windows reach. That is what the rooflights are for, and a deeper plan justifies more of them.

North facing. No solar gain to manage, so the constraint that caps the upper end does not apply. A north facing extension can carry more glazing comfortably.

Little other glazing. Where the extension has a small window and a solid side wall rather than a wall of bifolds, the roof is doing all the work.

What shifts it downward

South or west facing. A near horizontal pane collects the midday sun directly, and doubling the glazed area doubles the gain.

A lot of vertical glazing already. A rear wall of bifold doors is already admitting a great deal of light and heat, and the rooflights are supplementing rather than providing.

A shallow plan. An extension only two or three metres deep is largely lit by its own rear glazing, and the roof is filling in rather than carrying the room.

Several small or one large

This decides the quality of the light more than the total area does, and on most extensions several smaller units is the better answer.

A long narrow room wants a run of units spaced along its length. One large rooflight in the middle gives a bright centre with the ends noticeably darker by contrast, and the contrast makes the dark ends feel worse than they would with no rooflight at all.

A broad square room can take one generous unit, or a lantern, because a single pool of light reaches most of the space.

The practical advantages of several units are real too. Cost per square metre of glazing rises steeply rather than proportionally with pane size, so three moderate units are usually cheaper than one large one of the same total area. They are light enough to handle without a crane. They frequently need far less structural work. And they fail independently, so a seal failure in twenty years means replacing one modest unit rather than a very large and expensive pane.

Spacing them

Even spacing along the length of the room, set in from the walls rather than tight against them, gives the most consistent result.

A unit hard against the wall of the original house throws light down that wall and does less for the room. A unit set a metre or so in lights the floor.

Where the extension has a kitchen run along one side, positioning a unit over the working length rather than centrally in the room is worth more than symmetry, and north facing light over a work surface is the best light in the building.

Structure decides what is possible

Before any of the above, establish which way the joists run relative to the proposed openings.

An opening set within the span of existing joists rather than cutting across them frequently needs no structural work beyond doubling up either side. Cutting across joists means designed trimmers and, beyond a certain span, steel and an engineer’s fee.

On a new build extension this is designed in and costs almost nothing. On a retrofit it is frequently the largest single line on the quotation and it has nothing to do with the glazing.

That constraint frequently makes the case for several smaller units by itself, because three openings between joists can be cheaper than one that cuts three of them.

The kerb, which applies to every one of them

Each unit needs a kerb at least 150mm above the finished roof surface, with the deck falling away from it so water cannot pond against the seal, rigid insulation on its outer face, and a continuous vapour control layer sealed to the frame.

That last pair matters more than people expect. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punches straight through the roof insulation around all of it.

More units means more perimeter in total, so the kerb detailing matters more on a run of three than on a single large unit. It is worth asking directly whether the kerbs are insulated, on every one of them.

How many should open

At least one, and it should be at the highest point of the roof.

An opening at the top lets accumulated heat leave and draws cooler air in through anything open lower down. On a south or west facing extension that is what makes the room usable in July, and with a rain sensor on a motorised unit it happens overnight without anybody thinking about it.

There is rarely a case for all of them opening. One opening vent among three fixed units gives you the ventilation without paying for three mechanisms, three actuators to maintain, and three future actuator replacements at ten to fifteen years.

Where the extension is habitable, check the Part F purge ventilation requirement, which is calculated on free open area rather than on the size of the unit.

Rooflights or a lantern

The honest test is the ceiling. Where the extension has a flat ceiling at normal head height and the room is broad, a lantern gives volume the room does not otherwise have and it is worth the money.

Where the ceiling is already vaulted, or the room is long and narrow, or the budget matters, flat rooflights do the same job for less and frequently light the space more evenly.

A lantern is also considerably heavier, which on a retrofit means more structural work, and it stands proud of the roof, which matters where the extension is overlooked from an upper window.

A worked example

A five by four metre kitchen extension, twenty square metres, facing south west, with bifold doors across the rear.

Glazed area target: three to four square metres, at the lower end because of the orientation and the existing bifolds. Three units of roughly 1000 by 1200 spaced along the length, set in from the walls, with the run positioned over the kitchen working side rather than centrally.

Solar control glass on all three, because a near horizontal pane on a south west facing extension collects the midday sun directly. One of the three motorised with a rain sensor, positioned at the highest point of the fall. Insulated kerbs on all three, named on the quotation.

That specification costs less than a single large lantern, lights the room more evenly, and will not make it unusable in July.

Distribution beats quantity

The instinct is to ask how much glass. The better question is where the light needs to land.

One large rooflight produces a bright zone and dark corners. Two or three smaller ones spread across the plan produce a more even result and read as better lit, frequently with less total glass area.

That is why the number matters more than the total.

The specific problem of a rear extension

A single storey rear extension is deep, its only walls are at the edges, and the far end sits against the original house where no window can reach.

The doors at the garden end deliver plenty of light to the first couple of metres and almost none beyond.

So the rooflights belong at the back of the plan, over the part against the original wall, not distributed evenly for symmetry. Putting one near the doors adds light where there is already plenty.

What decides the number

The structure. Joist spacing and the position of any steel frequently make certain positions much cheaper than others, and it is worth asking where the cost steps are before fixing a layout.

The rooms below. An open plan kitchen and dining space wants light over both the island and the table, which is two positions rather than one.

Overheating. On a south or west facing extension, total glass area drives solar gain linearly while perceived brightness rises far more slowly. That argues for fewer, better specified units.

Cost. Each unit brings its own kerb, its own waterproofing detail and its own installation. Three small units cost more than one of the same total area.

A working approach

Start with the positions the room needs, usually one over the main working or gathering point and one at the deep end against the original house.

Check them against the structure and adjust to the cheaper positions where the difference in the room is small.

Then size for the orientation: modest and well glazed on south and west, more generous on north where there is no overheating penalty.

The alternative worth considering

A lantern rather than flush units, where the ceiling would benefit from lifting as well as lighting, and where the roof has the height for it under permitted development.

A lantern over the island and a flush rooflight at the deep end is a common and effective combination.