How many rooflights a flat roof extension needs

The short answer, expressed as a proportion of the floor

The proportion, not the count, is the number that matters. Two rooflights in a 12 square metre utility room and two in a 40 square metre open plan kitchen are not the same decision.

As a starting position for a habitable room under a flat roof with limited side windows, plan on 15 per cent of the floor area in glass, rising towards 20 per cent where the room is deep, north-facing, or has a boundary wall close to its only window. Below about 10 per cent the room will read as artificially lit for much of the day. Above about 25 per cent you are into a different set of problems, which are heat and glare rather than gloom.

Daylight factor, and the arithmetic underneath the rule of thumb

The rule of thumb comes from daylight factor, which expresses indoor illuminance as a percentage of the unobstructed outdoor illuminance available at the same moment.

Commonly quoted targets are an average daylight factor of around 2 per cent in a kitchen, 1.5 per cent in a dining room and 1 per cent in a living room. Those are averages across the working plane, and they are modest numbers: 2 per cent of a 10,000 lux overcast Gloucestershire sky is 200 lux, which is comfortable working light without any lamps on.

Run the calculation for a horizontal rooflight with a clear view of sky and the glazed area needed to hit 2 per cent in a typical room lands close to that 15 per cent figure. The rule of thumb is not folklore, it is the calculation rounded off.

Why glass in a roof beats the same glass in a wall

A horizontal rooflight sees the entire sky dome. A vertical window sees half of it at best, and much of that half is occupied by the ground, a fence, a neighbouring wall or a tree.

The practical consequence is that a square metre of rooflight delivers roughly two to three times the daylight of a square metre of window in the same room. It also delivers it further into the plan, and it delivers it more evenly through the day because the sky brightness overhead varies less than the brightness on any one facade.

This is why an extension with a large set of bifolds can still be dark six metres in, and why adding rooflights fixes it where adding more glazing at the back does not.

Room depth, and how far a rear window actually reaches

Useful daylight from a vertical window extends into a room roughly one and a half to two times the head height of that window. A door head at 2.1 metres gives useful light for about 3 to 4 metres.

Most rear extensions in this county are between 3 and 5 metres deep, and they are usually knocked through to the original back room behind them, giving a combined depth of 6 to 9 metres. The far half of that plan, which is generally where the kitchen ends up, is beyond the reach of the glazing at the back.

Measure the distance from your rear glazing to the furthest point of the combined room. Anything beyond about 4 metres is a zone that needs its own daylight from above, and that measurement usually decides the number of units on its own.

One large unit against three smaller ones

Three units of 1200 by 1000 and one unit of 2400 by 1500 both give 3.6 square metres of glass. They do not give the same room.

A single large unit gives one strong pool of light with the rest of the ceiling in comparative shade. It reads as dramatic, it works well over a specific object such as an island or a dining table, and it needs less perimeter detailing because there is one kerb rather than three.

Three smaller units spread the light, so the ceiling brightness is more even and the room feels larger. They cost more in total because kerbs, flashing and labour multiply, and they need the joist layout to co-operate. They are usually the better answer in a long narrow room, and the single unit is usually better in a squarer one.

Even light against a bright band, and what spacing does

There is a spacing convention borrowed from lighting design that transfers well: for reasonably even illumination, the gap between units should be no more than about one and a half times the height of the ceiling above the floor.

In a room with a 2.4 metre ceiling, that puts units at no more than about 3.6 metres apart. Space them further and the room develops bright zones with darker ground between, which the eye reads as unevenness rather than as light. Space them closer and the light merges into a single wash.

Setting units back from the walls by roughly half the spacing keeps the wall surfaces lit, and lit walls are what make a room feel bright. A rooflight in the exact middle of a ceiling lights the floor and leaves the walls dim.

Where the joists are, and the layout the structure will allow

Whatever you draw, the joists get a vote. On a typical flat roof they run at 400 or 600mm centres in one direction across the shorter span.

A unit whose width fits between two joists needs no structural work at all beyond noggins. A unit that interrupts one joist needs trimmers. A unit that interrupts two or more needs trimmers sized as a structural exercise, and possibly a beam.

So three units of 900mm width across joists at 600 centres, each interrupting one joist, is a straightforward job. One unit at 2.4 metres wide across the same joists interrupts three, and the trimming and the load path become the main item on the quotation. The number of rooflights and the amount of carpentry are not proportional.

Kerb clearance, and the gap two units need between them

Two rooflights side by side need enough roof between them for the covering to be detailed properly on both, for water to drain along the channel formed between them, and for the insulation to return onto each kerb.

Allow a minimum of 300mm of clear roof surface between the outside faces of two kerbs, and preferably 400 to 500mm where the roof falls across the gap rather than along it. Anything tighter creates a narrow gutter that collects debris, holds water and is awkward to work in. The same clearance applies between a kerb and a parapet or an abutment wall.

On a 2 metre wide Victorian rear addition roof, that clearance rule frequently means one unit rather than two, whatever the daylight calculation says.

Glare, contrast, and the ceiling that reads dark

More glass is not automatically better light. A single very large rooflight in a low ceiling produces a surface far brighter than anything around it, and the eye adapts to the brightest thing in view. The rest of the ceiling then reads as dark by comparison, and the room can feel gloomier than one with less total glass spread more widely.

Two things soften it. Splaying the internal reveal so the plasterboard around the opening catches light and grades the transition rather than presenting a hard edge. And a matt, light coloured ceiling finish, which bounces the light around instead of leaving it as one bright rectangle.

Overheating, and the point where more glass stops being an improvement

Horizontal glass takes the full force of a summer sun. In late June the sun at Gloucester reaches around 61 degrees above the horizon at midday, which puts it almost square on to a flat rooflight for several hours.

Past roughly 20 to 25 per cent of floor area in horizontal glass, summer heat gain in a west or south facing room becomes the governing problem rather than winter daylight. The answers are a solar control coating, which trades some daylight for a lower g-value, or ventilation, or an opening unit that lets hot air out at the top of the room. All three are worth deciding at the same time as the number of units, not afterwards, and they are covered on the energy efficient rooflights page.

How the glazed area counts toward the allowance for an extension

There is a limit on the total glazed area in a new extension under the energy regulations. As a working position, the glazing in a new extension is expected to be no more than about a quarter of its floor area, with the area of any existing openings that are being covered up added back into the allowance.

Rooflights count toward that figure alongside windows and doors, which is what catches people out on an extension with a full width rear opening. Going beyond the allowance is possible, but it has to be balanced by better performance elsewhere and demonstrated by calculation rather than assumed. Where an extension is being designed with a lot of glass at the back and a lot overhead, that arithmetic is worth doing at design stage. The Building Control notification for the rooflight work is handled as part of the job.

Zones under one roof, lit differently

Most flat roof extensions in this county contain a kitchen, an eating area and a sitting area in one space, and the three want different light.

Over a worktop or an island, light directly overhead is genuinely useful, and a unit centred on the run removes the shadow that a person standing at the counter casts on their own work. Over a dining table the same directness makes the table a focus and the room feel composed. Over a sofa, direct overhead light is less welcome, and a unit offset towards the wall behind gives the same brightness without the sky in your eyes when you look up from a book.

Positioning a large unit over an island is a related decision covered on the positioning a lantern over a kitchen island page, and the same reasoning applies to flat glass.

The sun through a Gloucester day, and where the light lands

Rooflights take most of their light from the sky rather than from the sun directly, which is why they perform so consistently. But direct sun still moves through the room, and it moves in a predictable way.

In midwinter the sun at this latitude only reaches about 15 degrees above the horizon at noon, so direct sun through a horizontal rooflight is weak and low, entering at a steep angle and landing on a wall rather than the floor. In midsummer it is almost overhead and lands directly below the opening. A rooflight positioned for a bright pool on the dining table in August will put nothing on it in December.

Orientation matters less for a flat rooflight than for a window, which is one of its advantages on a north-facing rear extension where a window would deliver very little.

Cost per unit against cost per square metre of glass

Glass is priced by area, and the price per square metre falls as the pane gets bigger up to the point where handling and craneage take over. Everything else on a rooflight job is priced per opening.

Each additional unit means another kerb, another set of trimmers, another perimeter of covering detail, another reveal to board and finish, and another day of labour spread across the programme. So three small units cost appreciably more than one unit of the same total area, and the difference is in carpentry and roofing rather than in glass.

That trade is worth understanding before the layout is fixed, because the daylight argument and the cost argument pull in opposite directions and the sensible answer is usually two or three units rather than either extreme. How a figure is built up is on the costs page.

Replacing one old dome with a properly planned set

A great many Gloucestershire extensions from the 1970s and 1980s have exactly one rooflight, a 600 by 900 acrylic dome, yellowed and crazed, delivering a fraction of the light it did when new.

Replacing that with three modern flush glazed units is not a like for like exchange. The glass area might triple, the transmission of each square metre improves substantially because clear glass outperforms an aged dome by a wide margin, and the room becomes a different space. It also means opening the covering in three places instead of one, and the decision usually turns into a discussion about renewing the whole roof surface at the same time. Where a single unit is being swapped for a single unit, the replacing a failed or misted rooflight page covers it.

Gloucestershire extensions, and the shapes that recur

Victorian rear additions through Gloucester, Tredworth and Kingsholm are narrow, often 1.8 to 2.4 metres wide, with joists spanning the short way. On those the clearance rule usually gives one line of units down the middle, sized to sit between the joists, and the answer is commonly two or three modest units rather than one large one.

Full width rear extensions on the estates at Quedgeley, Abbeydale, Bishops Cleeve and Yate are 5 to 7 metres wide and 3 to 4 deep, generally with a steel beam at the knock-through. Those take three or four units comfortably, spaced across the width, and the daylight target is easy to hit.

Behind Cotswold stone through Painswick, Northleach and the wolds villages, a flat-roofed rear extension is often the only place a rooflight can go at all, because the main roof is stone slate and any opening in it is a conservation question. That tends to concentrate the daylight ambition onto the flat roof, and it makes the number of units on it a more significant decision than it would be elsewhere.