Sizing a rooflight for the room

The formula, and the four numbers that go into it

The rough calculation used in practice is straightforward enough to do on paper. Average daylight factor equals the glazed area, multiplied by the visible light transmittance of the glass, multiplied by the angle of visible sky in degrees, divided by the total internal surface area of the room multiplied by one minus the square of the average reflectance, then divided by two.

The four numbers that move it are these. Glazed area is the actual glass, not the structural opening. Visible light transmittance is a property of the glass, commonly between 0.5 and 0.8. Angle of visible sky is how much sky the glass can see, which for a horizontal or near-horizontal rooflight with nothing overhead is close to the maximum available. Internal surface area is every surface of the room, walls, floor and ceiling added together, which is why a tall room needs more glass than a low one of the same footprint.

The practical point is not to run the formula on a napkin. It is to understand which lever does what. Doubling the glass doubles the daylight factor. Changing from a heavily coated pane to a clear one can lift it by half again for no extra area at all. Painting the room white lifts it a little. And a rooflight beats a wall window on the third term by a wide margin, which is the next section.

Why overhead glass is worth two or three times its area in a wall

A vertical window in a wall sees, at best, half a hemisphere of sky, and in practice a good deal less because the ground takes the bottom half and the building opposite takes a slice off the top. A rooflight looks straight up at the brightest part of the sky dome with nothing in the way.

The working consequence is that a square metre of roof glass delivers something in the region of two to three times the daylight of a square metre of wall glass in the same room. That figure is the reason rooflights exist as a solution rather than a decoration, and it is the reason a modest unit in the right place can change a room that four wall windows could not.

It also cuts the other way. The same geometry that collects daylight in December collects solar heat in July, which is dealt with further down and in full on solar gain, overheating and shading.

Visible light transmittance, and the share the coatings take

Manufacturers publish two glass numbers that matter here. The g-value is the fraction of solar energy that gets through, and it governs heat. Visible light transmittance, often written as Lt or Tv, is the fraction of visible light that gets through, and it governs the sums on this page.

A clear double-glazed unit with a soft low-emissivity coating sits around 0.75 to 0.80. Add argon and a second coating and it drifts down a little. Triple glazing takes another pane’s worth, landing around 0.65 to 0.70. A solar control coating, which is doing something quite different, can bring it to 0.50 or below. Laminated interlayers cost a couple of points. Self-cleaning outer coatings cost a couple more.

So a triple-glazed solar control unit can deliver two thirds of the daylight of a clear double-glazed unit of identical size. That is not a reason to avoid either specification. It is a reason to check the number on the datasheet and size the glass against it rather than against an assumption. The glazing options are set out on U-values and glazing specification and by slope orientation on specifying glazing by which way the slope faces.

Structural opening, clear aperture and glass area are three different figures

This catches people out because manufacturers quote whichever suits them. A roof window in size code MK06 is nominally 780 by 1180mm. That is the outer frame. The structural opening is a little larger. The visible glass, once the outer frame, the sash frame and the glazing bead are deducted, is smaller again, and on a small roof window the frame can take a quarter of the nominal area.

On a made-to-size flat rooflight the ratio is usually better because the frames are slim aluminium and the sizes are larger, so the glass might be 80 to 88% of the nominal. On a lantern with several panes and glazing bars it is worse. Do the sizing arithmetic on the glass figure, not the nominal one, and the small units in particular will come out bigger than expected. Codes and their real dimensions are on roof window sizes and how they are coded.

A maintenance factor for dirt, angle and a wet county

Published transmittance is a laboratory figure taken on clean glass. Roof glass does not stay clean. Overhead glazing collects what the air drops on it, and rain, which washes a vertical pane, arrives on a shallow rooflight and evaporates in place, leaving what it carried behind.

In practice it is sensible to assume a maintenance factor of 0.9 on a steep pitched roof window that sheds well, 0.8 on a flat rooflight at five or six degrees, and lower again where the glass sits under a tree or on a north-facing slope with moss on the covering around it. That is 10 to 20% off the daylight figure before anyone has done anything wrong, and it argues for sizing a shade generously rather than exactly. It also argues for cleaning, which is on cleaning and maintaining a rooflight.

How far the light has to travel, and the depth that defeats it

Daylight from a single rooflight falls off with the square of the distance from it, so the useful working figure is that a unit lights a zone roughly one and a half to two times its own width, spreading outward at the floor.

Ceiling height changes that. A rooflight in a 2.4 metre ceiling throws a tight, bright pool. The same unit in a 3.2 metre Cheltenham room throws a wider, softer one that reaches further but is dimmer at any given point. Deep plans are the hard case: a knocked-through Gloucester terrace can run 8 or 9 metres from the front bay to the back of the extension, and no single unit at one end will reach the middle. That plan wants glass distributed along it rather than concentrated, which is the next section.

Reveal geometry adds to this. A deep reveal in a converted roof, where the insulation build-up can be 250mm, throws real shadow unless the sides are splayed and the head is vertical. A splayed reveal can add a noticeable share of the daylight reaching the floor for the cost of half a day’s carpentry, covered on rooflight reveals and why splaying them matters.

One large unit against two smaller ones, at the same total area

Two units of half the area, spaced apart, do not light a room the same way one large unit does. They give lower peak illuminance and much better uniformity, they cut the contrast between the bright zone and the corners, and they usually make the room read as better lit even though the meter says the same average.

One large unit gives a stronger single pool of light, a more dramatic effect, and a better view of sky if the room has a seating position under it. It also concentrates the solar heat and the glare into one place.

The structural argument frequently settles it. Two narrow units in adjacent rafter or truss bays need no designed member, while one wide unit of the same total area needs a trimmer, an engineer and a Building Control structural submission. On a trussed estate roof at Quedgeley or a hipped semi at Longlevens that difference is decisive, and the pairing detail is on two rooflights side by side on one slope.

Orientation, and the slope that gives the steadiest light

Which way the slope faces changes the character of the light more than the quantity, but it changes the sizing decision all the same.

A north-facing slope gives cool, even, shadowless light that barely varies through the day and carries almost no solar heat. It is the steadiest light available and it is why studios face north. It is also the dimmest in absolute terms, so a north slope justifies going up a size. South-facing gives the most total daylight and the most heat, and it wants solar control glass and often external shading, at which point the transmittance loss argues for area back again. East gives morning light that suits a kitchen. West gives late afternoon and evening sun and is the hardest to manage in a loft room, because the sun is low, striking the glass at a steep angle at the hottest part of the day.

A flat roof unit faces up rather than any direction, so it collects the whole sky and behaves like a south slope for heat in midsummer when the sun is nearly overhead. North-facing performance is on do rooflights work on a north facing slope.

What the sky can actually see from that position

The formula’s angle-of-visible-sky term is the one people forget, and on a real house it can halve the answer.

Things that take sky away from a rooflight: a chimney stack upslope of it, a taller neighbouring house on a Gloucester terrace, the main roof of the house standing over a single-storey rear addition, a mature tree, a parapet on a Cheltenham flat, and the valley side itself in Chalford or Brimscombe where a hill fills a third of the sky. A rooflight tucked into the angle where a rear extension meets the back wall of the house is looking up a slot rather than at the sky.

Check it before choosing a position. Stand where the glass will be, look up, and see how much open sky is actually available. Where the answer is disappointing, moving the unit two metres out from the wall usually recovers more daylight than making it larger would.

Sizing by what the room is for

The ratios above are the arithmetic. What the room does adds constraints of its own.

A kitchen wants glass over the working areas rather than over the middle of the floor, and where there is an island the light should land on it rather than behind whoever is standing at it. A bathroom needs less area than people assume, because the room is small and the surfaces are reflective, and privacy is free overhead. A loft bedroom is sized for the standing zone along the ridge rather than the whole footprint, and the escape requirement often fixes the unit before daylight gets a say. A landing or stairwell is the highest return in the house: a small unit at the top of a stair transforms a dim core for very little money. A home office wants even light without glare on a screen, which favours two units or a north slope.

Where the room has no roof area to spare at all, a sun tunnel gets useful daylight down a reflective tube from a small dome, described on sun tunnel installation.

The heat that arrives with the daylight

Sizing for daylight without checking the heat is how a bright room becomes an unusable one in July. The two are linked but not fixed to each other, which is the useful part.

Glass area drives both. A solar control coating cuts the g-value from around 0.5 to roughly 0.25 or 0.30 while taking a smaller proportional bite out of the visible transmittance, so it separates the two. External shading, fitted outside the glass, stops the heat before it enters and outperforms any internal blind by a wide margin. And an opening sash at the high point of the room gives a stack effect that removes heat that has already arrived.

The practical rule for overhead glass: above roughly 15% of floor area in a west or south facing room, plan the shading at the same time as the glass rather than afterwards. External shading needs its fixings before the covering goes back, so it is an order-stage decision.

What the roof will actually give, by type

Step five. The room has asked for a number and now the structure answers.

On a traditional cut pitched roof, width is limited by the rafter bay unless a rafter is trimmed, and length is limited by the run of slope between the purlin and the eaves. Both are extendable with design, set out on trimming rafters for a pitched roof rooflight. On a trussed roof, the bay is around 550 to 570mm clear and going wider is a designed structure. On a flat roof, the joist span and direction set the clear opening, and the unit can be far larger, but the falls, the outlet positions and the kerb clearances constrain where it sits. On a Cotswold stone slate roof, the practical size is often set by what the conservation officer will accept rather than by the timber.

Where the roof gives less than the room wanted, the honest options are to distribute the glass across more openings, to accept a lower daylight factor and say so, or to design a structural opening. All three are legitimate. Choosing quietly and hoping is not.