What a roof light must do, and how big it should be

A roof light is usually drawn first as a rectangle on a roof plan, sized by eye. That is the wrong way round. Before the rectangle is drawn, the designer should know what the roof light must do, how much light the room needs, and what a larger opening will cost in summer heat, winter heat loss and condensation. Size is the decision that every later chapter depends on: the kerb, the bars, the glass, the hips, the gutters and the fixings all scale with it.

This chapter sets out the jobs a roof light must do, measures RL-24’s light-transmitting area against the daylight requirement for its room, and compares it with a smaller alternative on solar heat gain. The failure case is a roof light sized only for light, which made the room it served unbearable on summer afternoons.

The jobs a roof light must do

Every roof light, whatever its shape, has to do the same jobs. Listing them at the start stops the design from solving one and forgetting another.

Figure 1The jobs RL-24 must do, and the chapter that deals with each

• Avoid: Admit enough daylight for the roomChapter 1
• Avoid: Keep summer solar heat to an acceptable levelChapters 1 and 7
• Avoid: Carry rain from the roof above around itselfChapter 2
• Avoid: Carry its glass, its own weight and windChapter 3
• Avoid: Meet its geometry: hips, jacks, ridge and valleysChapters 4 and 5
• Avoid: Catch and drain condensationChapter 6
• Avoid: Accommodate thermal movement without leakingChapter 7
• Ask: Open for ventilation, if required, and close tightChapter 8
• Ask: Be safe to install, maintain and work nearChapter 8

Two of these jobs pull against the others. More glass gives more light but also more summer heat, more winter heat loss and more condensation. A taller, steeper roof light sheds water and condensation better but catches more wind and costs more glass. The design is a balance, and numbers make the balance visible.

How much daylight the room needs

For a house, the National Construction Code sets a minimum amount of natural light for habitable rooms. In the NCC 2022 Housing Provisions, clause 10.5.1 requires windows with a light-transmitting area of not less than 10% of the room’s floor area, or roof lights with a light-transmitting area of not less than 3% of the floor area, or a proportional combination of the two. The Code’s explanatory material notes that roof lights receive more sun than windows, which is why the percentage is lower. Other building classes have their own provisions, and the edition in force for the project is the one that applies; check it before relying on these numbers.

RL-24’s room is 7.5 m by 5.6 m, a floor area of 42 m². If the roof light alone had to provide the room’s natural light, it would need a light-transmitting area of at least 3% × 42 = 1.26 m². In practice the room also has windows and a glazed door, so the roof light is a supplement rather than the only source. The 3% figure is still a useful yardstick: it tells the designer what area of roof light the Code treats as equivalent to a full complement of windows.

Measuring light-transmitting area

Light-transmitting area is not the size of the hole in the roof. It is the area through which light actually passes, which is smaller than the kerb opening by the width of the frame, the bars and anything else that blocks light. How a certifier measures it on a pitched or hipped roof light should be agreed early. This book takes a simple, conservative approach: the clear opening seen from below, at ceiling level, less the frame sightlines, and ignoring the extra area of the sloping glass.

RL-24’s kerb opening is 2,400 mm by 1,200 mm, an area of 2.88 m². Its perimeter frame has a sightline of about 50 mm on every side, so the clear opening is about 2,300 × 1,100 mm = 2.53 m², ignoring the narrow hip and common bars. That is about 6.0% of the floor area, roughly twice the yardstick.

Figure 2Light-transmitting area as a share of RL-24's 42 m² floor

NCC yardstick for roof lights3.0%

1,800 × 900 mm kerb, 1.36 m² clear3.24%

RL-24, 2,400 × 1,200 mm kerb, 2.53 m² clear6.02%

Why bigger is not automatically better

On a clear summer day, the sun can deliver something like 1,000 W to every square metre of a surface facing it. A roof light is a near-horizontal surface that faces the high summer sun for most of the day. The heat that enters through the glass is the light-transmitting area multiplied by the irradiance and by the glass’s solar heat gain coefficient, the fraction of incident solar energy that ends up inside. Clear laminated glass lets most of it in; a tinted or coated performance laminate lets in perhaps half.

Figure 3Solar heat entering through a roof light at a summer peak

solar gain (W) = SHGC × irradiance (W/m²) × light-transmitting area (m²)

Clear glass, RL-24's 2.53 m²

0.80 × 1,000 × 2.53 ≈ 2,024 W

Performance glass, RL-24's 2.53 m²

0.40 × 1,000 × 2.53 ≈ 1,012 W

Clear glass, 1.36 m² alternative

0.80 × 1,000 × 1.36 ≈ 1,088 W

Performance glass, 1.36 m² alternative

0.40 × 1,000 × 1.36 ≈ 544 W

1. RL-24 in clear glass admits about 2 kW at the peak, roughly the output of a small split-system air conditioner

The irradiance used here is a round teaching value for a clear sky with the sun high. A designer checking a real roof light would use solar data for the site and the roof light’s orientation, and an energy assessor would model the whole house. The rough calculation is not a substitute for either; its job is to show, before anything is drawn in detail, whether a proposed size is in a reasonable range or wildly out of it. It also works in reverse in winter: the same glass that admits the summer sun loses heat on winter nights, at a rate set by its thermal transmittance, and a larger area loses more. Chapter 6 uses that loss to predict condensation, and Chapter 7 compares it with the summer gain.

Two kilowatts of sunshine poured into a 42 m² room at three in the afternoon is a lot. The air conditioner has to remove it, or the room overheats. The same roof light in a performance laminate admits about half, and a smaller roof light in performance glass admits about a quarter. The designer and the owner chose the size knowing these numbers; Chapter 7 returns to heat, winter loss and shading.

Where the roof light goes

Position matters as much as size. Light from a roof light falls in a patch below it, moving with the sun. Over a kitchen bench, the patch lights the work surface; over a television or a dining table, it causes glare. A roof light near a wall washes the wall with light and makes the room feel larger; one in the middle of the ceiling lights the floor.

The structure also limits position. RL-24’s opening is framed between rafters, and its 2,400 mm long side runs across the slope, parallel to the eaves. That orientation suits the rafters, which run down the slope at 600 mm centres: the opening removes three rafters and is trimmed by headers top and bottom. It has a consequence the designer must not miss: a roof light 2,400 mm wide across the slope intercepts all the rain from the roof above it, which Chapter 2 deals with. The same opening turned the other way would intercept a narrower strip of roof but cut more rafters’ worth of span.

The light well

Between the roof light and the ceiling there is usually a void: the depth of the rafters, insulation, battens and ceiling. Light must pass down a light well through that depth, and the well’s shape changes how much light arrives and how it spreads. A deep, narrow, dark-lined well acts like a tube and throws a tight patch of light on the floor. A shallow well, or one whose sides splay outwards towards the room, spreads the light and softens the edge of the patch.

RL-24’s roof is framed with 190 mm rafters on a 5° slope, and the raked ceiling below follows the rafters, so the well is shallow: the kerb, about 150 mm above the roof on its high side, plus the rafter depth, plus the ceiling lining, roughly 450 mm in all. The designer considered splaying the well at 45°. A 45° splay over a 450 mm depth moves each side of the ceiling opening out by 450 mm, so the ceiling opening would grow from 2,300 × 1,100 mm to about 3,200 × 2,000 mm. That spreads the light over a wider area and reduces glare from the bright glass seen against a darker ceiling, but it needs more trimming of the ceiling framing and more lining.

In the end the owner chose vertical well sides lined in white, which reflect most of the light that strikes them. The point is that the well is part of the roof light’s design, not an afterthought for the plasterer, and it is decided with the size.

Recording the brief

The sizing work ends in a short brief, kept with the drawings: the room’s floor area and the daylight yardstick; the kerb opening and how the light-transmitting area was measured; the glass type with its solar heat gain coefficient; the summer peak gain for the chosen size and the alternatives considered; the shape and pitch; the light well; and whether the roof light must open. RL-24’s brief fits on half a page. When a builder asks for “the same roof light, a bit bigger” on the next job, the brief shows what would change and why, and the chapters that follow show by how much.

The other things size drives

Size feeds every later calculation:

• Glass weight grows with area. RL-24’s sloping glass, at 25°, has an area of about 2.88 ÷ cos 25° ≈ 3.18 m², about 48 kg of 6 mm glass (Chapter 3).
• Wind uplift grows with plan area. The fixings to the kerb carry it (Chapter 7).
• Winter heat loss and condensation grow with glass area and fall with better glazing (Chapter 6).
• Thermal movement grows with the longest bar and the frame’s length (Chapter 7).
• Cost grows faster than area, because larger panes need thicker glass and stiffer bars.

A designer who doubles the area should expect every one of these to change, and should not reuse the details from a smaller roof light without checking them.

The shape: flat, pitched or hipped

The 1922 trade course that informed this book described flat skylights on pitched roofs, double-pitched skylights on ridges and hipped skylights, each with its own bars and gutters. The same three families exist today, with different reasons for choosing between them.

Figure 4Three roof light shapes for RL-24's 2,400 × 1,200 mm kerb

	Sheds water and condensation	Glass and bar count	Wind and appearance
Flat, at roof pitch, 5°	poorly; water ponds at bars and the frame	least glass, simplest bars	lowest profile
Pitched with gable ends	well on two faces; vertical gable glass	ridge, commons, two gable panes	moderate; gables catch wind
Hipped, 25° on all faces	well on all four faces	ridge, commons, four hips	lowest wind area for its pitch; tidy from every side

RL-24 is hipped. A pitch of 25° is steep enough for water and condensation to run reliably to the bottom edge of every pane, and low enough to keep the rise modest, about 280 mm. Chapter 4 works out its geometry and Chapter 5 the pitched and valley alternatives.

A failure case: sized for light alone

This is an illustration. On an earlier job, a homeowner asked for “as much light as possible” in a west-facing family room of 30 m². The designer drew a flat roof light 3,000 by 1,500 mm in clear laminated glass, about 4.06 m² of clear opening after 50 mm sightlines, 13.5% of the floor area and more than four times the yardstick.

In the first January, the owner complained that the room was unusable after two o’clock. A rough check, of the kind in this chapter, showed that the clear glass admitted about 0.80 × 1,000 × 4.06 ≈ 3.2 kW at the summer peak, more than the room’s air conditioner could remove. The detection came from the owner, which is the most expensive way to find a problem.

The correction was costly: the clear glass was replaced with a performance laminate and an internal blind was fitted, halving the gain but also halving the light that had been the point of the roof light. A ten-minute calculation at the design stage, comparing light against heat for two or three sizes, would have produced a smaller roof light in performance glass that gave the room more than enough light and a fraction of the heat. The designer now runs the light and heat numbers together for every roof light and records them on the drawing.

How this applies to a small Australian business

Before drawing a roof light, list its jobs and check the daylight requirement for the room in the edition of the NCC that applies. Measure the light-transmitting area as the certifier will, not as the hole in the roof. Run the summer solar gain for at least two sizes and two glass types, and show the owner the numbers. Think about where the patch of light will fall and about which way the opening intercepts water. Then fix the size, because the kerb, glass, bars, gutters and fixings all follow from it.

Exercise: a hallway roof light

A hallway of 9.6 m² needs a roof light. The owner wants the hall lit entirely from above. The kerb sightline is 50 mm each side. What is the minimum light-transmitting area by the 3% yardstick? Would a kerb opening of 700 × 700 mm meet it? What is the summer peak gain for that size in clear glass and in performance glass, using this chapter’s teaching values?

Worked answer. The minimum is 3% × 9.6 = 0.288 m². A 700 × 700 mm kerb with 50 mm sightlines has a clear opening of 600 × 600 mm = 0.36 m², which is 3.75% of the floor and meets the yardstick. Its summer peak gain is about 0.80 × 1,000 × 0.36 = 288 W in clear glass and 0.40 × 1,000 × 0.36 = 144 W in performance glass, both modest for a hallway. A 600 × 600 mm kerb would give only 0.25 m² clear, 2.6% of the floor, and would not meet the yardstick on its own.