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Roof Lights by the Numbers
Sizing, kerbs, glazing bars, hips and valleys, condensation, heat, movement and safe opening sashes, worked through one hipped roof light
Design and make a glazed roof light by numbers: daylight area, kerbs and flashings, glass and bars, hip and valley geometry, condensation, solar heat, movement, sashes and safety.
Start readingAbout this book
A roof light is a hole in a roof that is meant to let light in and keep everything else out. It has to carry the rain from the roof above around itself, hold its glass against wind and its own weight, catch the condensation that forms on its cold underside, keep the room below from overheating in summer, and survive decades of sun and temperature swings without leaking. It also has to be safe for the people who install and later work near it, because a roof light is one of the commonest places people fall through a roof.
Factory-made roof lights come with the maker’s tested details. Many roof lights are still designed and built for one opening: a hipped or pitched glazed light over a stair, a kitchen or a hall, made by a sheet metal shop and a glazier from folded flashings, extruded or folded bars and cut glass. This book is for that work. It sizes the roof light, details its kerb and flashings, works out the glass and bars, develops the geometry of hipped and pitched lights with their hips, jacks, ridges and valleys, and then deals with condensation, heat, movement, opening sashes, safety and testing. Every chapter works a problem through the running case with numbers that can be checked, shows how a mistake is detected and corrected, and ends with an exercise and its worked answer.
Prerequisites and how to use the book
It assumes basic trigonometry, some experience reading building drawings and an understanding of folded sheet metal. Chapters 1 and 2 cover size and the kerb, Chapter 3 glass and bars, Chapters 4 and 5 the geometry of hipped and pitched lights, Chapter 6 condensation, Chapter 7 heat and movement, and Chapter 8 opening sashes, safety and testing. Profiles, girths and flashing mitres are covered in Architectural Sheet Metal by the Numbers, and flows of rain in Roof Drainage by the Numbers; this book links to them rather than repeating them.
The running case
The book follows RL-24, a fictional hipped glazed roof light over the open-plan kitchen and living room of a single-storey house extension. The room is 7.5 m by 5.6 m, 42 m² of floor. The extension’s roof is metal deck at a 5° pitch. The roof light sits on a timber kerb whose inside opening is 2,400 mm across the slope by 1,200 mm down it. Its glass is pitched at 25° on all four faces, so it has two long trapezoidal faces, two triangular hipped ends, four hip bars and a ridge 1,200 mm long. It is glazed with laminated glass on aluminium bars, with folded aluminium flashings to the kerb.
All dimensions, loads, prices and results are invented for teaching. This is an illustration. None of it describes a KEVOS® client, supplier or project. Money, where it appears, is in Australian dollars including GST.
Source limits and professional responsibility
The supplied 1922 trade course informed the range of subjects: curbs, bars, condensation gutters, hips and valleys and opening sashes. The methods here are modern restatements with original examples. This book is educational. Glass selection for overhead glazing, structural adequacy, wind actions, energy performance and weatherproofing for a real building must follow the National Construction Code, AS 1288, the wind loading standard and the makers’ instructions, and be designed or checked by a competent person and approved where the law requires. Roof work is work at height and must be planned and carried out under the work health and safety law that applies.
Who this book is for
Sheet metal fabricators, roof plumbers, glaziers, drafters and building designers who detail, make or check site-built glazed roof lights, their kerbs, flashings and glazing bars.
What you will learn
- Size a roof light against the room's daylight requirement and test the size against solar heat gain.
- Detail a kerb and its flashings so that water from the roof above is carried around the roof light.
- Calculate common, hip, jack and valley bar lengths, pitches and backing angles for a hipped roof light.
- Predict when glass and bars will run with condensation and size the gutters and weep path that carry it away.
- Allow for thermal movement, wind uplift, sash operating forces and safe installation, then prove the roof light with a water test.
Inside this book
Chapters
- 01What a roof light must do, and how big it should beList the jobs a roof light must do, measure its light-transmitting area against the room's daylight requirement, and test a larger size against summer solar heat before drawing anything.
- 02Kerbs, upstands and flashingsSet the kerb heights on a sloping roof, carry the water from the roof above around the roof light with a back tray, and work out the girths of the apron, side and back flashings.
- 03Glass, glazing bars and the loads they carryChoose glass for overhead glazing, schedule RL-24's twelve panes, find their weights, and check the common bars and ridge for bending and deflection under wind uplift.
- 04Hipped roof lights: hips, jacks and backing anglesWork out RL-24's rise, ridge, common and hip bar lengths, the hip's pitch and plan angle, the backing angle its glazing faces must be folded to, and jack bar lengths and cuts.
- 05Pitched roof lights, gables, lean-tos and valleysCompare RL-24 as a gable-ended pitched light and a lean-to, work out a valley bar for an L-shaped version, and see why the valley's water and flashings decided against it.
- 06Condensation and the gutters that carry itCalculate the room's dew point, predict the inner surface temperatures of RL-24's glass and bars on a winter night, and size the condensation channels and weeps that carry the water outside.
- 07Heat, sun, movement and holding it downCompare RL-24's summer solar gain with its winter heat loss, allow for the thermal movement of its aluminium frame and glass, and size the fixings that hold it down against wind uplift.
- 08Opening sashes, safety and the water testSize the actuator for RL-24's opening sash against its weight and wind, compare its ventilating area with the room's needs, control falls through the roof light, and prove it watertight.
Apply This in Practice
Use these KEVOS® tools to try the method from this reading with your own numbers.
Applies this method
Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin)
Gives the room’s dew point and checks an inner glass or frame surface temperature against it.
References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 26 October 2026.
- William Neubecker, edited by Frank X. Morio, U. P. C. Book Company, New York. Home Instruction for Sheet Metal Workers (1922). Supplied source, read as a subject map of the flat, hipped and pitched skylights, curbs, glazing bars, condensation gutters, cross clips and raising sashes that sheet metal shops made by hand. Its engravings, imperial examples, sequences and prose are not reproduced; this book expresses the subject as modern calculations in SI units, with its own case, figures and exercises.
- Australian Building Codes Board. NCC 2022 Housing Provisions, Part 10.5 Light (clause 10.5.1) and Part 10.6 Ventilation (clause 10.6.2) (2022). Source of the natural light areas used in Chapter 1 (10% of floor area for windows, 3% for roof lights) and the 5% ventilating area quoted in Chapter 8. Check the edition and volume that apply to the building.
- Standards Australia and Standards New Zealand. AS 4285:2019 Rooflights; AS 1288 Glass in buildings: Selection and installation; AS/NZS 4389 Roof safety mesh; AS/NZS 1170.2 Wind actions (current editions). Named by number and scope only. AS 4285:2019 covers factory-made rooflights and, by its published scope, excludes site-built overhead glazing systems such as the one in this book, which falls to AS 1288 and the NCC. No values or clauses are quoted from any of these standards; loads, U-values and glass choices in this book are labelled teaching values.
- Oleg A. Alduchov and Robert E. Eskridge, Journal of Applied Meteorology 35(4). Improved Magnus form approximation of saturation vapor pressure (1996). Source of the Magnus coefficients (17.625 and 243.04 °C) used for dew points in Chapter 6.
- The Engineering ToolBox. Linear Thermal Expansion Coefficients of Materials (current web page). Source for the expansion coefficients of aluminium and glass used for movement in Chapter 7.
- Safe Work Australia. Safe Work on Roofs information sheet (2015). Background for the fragile-roof and roof safety mesh controls discussed in Chapter 8.
- SafeWork NSW. Falls through skylights and plastic roof sheeting (safety alert) (2018). Background for treating roof lights as non-trafficable during and after installation in Chapter 8.