Skylight | Chalkline

A skylight is a glazed opening in a roof admitting daylight (and sometimes ventilation) into the building below. In Australia, skylights are commonly used in kitchens, bathrooms, stair voids, and roof additions where natural light is desired but the wall plane cannot provide a window. Because the glass sits horizontally above an occupied space, it is classified as overhead glazing under AS 1288:2021 and must use laminated safety glass (typically 6.38 mm or 8.38 mm laminated): broken toughened-glass fragments would fall vertically into the room below, while laminated glass holds together on the interlayer even when shattered. Skylights are also a common point of roof waterproofing failure because they require complex flashings, kerbed framing, and seasonal thermal cycling at the perimeter. Verified per AS 1288:2021 and AS 4285:2017 (2026-05-16).

Types of skylight:

Type	Description	Use
Fixed	Sealed glass unit, no opening	Most common; pure daylight
Operable (manual)	Hinged glazed unit with handle/crank	Daylight + natural ventilation
Operable (electric/remote)	Motorised; rain sensor common	Premium installations
Tubular skylight	Reflective dome → tube → diffuser; daylight via small roof penetration	Internal rooms with no roof access; smaller glass area
Walk-on skylight (floor-level)	Glazed floor panel in roof terrace	Premium architectural; load-rated
Roof window	Larger operable skylight, typically dormer-style	Attic conversions, top-floor rooms

AS 1288 overhead glazing requirements:

Element	Requirement
Glass type	Laminated safety glass (NOT toughened); 6.38 mm minimum, 8.38 mm common, 10.38 mm for larger units
Wired glass	Acceptable in some heritage applications; uncommon in new
Polycarbonate	Allowed for some applications under AS 4285; not all certifiers accept it
Insulating glass unit (IGU) with laminated inner	Common in modern skylights for thermal performance; both panes typically laminated where over habitable space

The reasoning: a toughened pane breaks into thousands of pebble-sized fragments. Above a person, those fragments are projectiles. A laminated pane breaks but the PVB interlayer holds the fragments in place, so the broken glass remains as a sheet (even if cracked). The sheet may sag, but it does not rain glass.

Construction and flashing:

A skylight installation involves a kerb (a raised perimeter on the roof) and a flashing kit (typically supplied with the unit) that integrates the kerb with the roof system:

        ┌────────────────────┐
        │  GLAZED UNIT       │
        ├────────────────────┤  ← skylight perimeter seal
        │     KERB           │
        │     (raised)       │
        ├────────────────────┤
        │  STEP/CONT FLASHING│  ← integrates kerb with roof tiles or sheet
        │                    │
        └────────────────────┘
        Roof tiles or sheet

The flashing must:

Wrap up the kerb at minimum 75 mm (per NCC 2022 Housing Provisions 7.3).
Lap at least 75 mm at the head (top), with the upper roof course tucking over.
Step-flash the sides (per AS 2050 for tiles, AS 1562 for sheet).
Drain the apron at the base into the gutter system.

Common failure modes:

Failure	Cause
Leaks at upper edge	Flashing reverse-lapped; water enters under the head flashing
Leaks at sides	Step flashings too short or omitted; water tracks down the kerb
Leaks around penetration	Kerb-to-deck seal failed at corners
Condensation between IGU panes	Glazing seal failed; IGU end-of-life
Frame sealant perished under UV	Original silicone failed at 5-10 years; re-seal needed
Internal condensation on cold-side pane	Low-performance glass and high humidity; thermal break or low-e glazing helps

Energy and condensation:

A typical single-glazed skylight has a U-value around 5.0 W/m²K, much worse than a wall. In NCC Specification 42 (7-star plus WoH ≥60) assessments, skylights are commonly flagged as a thermal weak point. Two mitigation strategies:

Strategy	Effect
Double-glazed (IGU) skylight with low-e + argon	U-value can drop to 2.0-2.6 W/m²K
Triple-glazed skylight	U-value to 1.5 W/m²K; rare in residential, expensive

For warm-humid climates (NCC zones 1-3) condensation on the inner pane in winter is a known issue; in zone 7-8 (alpine) it’s near-constant without high-performance glazing.

Builder takeaway:

Specify laminated glass; never toughened for habitable-space skylights.
Use the manufacturer’s flashing kit; don’t fabricate.
Brief the roofer to step-flash the perimeter properly.
Allow for periodic gasket replacement (every 15-20 years).
For thermal performance, IGU + low-e is the residential standard now; single-glaze only on minimal-impact installations.

Also known as: roof window; rooflight; sky-window; lantern (architectural); roof glazing.

Category: Materials.

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