A skylight is not a hole in the roof with glass over it. It is a system: the glazed unit, a profile-specific flashing kit, a lined and insulated shaft, and a ceiling diffuser or frame. Specify one element without considering the others and you create problems that are expensive to fix after the plasterboard is on.
This guide works through each layer of that system, from glazing selection to shaft construction, with the compliance touchpoints a builder or roof plumber needs to know before ordering.
Fixed or Openable: Choosing the Right Unit Type
Fixed skylights are simpler to flash, cheaper to buy and carry no mechanical parts to fail. They suit spaces where ventilation comes from elsewhere and where the primary goal is daylight. Bathrooms, stairwells and south-facing rooms where direct sun is not a concern are typical applications.
Openable units, sometimes called roof windows, add a ventilation path that a fixed unit cannot provide. Manual crank-operated sashes work well in accessible locations. Electrically operated units with rain sensors are worth the premium in high or hard-to-reach positions, because a unit left open in a Queensland afternoon storm causes far more damage than the cost difference between manual and motorised. Either way, the opening mechanism adds a moving seal, and that seal is a maintenance item. Builders should note this in handover documentation.
Tubular daylight devices (TDDs) are a third category. They use a small roof dome, a highly reflective tube and a ceiling diffuser to deliver daylight into spaces where a full-height shaft is impractical: rooms under a hip end, spaces with a long horizontal run, or areas where a rectangular opening would compromise a structural rafter. The tube diameter typically ranges from 250 mm to 530 mm. Light output drops with tube length and bends, so manufacturer selection guides should be followed closely when the run exceeds two metres or involves an elbow.
Glazing: Heat, Glare and Acoustic Performance
Glazing choice drives thermal and acoustic performance more than any other single decision in skylight selection.
Single glazing is rarely appropriate in a habitable room. It has a U-value around 5.8 W/m²K and offers no meaningful acoustic attenuation. It remains common in agricultural and industrial buildings where NCC Section J energy provisions do not apply.
Double glazing with a low-emissivity coating is the baseline for residential and commercial habitable spaces. A typical low-e IGU (insulating glass unit) achieves a U-value in the range of 1.6 to 2.0 W/m²K, depending on the gap width and gas fill. The solar heat gain coefficient (SHGC) matters as much as the U-value in Australian climates. A unit with a low SHGC (around 0.2 to 0.3) limits summer heat load but can reduce useful winter solar gain in heating-dominated climates. The right balance depends on climate zone and orientation; a hydraulic or thermal consultant can model this if the project is large enough to warrant it.
Laminated inner panes are required by NCC Volume Two (Housing Provisions) and Volume One in locations where a person could fall onto the glazing from above, or where the glazing is overhead and breakage could cause injury below. AS 1288 governs glass selection for buildings. Where the skylight is in a trafficable roof or accessible area, the entire assembly must meet the fall protection requirements of AS 1170.1 and the relevant NCC deemed-to-satisfy provisions. This is a design and certification matter; if there is any doubt about whether a location is classified as trafficable or accessible, get a structural engineer's advice before ordering.
Acoustic glazing uses laminated panes with an acoustic interlayer. The improvement over standard laminated glass is meaningful in high-noise environments: near airports, busy roads or under flight paths. Manufacturers publish weighted sound reduction index (Rw) values for their assemblies. Compare these against the acoustic requirements in the project specification before selecting.
The Flashing Kit: Why Profile and Pitch Both Matter
The flashing kit is where most skylight failures begin. A kit that does not match the roof profile leaves gaps at the step and apron flashings that no amount of silicone will seal permanently. Silicone is not a substitute for correct geometry.
Roof sheeting profiles vary significantly in rib height and spacing. Custom Orb (corrugated), Trimdek, Spandek, Klip-Lok and Monoclad all have different profiles, and a flashing kit designed for one will not bed correctly on another. When ordering, specify the sheeting profile by name, not by generic description. A kit listed as suits corrugated may be dimensioned for a 762-cover corrugated profile that does not match the sheet on site.
Pitch matters for a different reason. Most flashing kits are rated to a minimum roof pitch, typically 15 degrees for standard kits. Below that pitch, water does not drain away from the upslope (head) flashing fast enough, and capillary action draws it under the flashing lip. Some manufacturers offer low-pitch kits rated to 5 or 10 degrees, but these use a different head flashing geometry and often require a secondary sealant bead specified in the installation instructions. Follow the manufacturer's instructions; they are the basis for any warranty claim.
For metal roofing, the flashing kit also needs to account for thermal movement. A skylight frame fixed rigidly through a long metal sheet restrains the sheet's ability to expand and contract, which can buckle the sheeting or pull fasteners over time. Kits designed for metal roofing include slotted fixing holes or flexible apron sections for this reason.
At CSA, skylight flashing kits are available to suit the major sheeting profiles we supply, including Lysaght and Stramit profiles. When you order sheeting and a skylight together, the trade desk can confirm kit compatibility before anything leaves the warehouse.
The Shaft: Insulation, Vapour Control and Condensation
The shaft connects the roof unit to the ceiling plane. It is the most frequently underbuilt part of the system, and the consequences show up months or years later as condensation staining, mould or plasterboard damage.
A shaft passes through the insulation layer. If the shaft walls are not insulated, they become a cold bridge in winter. Warm, humid air from the room below contacts the cold shaft lining and deposits moisture. In a bathroom or kitchen, this happens quickly. The shaft lining should be insulated to at least the same R-value as the ceiling insulation it displaces, and the insulation should be continuous with no gaps at the corners.
Vapour control is the second issue. In climate zones 1 to 3 (tropical and subtropical), the dominant moisture drive is inward: humid outdoor air trying to move into the cooled interior. In zones 6 to 8 (cool to alpine), the drive reverses in winter. The NCC does not prescribe a universal vapour barrier for shafts, but the principle is straightforward: place the vapour control layer on the warm side of the insulation. In most Australian residential construction, a reflective foil sarking on the warm-side shaft wall, taped at all joints, is sufficient. In cool climates, this means the room-facing side of the shaft lining.
The shaft geometry also affects performance. A straight vertical shaft delivers more light than an angled one, but an angled shaft can reach a ceiling opening that is not directly below the roof unit. Where the shaft is angled, the internal surfaces should be lined with a high-reflectance material (white-painted plasterboard or a proprietary reflective liner) to minimise light loss. A shaft that is dark inside defeats the purpose of the skylight.
The ceiling diffuser or frame closes the bottom of the shaft. In a fixed skylight, this is typically a flat or domed acrylic or polycarbonate panel. In a roof window, the sash itself extends to the ceiling. The diffuser should be sealed to the shaft lining to prevent air movement through the shaft cavity, which drives condensation independently of the vapour control layer.
Interaction with Roof Insulation and Sarking
Sarking under metal roofing serves two functions: condensation control on the underside of the sheeting, and a secondary weather barrier at laps and penetrations. Where a skylight penetrates the sarking, the cut edges must be dressed back to the skylight frame and taped. A loose or untaped sarking edge around a skylight becomes a funnel for any condensation that forms on the sheeting above.
Batts or blanket insulation in the ceiling must be cut back from the shaft and not compressed against it. Compressed insulation loses R-value and can trap moisture against the shaft lining. Leave a small clearance and use a rigid insulation board to cap the gap if the shaft wall insulation does not extend fully to the ceiling plane.
Compliance Touchpoints
Two areas attract the most compliance questions on skylight installations.
Glazing classification: AS 1288 requires that overhead glazing in a habitable space uses safety glass. Laminated glass is the standard solution. Toughened (tempered) glass alone is not suitable overhead because it disintegrates into fragments when broken; laminated glass holds together. Most skylight manufacturers supply laminated inner panes as standard on residential units, but verify this on the product datasheet before ordering, particularly for budget or imported units.
Fall protection: Where a skylight is installed in a roof that workers or occupants may access, the glazing must either support the design load or be protected by a guard. AS/NZS 1657 covers fixed platforms and walkways adjacent to roof penetrations. The NCC references these standards in the deemed-to-satisfy provisions. If the roof is trafficable by design (a maintenance walkway, a terrace, or a green roof), the skylight assembly needs to be specified and certified for that load case by a structural engineer.
For most residential installations on a non-trafficable pitched roof, the primary compliance obligation is correct glazing specification and a watertight installation to the manufacturer's instructions.
Ordering as a System
The practical takeaway is to order the unit, the flashing kit and the shaft lining materials together, after confirming the roof profile, pitch and climate zone. A flashing kit that arrives after the sheeting is laid, or that turns out to be the wrong profile, costs more in labour to rectify than the price difference between getting it right the first time.
Construction Supplies Australia stocks skylights alongside the sheeting profiles, sarking and insulation that the shaft installation requires. The trade desk can confirm flashing kit compatibility with the specific Lysaght, Stramit or BlueScope profile on your project. Visit constructionsupplies.group/au or contact the trade desk to put together a complete order.