Our Adelaide terrace design has three skylights in a roof that runs to within a metre of the party wall, and the certifier has raised fire separation. How close to a boundary or separating wall can a roof light sit, and how much of the roof can they take up?

The certifier is right to raise it. A combustible skylight must sit at least 900 mm from the vertical projection of the party wall, and the skylights together can take up no more than 20% of the roof or part of the roof they sit in. [1] "Within a metre" is right on the line: at 1 m you have 100 mm of margin, and anything under 900 mm fails. [1] [3]

Two clauses, two different lines to measure from. On a terrace these get blurred, but they are separate tests:

• 9.3.3 Roof lights (separating walls). A combustible roof light must be not less than 900 mm from the vertical projection of a separating wall extending to the underside of the roof covering. [1] Measure to the line where the party wall would continue up through the roof, not to the edge of the wall's footing or the gutter.
• 9.2.10 Roof lights (allotment boundaries). A combustible roof light must be not less than 900 mm from the allotment boundary, other than a boundary adjoining a road alignment or other public space. [2] If your party wall also sits on the title boundary, both tests bite, and both land on 900 mm.

The figure for 9.2.10 shows the two side by side: roof lights located not less than 900 mm from the allotment boundary or separating wall. [3]

How much of the roof they can take. Under both clauses the roof lights must have an aggregate area not more than 20% of the roof or part of the roof. [1] [2] That is the combined area of all three skylights, not each one on its own.

The 1.8 m rule is not about your three skylights. A common misreading is that skylights must be 1.8 m apart. The 1.8 m distance in 9.2.10 applies to a roof light in another building on the allotment, other than an associated building or a detached part of the same building. [2] It does not set spacing between skylights in the same roof.

When these limits apply. Both clauses are written for combustible roof lights, skylights or the like installed in a roof or part of a roof required to have a non-combustible covering. [1] [2] The figure notes confirm the roof lights it shows are combustible. [3] That trigger is what switches the limits on, so ask the certifier to confirm in writing that your roof, near the party wall, is one required to have a non-combustible covering, and what the skylight product is made of. Do not assume a different product gets you out of the rule without that confirmation.

A separate test from the energy side. Roof lights serving a habitable room also face an energy limit: a total area of not more than 5% of the floor area of the room or space served, plus thermal performance requirements. [4] That is measured against the room's floor, not the roof, so a design can pass the 20% fire cap and still fail the 5% energy cap. Check both.

What to do with the design. Get a roof plan showing the vertical projection of the party wall and the title boundary, dimension each skylight's nearest edge to both, and total the skylight area against the roof or part of the roof it sits in. If any skylight is inside 900 mm, it has to move or change.

SA edition note. SA building work is on NCC 2022 until 30 April 2027, with SA variations in Ministerial Building Standard MBS 007. No MBS 007 modification to these roof light clauses was found, so confirm with your certifier before relying on the national figures alone. NCC 2025 applies to SA building work from 1 May 2027 and carries the same 900 mm, 1.8 m and 20% figures, so the answer does not change when the edition moves. [5]