The rules themselves
The provisions in full.
The same figures the calculator uses, written out — so the page answers with scripting off, and so you can check the calculator against the rule rather than the other way round.
10.8.3 — insulation not parallel to the roof plane
In climate zones 6, 7 and 8, a roof with the primary insulation layer not parallel to the roof plane must have a roof space that has a height of not less than 18 mm at any point between the primary insulation layer and the underside of the roof or a control layer, is located immediately above the primary insulation layer, and is ventilated to outdoor air in accordance with Table 10.8.3.
Table 10.8.3 — roof space ventilation requirements| Roof pitch | Ventilation openings |
|---|
| Less than 10° | 20,000 mm²/m provided at eaves or low level, or at each of two opposing ends for gable roofs |
| For roofs less than 10 m², 7,000 mm²/m provided at eaves or low level roof perimeter and 5,000 mm²/m provided at ridge or high level for skillion roofs |
| 20,000 mm²/m provided at the eaves or low level and 5,000 mm²/m provided at ridge or high level for skillion roofs |
| 10° or more and less than 75° | 7,000 mm²/m provided at the eaves or low level and 5,000 mm²/m provided at high level or ridge |
The table notes carry the calculation rules. Openings are a minimum free open area per metre length, and the longest total plan dimension is based on the roof length irrespective of the number of storeys beneath. Total low-level openings are calculated on twice the longest total plan dimension of the roof — except mono-pitch or skillion roofs, which use the dimension once. Total high-level ventilation is calculated on the longest total plan dimension once.
The requirements do not apply to a concrete roof, a roof made of insulated sandwich panels, a roof subject to Bushfire Attack Level FZ requirements in accordance with AS 3959, or a tiled roof without a control layer located above the primary insulation layer.
10.8.4 — insulation parallel to the roof plane
In climate zones 6, 7 and 8, a roof system with the primary insulation layer installed parallel to the roof plane — a cathedral ceiling, or an insulated flat or skillion roof — must have a roof space immediately above the primary insulation layer or any control layer immediately above it, not obstructed by insulation, with a height of not less than 18 mm measured perpendicular to the plane of the roof, ventilated to outdoor air through openings not less than: for roofs less than 10 m², 7,000 mm²/m at the eaves or low level and 5,000 mm²/m at the high level or ridge; for all other roofs, 20,000 mm²/m at the eaves or low level and 5,000 mm²/m at the high level or ridge.
Where a control layer is installed immediately above the primary insulation layer in climate zones 6, 7 and 8, it must have a vapour permeance of not less than 1.14 µg/N.s. The same four exemptions apply, with sandwich panels excluded with or without an additional ceiling lining.
What the explanatory information adds
Labelled explanatory, not operative, in the NCC itself: the section does not apply to Class 10 buildings, though an attached private garage may be included when determining the longest plan dimension; openings are to be evenly distributed to avoid pockets of stagnant air; for pitches over 10° the total low-level openings must exceed the total high-level openings, so the space draws outdoor air rather than air from the rooms below; for pitches under 10° other than skillions, cross-ventilation at opposing ends does the work and high-level ventilation is not necessary; openings formed by the roof cladding profile can count where they connect to the ventilated void and are not blocked by sarking or insulation; and vermin or ember mesh may be used where it still lets the air through.
What changed from NCC 2022
If the notation block on the plan reads like the old code, it probably is the old code. The differences that matter:
NCC 2022 to NCC 2025 changes for roof space ventilation| NCC 2022 (superseded in Victoria from 1 May 2026) | NCC 2025 (current) |
|---|
| Below 10°: 25,000 mm²/m at each of two opposing ends | Below 10°: 20,000 mm²/m at eaves or low level, or at each of two opposing ends for gable roofs; skillion cases separated out with their own rates |
| 10° to 15°: its own band — 25,000 mm²/m eaves and 5,000 mm²/m high | Band removed — 10° to 75° is one band at 7,000 and 5,000 mm²/m |
| Cathedral ceilings in the 15° to 75° band: an extra 18,000 mm²/m at the eaves | Replaced by clause 10.8.4 — 20,000 and 5,000 mm²/m (7,000 and 5,000 under 10 m²) |
| Roof space at least 20 mm | At least 18 mm (10.8.4: measured perpendicular to the roof plane) |
| Per-metre rate applied to the longest horizontal dimension | Table notes now say it plainly: low-level totals on twice the plan dimension for dual-pitched roofs, once for mono-pitch and skillion; high-level totals once |
| Unsarked tiled roof satisfied the ventilation limb | Tiled roof without a control layer above the insulation is exempt from the clause |
The rest of Part 10.8 at a glance
10.8.1 — external walls. Pliable membranes and sarking used as control layers comply with AS 4200.1 and are installed to AS 4200.2. Control layers between the cladding and the outside of the primary insulation must meet the vapour permeance in Table 10.8.1 — in climate zones 6, 7 and 8 a wall without a drained and ventilated cavity cannot comply, so most walls there need a cavity at least 12 mm deep, drained to the exterior, with openings of at least 1,000 mm²/m at the base and top. Excluded: single-skin masonry and concrete walls, walls of insulated sandwich panels for their full extent, walls outside the building envelope, and any portion of an external wall below natural ground level — a wall running from below ground to above it is excluded only for the below-ground portion.
10.8.2 — exhausts (unchanged from NCC 2022). Minimum flow rates of 25 L/s for a bathroom or sanitary compartment and 40 L/s for a kitchen or laundry; exhaust and venting clothes dryers discharge directly or by duct to outdoor air; an intermittent exhaust serving a bathroom or sanitary compartment that is not naturally ventilated is interlocked with the room's light switch with a 10 minute run-on, and rooms without natural ventilation get make-up air — an opening of 14,000 mm² to an adjacent room does it, which is a 20 mm undercut on a 700 mm door.
Every figure on this page was read against the NCC 2025 ABCB Housing Provisions, Part 10.8 (as published at ncc.abcb.gov.au, read 26 August 2026), and against the ABCB's state adoption table for the 1 May 2026 Victorian adoption date. The NCC is amended from time to time — if you are reading this well after that date, ask us to confirm the current text.
