At first glance, the 2022 regulations look to have changed considerably in NCC 2025. It certainly looks a lot more complex – as 10.8.3 has been expanded into a new sub section – 10.8.4. This article examines what has actually changed and what this means to builders, contractors and architects.
Whilst the regulations are more detailed, much of this comes from expanded explanatory information and diagrams. Additionally, the principles have not changed between the 2022 and 2025 regulations. The focus on providing sufficient ventilation to reduce condensation in areas prone to moisture issues remains intact.
Calculations have been simplified in that there is no separate category for roofs with a pitch of between 10 and 15 degrees. Now the two pitch ranges are under 10 degrees and 10 to 75 degrees. This more streamlined approach appears to have been well embraced by the industry.
References to cathedral and non-cathedral roofs have been removed and replaced with roof spaces that either have the primary insulation layer parallel (less common in practice) or not parallel (more common) to the roof. Subsection 10.8.3 has been separated into 10.8.3 and 10.8.4. 10.8.3 covers roofs with non-parallel insulation layers and 10.8.4 covers roofs with parallel insulation layers.
There is an additional exemption for both 10.8.3 and 10.8.4 – buildings with a tiled roof but no additional control layer located above the primary insulation layer do not need to comply with this regulation.
10.8.3 will be the more commonly used regulation for venting the roof space. It adds some detail by differentiating between skillion and non-skillion (gable) roofs. Non-skillion roofs under a 10 degree pitch do not require high level ventilation whereas skillion roofs do. Additionally, there is a new category for roofs under 10m2 to simplify calculations and not require excess ventilation in smaller areas. 10.8.3 explanatory information provides additional detail on 1.) the reason for requiring evenly distributed openings, 2.) confirmation that structural openings can assist in meeting ventilation requirements and 3.) that use of meshes are allowable as long as they do not obstruct the main purposes of ventilation and drainage. Additionally 3 diagrams are included – one with a roof just requiring low level ventilation, one requiring low and high level ventilation – and one multi-pitched roof. There is also additional clarification on how close to the top of the roof the high level ventilation system needs to be positioned.
| Category | Detail |
|---|---|
| Scope | 10.8.3 will be the more commonly used regulation for venting the roof space |
| Roof type differentiation | Adds detail by distinguishing between skillion and non-skillion (gable) roofs |
| Non-skillion (gable) roofs | Under 10° pitch — high level ventilation not required |
| Skillion roofs | High level ventilation required, regardless of the under-10° pitch exemption |
| Small roof exemption | New category for roofs under 10m² — simplifies calculations and removes need for excess ventilation in smaller areas |
Subsection 10.8.4 applies to roofs where the primary insulation layer is parallel to the roof plane (previously referred to as a cathedral ceiling). This subsection also includes special provisions for roofs under 10m2 which are effectively the same as 10.8.3. For these roofs above 10m2, the high level requirements are the same but the requirements for venting at the eave have effectively been reduced from 25,000m2/m to 20,000m2/m.
Another change is that for roofs needing to comply with 10.8.4, any control layer installed above the primary insulation layer must have a minimum vapour permeance of 1.14ug/N.s. This wasn’t previously specified in the 2022 regulations and seems like a logical addition in these tighter roof cavities.
In summary, the changes regarding ventilating the roof space aren’t as significant as they initially appeared to be. It is certainly easier for smaller roof sections – and more explanatory notes assists in providing practical installation advice.