Flat Roof-NCC 2022 Hygrothermal Design and Installation of Timber Roofs

Introduction

Long run metal roofing in Australia and New Zealand if often built at 2- or 3-degrees pitch. Although the Australian Building Code, Australian Standards, the New Zealand Building Code, and the New Zealand Standards allow this to happen. The question is if it is actually feasible to get a high-performance roof system at these low pitches. 

Low pitch roofing must firstly drain the rain, secondly drain any condensate, keep the heat out, and keep the heat in, all while remaining free from mould.

Market drivers, planning laws and architectural aesthetics drive the desire for low pitch roofing. As we move to next zero buildings it is essential that we lock down the airflow in these roof systems to retain heat in and block heat out.

As we build more airtight skillion and cathedral roofs we can no longer rely on volumetric airflow through that rafters to make sure “she’ll be right” from a moisture perspective. The building code is now moving to address this for better allowable solutions in the NCC 2022.

It is then essential that the mechanisms by which water gets in and water gets out is critical to the overall durability of these flat roof structures. It’s not rocket science, it’s building science.

At the end of this presentation, attendees will:

• • Why is important – Flat roofs are prone to moisture buildup due to condensation and trapped water. This can damage the roof and create unhealthy conditions inside your home. A ventilated cavity helps prevent these problems by letting the roof “dry out.”
  • What to watch out for – Even with a cavity, condensation can still happen. To prevent this, a separate waterproof layer below the roof panels is crucial. This stops leaks and protects the insulation.
  • Why vapour diffusion and ventilated cavities are important reduce the risk of moisture related issues in timber boards.
  • Why vapour control layers are required in low pitch skillion roofs
  • Why promoting airflow as ventilation through insulation layers is no longer considered a good design practice.

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