Roof Ventilation

Achieving calculated, continuous and unimpeded airflow through the roof cavity is relatively straight forward. Whilst it is possible to retrofit passive ventilation solutions to existing buildings, it is highly recommended to incorporate passive ventilation during the design phase of any new building project.

Recommended airflows of either 10,000mm² per linear metre (equivalent to 1/300) or 25,000mm² per linear metre (equivalent to 1/150) are determined by the pitch or the design of the roof and can be easily achieved using recommended product combinations.

Recommended airflow is determined according to the pitch and the type of roof. Whilst Vent Systems recommendations are based on BS5250, Australian National Construction Code, Australian Standards and local State guidance is leading when considering what volume of ventilation is considered in the design of the roof.

Recommended airflows below indicate airflow rate requirements as per the NCC and CBOS (Tasmania):

National Construction Code Vol 2, Part 3.8.7.4.b requires:

  • Traditional Roof Pitches >22° require a calculated airflow of 10,000mm² per linear metre or 1/300:
    • Product combination – G1200N and G502
  • Traditional Roof Pitches >30° require a calculated airflow of 10,000mm² per linear metre or 1/300:
    • Product combination – G1200N, G502 and RV10P/DT
  • Traditional Roof Pitches <22° require a calculated airflow of 25,000mm² per linear metre or 1/150:
    • Product combination – G2500N, G502
  • Cathedral Roof Pitches any degree pitch require a calculated airflow of 25,000mm² per linear metre or 1/150:
    • Product combination – G2500N, VB20 and RV10P/DT (Pitched) or AB20/DT (Mono Pitched)

Control of Condensation and Mould in Tasmanian Homes (CBOS vs 2):

  • Traditional Roof Pitches >16° require a calculated airflow of 10,000mm² per linear metre or 1/300:
    • Product combination – G1200N and G502
  • Traditional Roof Pitches >30° require a calculated airflow of 10,000mm² per linear metre or 1/300:
    • Product combination – G1200N, G502 and RV10P/DT
  • Traditional Roof Pitches <16° require a calculated airflow of 25,000mm² per linear metre or 1/150:
    • Product combination – G2500N, G502
  • Catherdral Roof Pitches  any degree pitch require a calculated airflow of 25,000mm² per linear metre or 1/150:
    • Product combination – G2500N, VB20 and RV10P/DT (Pitched) or AB20/DT (Mono Pitched)

When considering passive ventilation consideration should be given to other Australian Standards for example AS3959 for buildings in bushfire zones.

Primary roof ventilators fit externally within the eaves, usually within either the soffit board or fascia board, and provide an appropriately sized opening for air to flow into and out of the roof space.

They incorporate rows of slots or holes which are large enough to allow the required volume of air to flow freely without danger of blockage by dirt, dust, cobwebs etc but are small enough to prevent the entry of vermin or large nest-building insects such as bees and wasps. All ventilators have 3.5 – 4mm wide slots or holes in accordance with recommendations.

Secondary roof ventilators fit higher up within the eaves, usually between the rafters above the wall plate position. They hold down the quilt of roof insulation material and prevent it from restricting the flow of air to and from the primary roof ventilator.

In most cases it is essential that both primary and secondary roof ventilators are used in conjunction with each other in order to provide a reliable ventilation system.

Products distributed by TCL Hoffman, available through:

Mitre 10