Timber Cladding for Buildings in Bushfire Areas20 May 2020
The devastation of the 2019 bushfires right across Australia was beyond comprehension and has raised questions of where and how to build or re-build to minimise risk. We've put this article together using key points that relate to our products from other information sources to help with the use of external timber cladding in bushfire rated propeties.
BAL – Bushfire Attack Level
Bushfire Attack Level (BAL) is a means of measuring the severity of a building's potential exposure to ember attack, radiant heat, and direct flame contact. It is measured in increments of radiant heat. Determining the level of risk for a property requires an assessment from an accredited BAL assessor in your local area. Plans, material specifications, construction documents and building methods need to be in accord with the bush fire attack level listed in that assessment.
The 6 Bushfire Attack Level Ratings:
BAL–LOW: Standard construction materials and methods, including timber framing and cladding materials can be used. These sites have no special requirements as there is such a low risk of bushfire attack.
BAL–12.5: Some possibility of ember attack has been identified from looking at the closeness of vegetation, the site itself and local conditions with construction elements expected to be exposed to a radiant heat flux not greater than 12.5 kilowatts per square metre (kW/m2).
BAL–19: Sites identified as having an increasing level of predicted ember attack and burning debris ignited by wind-borne embers together with an increasing radiant heat flux, not greater than 19 kW/m2.
BAL–29: Increasing level of chance of ember attack and burning debris ignited by wind-borne embers, together with an increasing radiant heat flux not greater than 29 kW/m2.
BAL–40: Further possibility of ember attack and burning debris ignited by wind-borne embers; together with an increasing radiant heat flux, not greater than 40 kW/m2, and an increased likelihood of exposure to bushfire flames.
BAL–FZ: Extremely bushfire-prone, probably in a picturesque bushland setting, a home with this BAL has a predicted direct exposure risk to flames from a fire front, ember attack and a radiant heat flux greater than 40 kW/m2.
This information on the six levels is based on Appendix G of AS 3959. This information is sourced from Wood Solutions Guide #04, link to this document is at the end of this article.
Below: Trendplank Spotted Gum Timber Cladding in a Bushfire Rated Area
What timbers can I use in BAL rated areas?
BAL LOW – Burnt Ash, Pacific Teak, Spotted Gum, Ironbark, Blackbutt, Jarrah
BAL 12.5 – Pacific Teak, Spotted Gum, Ironbark, Blackbutt, Jarrah
BAL 19 – Pacific Teak, Spotted Gum, Ironbark, Blackbutt, Jarrah
BAL 29 – Spotted Gum, Ironbark, Blackbutt
The above timbers can be used for higher ratings using products like Boral’s OutRWall. Boral state that any cladding, including timber, can be used on their system. The fire-grade plasterboard provides the fire protection and the timber cladding protects the plasterboard from external weather (i.e. rain, UV).
Using fire retardant lining systems like the OutRWall means timbers like Burnt Ash with low BAL rating can be used for BAL 12.5, 19, 29, 40 and FZ.
For BAL-FZ, the wall system needs to achieve a FRL 30/30/30 – the Boral system achieves twice the performance at FRL 60/60/60. This method needs careful attention to the detail of eaves and behind the fascia to also include fire-grade plasterboard and then an appropriate flame zone roof. (See diagram below) We recommend contacting Boral directly to ensure the correct use of their product.
Our Trendplank Timber Cladding and Shou Sugi Ban range is a suitable product for use in bushfire rated areas, further information can be found at the following links:
- Trendplank Timber Cladding - https://www.mortlock.com.au/product/trendplank/
- Shou Sugi Ban Timber Cladding (Charred Timber) - https://www.mortlock.com.au/product/shou-sugi-ban/
The cost to build in BAL rated areas.
An interesting study on this subject undertaken by Julie de Jong, Director of H+H Architects and Kathryn Kinnear, Director of Bio Diverse Solutions has sought to address the misinformation and confusion about the cost of building bushfire-resistant houses.
It confirms that building to AS3959 and BAL is good building practice; it prioritises resilience, durability, building performance and site-responsive design.
While this study is related to building in West Australia, it remains relevant for the rest of Australia, this article was published in FIRE Australia Issue 2, May 2020, you can download a copy of this at: VIEW ARTICLE HERE>
Further information can be found at the following links:
- Wood Solutions Guide #04 – https://www.mortlock.com.au/wp-content/uploads/2020/05/Wood-Solutions-Building-with-Timber-in-Bushfire-prone-Areas.pdf
- Boral OutRWall® -https://www.usgboral.com/content/dam/USGBoral/Australia/Website/Documents/English/tech-guide/04685_T_OutRwall_Oct09.pdf
- Forest & Wood Products Australia Limited
The Australian Standard AS 3959–2018 Construction of buildings in bushfire-prone areas and Wood Solutions Building with Timber in Bushfire-prone Areas Technical Design Guide provide an extensive guide to help architects, designers, builders and owners to understand the Standard and what is required for each of the Bushfire Attack Level (BAL) areas.