BUSHFIRES & CONSTRUCTION
OVERVIEW
In this post, I’ll discuss what I learned about planning a build and what is required in terms of Bushfire protection and assessments when submitting a Development Application (DA).
BACKGROUND
Our smart shipping container homes that we are building are going on a vacant block of land that we recently purchased in the Blue Mountains, NSW, Australia. The land is in what is known as a Bushfire Prone Area. In 2019 the Australian Bushfires were so bad that they made global headlines. Sadly, 33 people died and over 17 million hectares were burnt! (for scale the whole of the United Kingdom is 24 million hectares)
This means that as we plan our build we must take bushfire precautions and build to the relevant bushfire codes. In order to understand these requirements, a bushfire assessment needs to be completed to determine the Bushfire Attack Level (BAL) from each of the four sides of the dwellings (North, South, East and West).
BUSHFIRE ATTACK LEVELS (BAL)
In NSW, there are 5 different BAL’s which are denoted by their heat flux (flow of heat energy per m2). The image below outlines these five levels — which most intense, direct fire exposure to the left and decreasing to the right.
A BAL rating assessment must be made for all four sides (North, South, East and West) of each dwelling proposed to be built. To calculate the BAL the following factors must be considered: vegetation type, the distance of the vegetation from the dwelling and the slope of the land.
VEGETATION TYPES
The NSW Rural Fire Service (RFS) has a Single Dwelling Application Kit which defines the 5 different vegetation types as shown below in order of impact:
So we need to take each of four directions (N,S,E,W) and determine the vegetation type and distance from the dwelling. This can be quite tricky as more often than not there will be a variety of vegetation types. The guidelines are to use the worst-case scenario vegetation type.
SLOPE OF THE LAND
The slope of the land is important because fires like to travel uphill (heat rises) more than down. Therefore if your dwelling is high up on a hilltop with a forest below you are at higher risk from Bushfire attack
Thee slope refers to where the vegetation is in relation to the dwelling — which often causes confusion for some people.
e.g. Downslope — means the vegetation is down the slope (not good)
So, the worst-case scenario BAL rating (BAL-FZ) would involve a Forest of trees surrounding, in close proximity and downslope of the dwelling.
Best case scenario (No BAL rating) would be Managed Land (lawns) for 100m or more around the property on all four sides on flat ground.
FOREST FIRE DANGER INDEX — FFDI
Finally, we need to use the Forest Fire Danger Index that has been determined for our overall area. This list is available by council or city and currently is either 80 or 100 — the latter being more dangerous.
For our block of land, the FFDI is 80 — a good start for us!
CALCULATING A BAL RATING
Once we have our FFDI (of 80) we are then able to look-up the BAL rating for each direction of our dwelling using the tables provided (see below) There are two sets of tables one for FFDI of 80 and one for FFDI of 100.
Follow these steps for each direction (N,S,E,W):
- Select the correct FDI table (Table 4)
- Identify the land slope (first blue column)
- Using that row, select the vegetation types and distances
- From the range of BAL’s obtained (for different vegetation types) select the highest category (worse case)
- Repeat for the other sides
ASSESSING OUR DWELLING’S BAL RATINGS
I utilised my drone aerial photographs and the DroneDeploy platform (see my post here) to determine the 100m distances in all four directions as shown below. As you can see there is a variety of vegetation ranging from a small number of trees (not a Forest) and a lot of Managed Land.
After spending a while trying to determine what vegetation type was correct, I decided that it would be better (and safer) to get the whole property assessed by a qualified Bushfire Assessor. This should not only give me a professional, expert appraisal but should remove any doubt in the planner's minds about bushfire risk and whether they would need to review my self-assessment.
Scott Jarvis at Sydney Bushfire Consultants came highly recommend by my planning/building design guy. Scott holds qualifications in residential construction, building surveying, design in bushfire-prone areas and firefighting management/operations. Employed as a professional firefighter with 14 years experience in firefighting operations and management, supplemented by 8 years of practical experience in the residential building construction industry,
I don’t think you can get much better experience and qualifications than that and so I engaged Scott to do a site visit and full bushfire assessment of our property and the two potential dwelling sites we had chosen.
BUSHFIRE REPORT FOR OUR PROPERTY — BY A PROFESSIONAL
Firstly, I have to say Scott was great! He was very easy to work with and turned around the report in a very short period of time. The final report was very comprehensive and totalled 27 pages including, site photos, assessment process, zone protection, recommendations and other critical information.
Below you can see the area studied and how it was mapped for each dwelling.
A great level of detail is contained within the report but to summarise the key BAL ratings for each side of each property are defined in the tables below. As you can see the primary dwelling is rated at BAL 12.5 and the secondary would be BAL 19.
Construction Requirements
Given that both dwellings have BAL ratings the following levels of construction must be followed as per AS 3959:2018 Construction of buildings in bushfire-prone areas;
Primary Dwelling — All Elevations
Construction for Bushfire Attack Level 12.5 (BAL — 12.5) — Section 5 (AS 3959:2018)
Secondary Dwelling — North, East & West Elevations
Construction for Bushfire Attack Level 19 (BAL — 19) — Section 6 (AS 3959:2018)
Secondary Dwelling — South Elevation
Construction for Bushfire Attack Level 12.5 (BAL — 12.5) — Section 5 (AS 3959:2018)
I’m yet to fully understand the details of these construction requirements but they do include bushfire rated windows and doors and determine the allowed external exposure of wooden materials etc.
Summary
It’s important to stay safe and build to code when living in a bushfire prone area. Construction requirements are only part of staying safe — on-going land management and an up to date bushfire plan is essential.
It can be quite complex to do self-assessments if the slope and vegetation vary across your property. I would highly recommend getting a bushfire consultant (like Scott) in to do your assessment — any costs involved will likely be recouped through the DA and build process should you self-assess in-correctly.
I will follow up in another article with more details on the construction impacts of the BAL ratings and what our bushfire plan looks like.
Stay safe!
For more shipping container build check out my blog site here -creationstation.build
