New Australian houses in a bushfire prone area must have a BAL rating. This is to minimise the risk of the house burning down if a bushfire occurs. The levels are defined by the amount of heat radiation the house is expected to withstand, and each level has specific construction requirements for radiation and ember attack, all the way up to flame impingement.
To get a BAL rating for your property a bushfire management consultant must be employed to assess your site, and prepare a report and management plan. BAL ratings depend on the type of vegetation, its proximity to the dwelling and the slope of the land. Our consultant recommended clearing the land to reduce heat radiation on the house to 12.5 kW/m2, but building the house to BAL 29 (29 kW/m2) standard, to offset the increased risk of not building on a directly on a road. Blog post: Bushfire rating. This is a standard approach allowed under the regulations – if all of the proscribed conditions can not be met, then additional risk mitigation measures can be proposed to compensate.
Once assessed, vegetation around the house must be managed in accordance with the BAL rating. eg you can’t plant a large tree close to your house if your BAL rating requires low or limited vegetation. The area impacted is known as the defendable space, In our case, we must have a clearing of 41m around the house, with individual tree canopies separated by at least 5 m, with no tree branches closer to the ground than 2m. Shrubs can not be larger than 5m in diameter. Other restrictions apply to vegetation near windows, and the distance trees must be from the house.
Vegetation clearance is allowed without offset or assessment to meet the BAL requirements. Our council initially tried to insist offsets were required for the additional vegetation clearance for us to achieve BAL 12.5 (41m) since we were building to BAL 29 (~30m defendable space). DELWP eventually confirmed to them that this was not required.
In some circumstances, building to a lower BAL level may be allowed if a bushfire shelter is incorporated into the development.
There are six BAL rating levels, with the specific construction requirements contained within AS3959. It should be noted that some materials are only tested/certified to be non-combustible upto specific heat flux levels, so may be able to be used in BAL 12.5 construction for example, but not BAL 29.
This is a rough guide, but certainly not exhaustive. A little more detail on BAL 29.
Note: Bushfire resisting timber – BFRT – Ash silvertop, Blackbutt, River Red Gum, Spotted Gum, Red Ironbark, Kwila (Merbau), Turpentine
BAL-Low : Risk is minimal
Usually all that is required is a 100 metre distance between the buildings and any classified vegetation around the property or 50m from grasslands. There are no specific construction requirements.
BAL-12.5: Low risk of ember attack and heat radiation from 0 – 12.5kW/m2
In Victoria, a minimum of BAL 12.5 is required for all new homes being constructed in a bushfire prone area as a precaution against burning embers.
Summary construction requirements:
| Subfloor Supports | No special construction requirements |
| Floors | No special construction requirements |
| External Walls | Parts less than 400mm above ground or decks etc to be of non-combustible material, 6mm fibre cement clad or bushfire resistant/naturally fire resistant timber (BFRT) |
| External Windows | Protected by bushfire shutter, OR completely screened with steel, bronze or aluminium mesh or 4mm Grade A safety glass or glass blocks with 400mm of ground, deck etc Openable portion metal screened with frame of metal or metal reinforced PVC-U or bushfire resisting timber |
| External Doors | Protected by bushfire shutter, or screened with steel, bronze or aluminium mesh, or glazed with 5mm toughened glass, non combustible or 35mm solid core timber for 400mm above threshold, metal or bushfire resisting timber framed for 400mm above ground, decking etc tight fitting with weather strips at base. |
| Roofs | Non-combustible covering. Roof/wall junction sealed. Openings fitted with non-combustible ember guards. Roof to be fully sarked. |
| Verandahs Decks etc | Enclosed sub-floor space – no special requirement for materials except within 400mm of ground. No special requirements for supports or framing. Decking to be non-combustible or bushfire resistant with 300mm horizontally and 400mm vertically from a glazed element. |
A Bushfire Attack Level of BAL-12.5 is not particularly difficult to achieve, and does not add much cost to the construction. Most of the requirements are something a solid construction will already follow. The key element for Bushfire Attack Level of BAL-12.5 bushfire construction is avoiding an ember attack hence the requirement of ember guards/mesh and bushfire shutters/metal screening (fly wire!) on all openings.
BAL-19: Moderate risk of ember attack with heat radiation to 19kW/m2
Construction requirement summary:
| Subfloor Supports | No special construction requirements |
| Floors | No special construction requirements |
| External Walls | Parts less than 400mm above ground or decks etc to be of non-combustible material, 6mm fibre cement clad or bushfire resistant/naturally fire resistant timber |
| External Windows | Protected by bushfire shutter, completely screened with steel, bronze or aluminium mesh or 5mm toughened glass or glass blocks with 400mm of ground, deck etc Openable portion metal screened with frame of metal or metal reinforced PVC-U or bushfire resisting timber |
| External Doors | Protected by bushfire shutter, or screened with steel, bronze or aluminium mesh or glazed with 5mm toughened glass, non combustible or 35mm solid core timber for 400mm above threshold, metal or bushfire resisting timber framed for 400mm above ground, decking etc tight fitting with weather strips at base. |
| Roofs | Non-combustible covering. Roof/wall junction sealed. Openings fitted with non-combustible ember guards. Roof to be fully sarked. |
| Verandahs Decks etc | Enclosed sub-floor space – no special requirement for materials except within 400mm of ground. No special requirements for supports or framing. Decking to be non-combustible or bushfire resistant with 300mm horizontally and 400mm vertically from a glazed element. |
BAL 19 is still easy to achieve, with only a small upgrade to BAL 12.5, highlighted.
BAL-29 : High risk of ember attack and burning debris with heat radiation to 29kW/m2
Construction Requirements summary:
| Subfloor Supports | Enclosed: slab on ground (no special requirements), enclosed by external wall or by steel, bronze or aluminium mesh. Unenclosed: non-combustible supports or naturally bushfire resistant timber (BFRT) stumps. |
| Floors | Enclosed: no special requirements if enclosed as above Unenclosed: bearers, joists and flooring less than 400mm above ground level to be non-combustible, BFRT or protected on the underside with sarking or mineral wool insulation. |
| External Walls | Non-combustible material (masonry, brick veneer, mud brick, aerated concrete, concrete), OR Timber logs, density >680kg/m3 at 12% moisture with minimum nominal thickness of90mm with minimum thickness of 70mm, OR Cladding that is externally fixed to timber framed, steel framed walls and is 6mm fibre content sheeting or steel sheeting or BFRT. Any combination allowed. All gaps must be less than 3mm. Vents and weepholes must be screened if 3mm or larger. |
| External Windows | Protected by bushfire shutter, OR Frames made from BFRT, Metal, metal reinforced PVC-U AND 5mm toughened glass minimum (only external glazing for double glazed windows) AND Completely screened with steel, bronze or aluminium mesh within 400mm of ground/deck/roof etc AND Openable portion screened. |
| External Doors | Protected by bushfire shutter, OR Screened with steel, bronze or aluminium mesh OR Non combustible or solid core, laminated or reconstituted timber, having a minimum thickness 0f 35mm for 400mm above threshold or 6mm toughened glass glazed door where frame is made from BFRT or non-combustible materials. Doors must be metal or bushfire resisting timber framed, tight fitting with weather strips at base. |
| Roofs | Non-combustible covering. Roof/wall junction sealed. Openings fitted with non-combustible ember guards. Roof to be fully sarked. |
| Verandahs Decks etc | Enclosed sub-floor space or non-combustible or bushfire resistant timber supports. Decking to be non-combustible. |
Somewhat more onerous, but nothing too disastrous. Some materials are tested to be non-combustible only up to specific heat flux levels
BAL-40: Very high risk from ember attack and burning debris with increased possibility of exposure to flames with heat radiation to 40kW/m2
Construction requirements summary:
| Subfloor Supports | If enclosed by external wall refer to external walls below, or non-combustible subfloor supports or tested for bushfire resistance to AS1530.8.1 |
| Floors | Concrete slab on ground, enclosed by external wall, or protection of underside with non-combustible material such as fibre cement sheet or be tested for bushfire resistance to AS1530.8.1 |
| External Walls | Non-combustible material (masonry, brick veneer, mud brick, aerated concrete, concrete), timber framed, steel framed walls sarked on the outside and clad with 9mm fibre content sheeting or steel sheeting or be tested for bushfire resistance to AS1530.8.1. |
| External Windows | Protected by bushfire shutter or 5mm toughened glass. Openable portion screened with steel or bronze mesh. |
| External Doors | Protected by bushfire shutter, non combustible or 35mm solid timber, metal framed tight fitting with weather strips at base. |
| Roofs | Non-combustible covering. Roof/wall junction sealed. Openings fitted with non-combustible ember guards. Roof to be fully sarked and no roof mounted evaporative coolers. |
| Verandahs Decks etc | Enclosed sub-floor space or non-combustible supports. Decking to be non-combustible. |
Getting really onerous – many products are not rated for BAL 40.
BAL-FZ: Extreme risk with direct exposure to flames and heat radiation above 40kW/m2
Construction requirements summary:
| Subfloor Supports | Enclosure by external wall or non-combustible with an FRL of 30/-/- or be tested for bushfire resistance to AS1530.8.2. |
| Floors | Concrete slab on ground, or enclosure by external wall or an FRL of 30/30/30 or protection of underside with 30 minutes incipient spread of fire system or to be tested for bushfire resistance to AS1530.8.2. |
| External Walls | Non-combustible material (masonry, brick veneer, mud brick, aerated concrete, concrete), with minimum thickness of 90mm ir an FRL of -/30/30 when tested from outside or be tested for bushfire resistance to AS1530.8.2. |
| External Windows | Protected by bushfire shutter or FRL of -/30/- and openable portion screened with steel or bronze mesh or be tested for bushfire resitance to AS1530.8.2. |
| External Doors | Protected by bushfire shutter, or tight fitting with weather strips at base. and an FRL of -/30/- |
| Roofs | Roof with FRL of 30/30/30 or tested for bushfire resistance to AS1530.8.2. Roof/wall junction sealed. Openings fitted with non-combustible ember guards. No roof mounted evaporative coolers. |
| Verandahs Decks etc | Enclosed sub-floor space or non-combustible supports. Decking to be non-combustible. and have no gaps |
You really really don’t want to go here. Mega expensive, difficult to do, and not nice to live in.
FRL is the Fire Resistance Level, with the three elements following the number of minutes the item must survive
So for FRL 30/30/30
30 minute structural adequacy – it will not collapse in 30 minutes
example)
30 minutes integrity – no deterioration, no holes in panels/joints for example
30 minutes Insulation – heat transfer from the fire side to the non-fire side. This is 180 degrees above ambient temperature
Where a – occurs this means it is not required.
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