Roofs

Preventing external roof ignition

Flame and burning debris can ignite combustible elements of a building like walls and decking. While roof cladding is required to be non-combustible in Australia, many typical roof designs include combustible eave facias, eave linings, and framing elements. These can be directly exposed to flames or the build up of burning debris on the roof or in the guttering.

Unfortunately, conventional sarking products, including those specified as compliant in AS 3959:2018 are insufficient to limit the spread of flame to underlying framing elements. Hence using non-combustible materials for all exposed elements is essential.

Install gutter protection or remove leaves and other debris from gutters and other roof spaces (source: xshot/shutterstock)

Preventing roof cavity ignition

Roofs contain many gaps to the outside environment; this allows for air and built-up moisture to flow between the roof cavity and the outside area. Unfortunately, these gaps also allow embers and debris to enter the roof space. When this happen small flame source can occur in the roof space which can either burn out or ignite adjacent materials leading to a larger fire and possible house destruction.

Efforts to limit the amount and size of embers and debris that can enter a roof cavity helps to limit the size of small flame sources. However, it is technically difficult to design and build a roof that has no gaps greater than 2mm. Wind actions, debris impact, modifications, thermal and moisture expansion, and contraction all combine to make it difficult to completely manage gaps to less than 2mm, particularly as a roof ages. Hence it is essential to use non-combustible material for all elements within the roof cavity including the framing, roof battens, stored items, ceiling and insulation. It is also essential to design any penetrations in the ceiling with gaps finer than 2mm.

Debris naturally accumulate within the roof cavities of homes over their lifespan. However, minimising the number and size of gaps and vents in the roof design can reduce the rate and extent of this accumulation. The risk posed by the accumulation of debris can also be reduced by constructing the roof cavity using non-combustible materials.

Complex roof designs

The more complex a roof is the more opportunity for debris to build up in the valley lines and complex details. If the recommendations of construction a fully non-combustible roof and roof cavity are followed then the ignition of debris built up in these is very unlikely to cause ignition of the interior of your home (unless these details are immediately adjacent to windows or skylights). However the flames from this debris may be sufficient to damage paints and finishes of adjacent non-combustible materials leading to the need to replace those material at some point in the future.

Gutter protection

If using a fully non-combustible roof and roof cavity, the ignition of debris built up in gutters is very unlikely to cause ignition of the interior of your home. However, the flames from this debris may be sufficient to damage paints and finishes of adjacent non-combustible materials leading to the need to replace those materials in the future. If using combustible fascias or roof framing, gutter design and protection becomes very important.

Gutters that are non-combustible and are entirely covered with metal mesh limits debris build up in the gutter and helps to limit the size of fire that can occur in the gutters. Using mesh with an aperture of less than 2mm and tightly fixed to the gutter line and roof further reduce the likelihood that embers may ignite any fine debris. This is also a way to reduce further the entry of embers and debris into the roof space. Noting that these approaches are far more effective than physically remove debris from the gutter before a fire, unfortunately, a fire event brings with it new debris and embers that will at least partly refill your gutters during the fire event.

Do’s

• A solid concrete-slab or earth-covered roof will provide good bushfire protection when combined with a non-combustible support structure.
• Other suitable options include a steel sheeted or tiled roof with non-combustible cavities.
• Build to a higher wind-loading code than is required at your location (aim for one or two ratings above the regulated requirement).