Optimising fire resistance in timber for interior applications

Optimising fire resistance in timber for interior applications

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photographs: Adrien William

Timber is conventionally thought of as a material best suited for use in structural applications or as external cladding. However, the recent popularity of the ‘Scandinavian’ (or ‘natural’) aesthetic has seen an increasing use of timber in decorative or architectural applications. The most popular examples of this are the decorative use of cross-laminate timber (CLT), and the use of timber panels as wall or ceiling linings.

Before specifying timber for interior applications, a number of factors must be considered. Crucially, timber that is used as a wall or ceiling lining is subject to a number of fire-resistance standards which, while different to the standards for structural or exterior use, are just as stringent. While fire performance for structural or exterior timber is oriented toward resistance to heat sources and preventing the spread of fire along the building envelope, fire performance for interior timber has a different focus.

Timber used as a wall or ceiling lining must limit the internal spread of fire and inhibit the production of smoke. Performance in this regard is measured as a combination of three key indices: resistance to ignition, ability to limit or stop the spread of flames, and minimisation of smoke production.

Why Timber Burns

When timbers are exposed to temperatures above 150 degrees Celsius, the cellulose structures start to decompose and release volatile, flammable gases. At 250 degrees Celsius, the accumulation of these gases will ignite in the presence of a spark or flame. When temperatures of 500 degrees Celsius are reached, they may self-ignite. If timber is carefully heated to release most of the volatile gases without igniting the timber, the result is charcoal.

Resistance to Ignition

As with timber used in structural or exterior applications, resistance to ignition is a central requirement of timber used as an internal wall or ceiling lining. This is understandable, as timber with a strong flame resistance can make a critical difference when it comes to preventing the spread of fire that is not caused externally (for example, a bushfire or electrical fault), but rather is caused by flames or heat inside a space.

The resistance of a material to ignition is assessed using the ISO 9705 full-scale room test. Results are divided into four ‘Groups’, with Group 1 being the highest performing classification and Group 4 the poorest. Timber generally achieves a Group 3 rating, which can be enhanced through the use of certain surface treatments and finishes.

The BCA (Building and Construction Authority) states that materials used as a finish, surface lining, or attachment to a wall or ceiling must have a fire resistance rating of Group 1-3. Group 4 materials are deemed unsuitable for these uses. The type of building and use of any other fire prevention measures – such as sprinklers or fire curtains – affects the required minimum fire resistance rating. Simply put, public crowded spaces and egress paths require higher ratings, while lower risk areas such as offices and well-sprinklered areas have lower requirements.

When determining the fire requirements of any building, the building class must first be identified. Part A3 of the BCA categorises buildings according to use/risk. Buildings are divided into classes 1-10. Certain of these classes have additional requirements. For example, schools are considered class 9b buildings and are – in most instances – not sprinklered. As per Table 3 of spec C1.10-4, these buildings would require Group 1 interior linings to fire isolated exits and fire control rooms. All public corridors would require Group 1 or 2 linings to walls and ceilings. Since untreated timber products only achieve Group 3 or 4 ratings, their use in schools is greatly restricted unless they are properly treated.

Ability to Limit or Stop Spread of Flame

Once a fire has already started spreading, the next line of defence is to contain the flames. As much as possible, timber used for wall and ceiling linings should prevent flames without sustaining significant damage.

Fire Resistance Level (FRL) is tested in accordance with AS 1530.4 – Methods for fire tests on building materials, components and structures. This test measures the ability of a material to contain live fire based on three criteria: structural adequacy, integrity, and insulation. Results are given in terms based on how long a material can resistant flames during a standard fire test before failing. These results are measured in minutes.

For example, a material with an FRL of 60/60/30 can be expected to withstand fire for 60 minutes before its structural adequacy and insulation are compromised, though its insulation capabilities will begin to deteriorate after only 30 minutes. The higher the FRL, the better.

The Spread of Flame Index (SFI) is a measure from 0 to 10 that determines how quickly flames grow in a room lined with a particular material. The lower the index, the better. For example, an SFI of 0 indicates a non-combustible surface, while an SFI of 10 means that flames would reach the ceiling within 10 seconds of ignition. Materials used in class 2 to class 9 buildings must not have an SFI of more than 9. In their untreated forms, most timbers commonly used as wall and ceiling linings (including Radiata, Hoop, and Slash Pine) have a Spread of Flame Index of 7 or 8.

Minimising Smoke Production

It is important that materials used to line walls and ceilings minimise the production of smoke, which creates an additional, significant risk during a fire. This is particularly true within enclosed spaces where wayfinding or the location of emergency exits may be difficult.

Smoke Developed Index (SDI) measures the visual density of smoke, which is toxic and can greatly reduce visibility for those exiting a building. Like SFI, it is measured on a scale of 0 to 10. An SDI of 0 indicates the best possible performance, while a rating of 10 is very poor.

courtesy: BOSS Fire & Safety, Australia