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22 February 2018

Understanding Different New Zealand Fire Ratings

Matthew Hughes Timber Fire Ratings

For as long as we have stood upright, we have built with timber. It’s light, strong, easily worked and beautiful. With advent of modern fire design, the use of timber has slowly declined. Even the use of timber architectural panelling and acoustic products has become difficult or even impossible in certain situations.

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

With the move towards sustainable construction, materials that support local economy, and a softer, more natural aesthetic are growing in favour: timber is back! Glulam, Cross Laminated Timber (CLT) and Laminated Veneer Lumber (LVL) are rapidly increasing in popularity for all the right reasons. They’re fast, green and affordable.

There are many considerations for anyone specifying timber for structure, linings or claddings. Appearance, structural capacity, acoustics and obviously the fire ratings required. Specifiers should understand there are multiple different types of fire ratings, and most buildings are required to meet at least one of these.

Fire Ratings

Group Ratings of Interior Ceiling and Wall Linings

Group rating of interior ceiling and wall linings is effectively the time it takes for flame to spread over a surface and 'flashover'. This is expressed as Group 1-4, with Group 1 surfaces typically consisting of non-combustible products such as concrete or plasterboard, while most standard timbers would be considered Group 3. In simple terms, public crowd spaces and egress paths require higher group ratings, and lower risk areas such as offices and well sprinklered areas have a lesser requirement. The addition of 'S' to a group rating, requires minimal smoke offput too.

Fire Resistance Rating (FRR)

Fire Resistance Ratings (FRR) relate to structural elements and separations. These ratings are expressed in minutes of resistance across three different factors: Capacity / Integrity / Insulation. For example, an inter-tenancy wall will have a requirement of 60/30/30. This wall must maintain structure capacity for 60 minutes, integrity to prevent smoke and flame passing through for 30 minutes, and insulation to prevent the temperature of the opposing side increasing more than 140°C for 30 minutes. If the wall was non-structural this may be written as -/30/30. Structural beams and columns cannot offer integrity or insulation, so would simply require a 60-minute rating.

Exterior Cladding Requirements

Commercial buildings taller than seven metres and residential buildings greater than 10 metres are required to meet cladding combustibility requirements. If either of these building types are within one metre of a boundary, all claddings on all levels must be non-combustible or achieve the 'Type A' cladding requirements. If more than one metre and even up to 30 metres from a boundary, you will still need to meet the 'Type B' requirements. Ultimately this means that timber air barriers, battens and claddings cannot be used without an appropriate coating.

We love timber, talking about timber, and finding solutions to allow its use in more situations. For design advice and solutions for your projects, myself and the Fireshield team are here to help.

View more information on Fireshield Fire Protection Coatings, including contact details.
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