How to select a fire rated wall or ceiling
One of the most common questions I get asked is “how can one system have two different Fire Resistance Levels (FRLs) and what does each one mean?” For example a wall system can have -/90/90 and 60/60/60. Even for architects, understanding the requirements of the Building Code Australia (BCA), can be confusing. But when it comes to selecting fire rated systems, including fire rated walls and fire rated ceilings, there is some key terminology you need to know. For example, Fire Resistance Level or Fire Rating? What is an FRL and why are there three numbers? What are Fire Hazard Properties? What is Insipient Spread of Flame? What is the difference between loadbearing and non-load bearing, and how does this relate to fire resistance? In this article we explain what each one means and how it relates to selecting a wall or ceiling system.
Usually a building surveyor or certifier will specify the required FRL. It’s handy to know what they are asking for and have an understanding of the appropriate system. Then it’s a matter of selecting a system from the large range available.
1. FRL (Fire Resistance Level)
Although commonly referred to as fire rating, the correct term to describe the fire resistance of a building element is FRL (Fire Resistance Level). The FRL is the ability of a building element to withstand a fire under test conditions for a certain period of time and consists of the three criteria listed below:
- structural adequacy
Figure 1 below shows an FRL of 60/60/60. This means that if a building element were exposed to a standard fire test, it would not be expected to fail for 60 minutes in each of the three criteria. The BCA specifies FRLs for building elements such as walls, columns, roofs and floors. These FRLs can be many combinations of the three criteria, eg. 90/-/-, 90/60/30 or -/60/60. A dash in the FRL means there is no requirement for that criterion.
There are many ways in which an FRL can be determined. They are listed in BCA Specification A2.3 and include the following:
- A standard fire test in accordance with AS 1530.4 Methods for fire tests on building materials, components and structures
- Australian Standards for steel, timber, concrete or masonry structures
- Table 1 within BCA Specification A2.3
- A Registered Testing Authority report which is based on a fire test
- Calculation based on a fire test
2. Fire Hazard Properties
FRLs are about how well a system of components contains a fire. Fire hazard properties on the other hand refer to a material’s properties, such as how easily it burns.
One way of reducing the effects of a fire is to use materials on the walls, floor and ceiling that don’t encourage smoke and fire. Fire trying to spread along a concrete corridor would do so more easily if it were lined with flammable materials. Therefore the BCA regulates the fire hazard properties of coverings and lining materials in buildings according to BCA Specification C1.10.
Floor linings and coverings must have a high enough critical radiant flux to comply with BCA Specification C1.10, while wall and ceiling linings must have a low enough group number. The group number indicates how quickly wall and ceiling linings spread fire, with Group 1 products ranked the slowest and Group 4 the fastest.
3. Resistance to Incipient Spread of Fire (RISF)
Floor/ceiling systems or roof systems with an FRL include the roof lining or flooring as part of the fire barrier. Ceilings with a Resistance to the Incipient Spread of Fire state the ability of a ceiling to limit the temperature rise in the ceiling cavity [shown below]. Ceilings with 60 minutes of RISF are specified by the BCA and are appropriate where the ceiling is the primary fire barrier that limits fire spread by way of the ceiling void.
An example of where this type of ceiling is required is a Class 2 residential building which has several apartments on the top floor. The apartments have fire resisting separating walls between them which finish at the ceiling under a shared roof space. The RISF 60 ceiling prevents fire spread from one apartment, up into the roof space and then down into a neighbouring apartment.
4. Loadbearing or non-load bearing?
Is it 60/60/60 or -/60/60? If a building element is load bearing then it must have a Structural Adequacy component to the FRL.
The definition of loadbearing from the BCA: ‘intended to resist vertical forces additional to those due to its own weight’. Therefore walls such as those holding up a floor or roof above are loadbearing. While (in general) walls that span between concrete slabs and are not holding up the slab, are considered non-loadbearing. The BCA deemed to satisfy provisions specify FRLs based on whether the building element is load bearing or not (Refer to BCA Specification C1.1). For example, walls separating sole occupancy units in a Class 2 building of Type A construction (residential high rise) need an FRL of -/60/60 if they are non-load bearing and 90/90/90 if they are load bearing. Residential high rise buildings are usually slab to slab construction in which case the concrete columns are load bearing but the plasterboard infill walls are not.
If an FRL with Structural Adequacy is specified (e.g. 90/90/90) where there is no additional vertical load, a building element without Structural Adequacy may be used (e.g. -/90/90). Refer to BCA Vol. 1 Specification A2.3 (6).
Selecting a system
Fire resistance is a big subject but hopefully now a little better understood. If you’d like even more detail about these terms or fire rating requirements, they’re defined in the BCA which is now part of the National Construction Code; free to download after registration at http://services.abcb.gov.au/NCCOnline/
Once you have the fire resistance requirements from the plans, building surveyor or building certifier, you can select the right system with a little more insight. Do you need structural adequacy or not? Does your ceiling need an FRL of 06/60/60 or an RISF of 60?
Double checking the fire requirements is possible by reading the BCA, although this can be complicated and is best left to the building surveyor or certifier. Well-presented information on FRL requirements specific to timber construction are found in the Wood Solutions design manuals.
And of course, don’t forget to consult the Knauf Technical Manual for plasterboard systems and the online Knauf system selector or contact Knauf either on 1300 724 505 or through the contact form and get project specific advice from one of our expert engineers.
Read more interesting facts about acoustics and design at the Sound of Architecture and download our free e-books on acoustic design for health & education.