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Understanding group numbers and non-combustibility

According to the National Construction Code, building materials used as internal linings need to comply with set fire hazard properties.

These fire hazard properties are regulated with one goal in mind: to minimise rapid fire and smoke spread in buildings to ensure the safe and timely evacuation of occupants.

In 2019, the NCC introduced tougher test requirements for fire hazard properties to not only stipulate what the fire hazard properties of materials must be, but also how these properties are to be determined by stringent fire testing.

In other words, having a report or certificate that indicates satisfactory fire hazard properties for a wall or ceiling lining may not be enough to meet the requirements of the NCC. It’s also important to check that these properties were determined by means of the tests that comply with the relevant Australian Standards.

In this blog we look at group numbers and non-combustibility, and what the relevant tests are to determine these ratings.

Important: The NCC 2022 came into effect on 1 May 2023 and while the NCC 2022 introduces key changes for fire safety, the requirements for internal linings largely remain unchanged. Read on for more details.

Q: What are group numbers?

A: According to the NCC a group number is “the number of one of four groups of materials used in the regulation of fire hazard properties and applied to materials used as a finish, surface, lining, or attachment to a wall or ceiling”.

The classification governs if and where the material can be used in the walls and ceilings of Class 2 to 9 buildings. Building materials with a low group number are required where reducing the spread of smoke and flame is critical, e.g. fire escape tunnels. (Refer to Specification C1.10 Fire hazard properties Table 3 for more information.)

In all cases the group number must be 1,2 or 3. If a material does not qualify for group 1,2 or 3, it is allocated to group 4 and cannot be used in the construction of commercial buildings in Australia.

Materials are given a group number based on the results of the specified fire tests, with group 1 reserved for the best performers (including regular gypsum plasterboard).

Q: What are the required tests to determine group numbers?

A: The NCC stipulates that the relevant wall and ceiling materials must have the group number determined in accordance with AS 5637 (AS 5637.1:2015 – Determination of fire hazard properties, Part 1: Wall and ceiling linings).

AS 5637 describes how the product must be tested by means of two possible tests. The two tests are either a full-size room burn test (ISO 9705) or a series of small-scale sample tests (AS/NZS 3837).

The data obtained in the small-scale tests can be used to predict how the lining would perform in a full-scale test and a group number can be allocated accordingly.

However, only certain well-known and predictable materials (including gypsum plasterboard and solid timber) may be tested to AS/NZS 3837, also known as the “Cone Calorimeter test”. Other materials must be tested to ISO 9705 (which is a significantly more expensive test).

Click here to download the latest Fire Hazard Properties Report for Siniat plasterboard lining products.

Q: What is meant by non-combustibility?

A: In construction, combustibility doesn’t only mean that a material will catch fire, but also refers to its ease of ignition, propensity to spread flame, flammability and smoke produced.

The BCA defines non-combustibility as follows:

  • Applied to a material: not deemed combustible as determined by AS 1530.1 – Combustibility Tests for Materials; and
  • Applied to construction or part of a building: constructed of materials that are not deemed combustible.

Non-combustible materials are required for external walls, fire-rated walls and some other building elements.

Q: What are the required tests to determine non-combustibility?

A: The non-combustibility test according to AS1530.1 is different to the two group number tests described above and includes putting five test samples into a furnace for 30 minutes. If any of the samples flame for more than 5 seconds during that time, it is deemed combustible.

According to CSIRO, the purpose of AS1530.1 is to provide a consistent and standardised test which conservatively categorises materials (primarily for the construction industry) which, although not completely inert, produce only a limited amount of heat and flame when exposed to temperatures of approximately 750°C and are expected not to burn substantially even when exposed to severe fire conditions.

However, some building materials, including plasterboard, do not have to be tested for combustibility in terms of AS 1530.1 as the BCA classifies them as deemed-to-satisfy. These materials may be used wherever a non-combustible material is required.

The full list of materials that are regarded as non-combustible under NCC 2019 Part C1.9 (e) are:

  1. Plasterboard
  2. Perforated gypsum lath with a normal paper finish.
  3. Fibrous-plaster sheet.
  4. Fibre-reinforced cement sheeting.
  5. Pre-finished metal sheeting having a combustible surface finish not exceeding 1 mm thickness and where the Spread-of-Flame Index of the product is not greater than 0.
  6. Sarking-type materials that do not exceed 1 mm in thickness and have a Flammability Index not greater than 5.
  7. Bonded laminated materials where—
    1. each lamina, including any core, is non-combustible; and
    2. each adhesive layer does not exceed 1 mm in thickness and the total thickness of the adhesive layers does not exceed 2 mm; and
    3. the Spread-of-Flame Index and the Smoke-Developed Index of the bonded laminated material as a whole do not exceed 0 and 3 respectively.

Some of these materials may still have combustible components, e.g. the liner paper of plasterboard, but the materials are well known in terms of their performance in fires and are not considered hazardous and exempt from testing to AS1530.1.

Note: The list above remains unchanged in BCA2022.

Q: What about the other elements used in an internal lining?

A: Clause C1.9(d) of the NCC2019 contains a list of other items and components excluded from the non-combustibility requirements. It includes elements used in the construction of internal linings, e.g. sealants.

This list is larger in NCC2022 (it's now Clause C2D10(4)) and includes many useful additions such as “isolated blocking for fixing fixtures”. This includes plywood or timber noggings used for attaching plumbing services within a wall.

NCC2022 provides further clarification of what is meant under “isolated”, stating that the blocking of an entire wall is not considered an isolated situation whereas isolated fixing and fixing accessories such as screws, anchors, wall plugs, nails and washers are.

This is a welcome addition to the Code as the use of timber noggings in fire-rated walls has led to many questions and confusion under NCC2019, seeing that timber is classified as a combustible material.

See the diagrams from the Siniat Blueprint below for illustration of where timber noggings are typically used:

Nogging 2Nogging

Q: Where do fire-resistant plasterboard and FRLs come into the picture?

A: All regular gypsum plasterboard is deemed non-combustible and allocated group number 1, but fire-resistant plasterboard such as Siniat Fireshield contains extra additives to make them even more fire resistant to help meet the BCA requirements for building elements or systems (walls, ceilings, column protections, ducts) where a Fire Resistance Level (FRL) is required. 

Remember that fire hazard properties (including group numbers and combustibility) describe materials. An FRL, on the other hand, is not a material property but a system property (e.g. a wall or ceiling system). The FRL describes how well the complete system (including all of its components) performs and behaves in a fire.

In simple terms an FRL describes how long the system will remain structurally adequate (not collapse), restrict the passage of fire or hot gasses (integrity) and prevent heat from passing from the fire side to the non-exposed side (insulation).

Read more in our blog How to select a fire-rated wall or ceiling

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. To find this yourself,  go here  to use the BCA 2019 Requirements Finder and then use the Siniat System Selector.

For more technical and project specific advice on fire hazard properties and FRL’s, contact Siniat on 1300 724 505 and ask for the technical team.

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