Material Group Numbers vs. CRF: What architects need to know
Fire performance is a critical consideration when selecting interior finishes — but in New Zealand, the terminology can easily cause confusion. One of the most common misunderstandings is the relationship between Material Group Numbers and fire ratings for flooring. Although both relate to fire behaviour, they apply to entirely different parts of the building.
What Material Group Numbers actually cover
Material Group Numbers (Group 1 through Group 4, including the “-S” smoke classifications) describe how wall and ceiling linings perform in fire conditions. These classifications are based on how quickly a lining contributes to flame spread and the likelihood of flashover. For architects and designers, this system provides clear guidance when selecting internal linings to meet NZ Building Code Clause C Protection from Fire.
But Material Group Numbers do not apply to flooring. Floors behave differently in a fire scenario and require their own dedicated test method.
Flooring Is Rated Using CRF / CHF — Not group numbers
In New Zealand, flooring fire performance is measured by Critical Radiant Flux (CRF) — also commonly referred to as Critical Heat Flux (CHF). Importantly, CRF and CHF are interchangeable terms describing the same test result.
CRF is determined using the ISO 9239-1:2010 radiant panel test, which measures how much radiant heat is required for flames to spread across a flooring surface. Put simply, the higher the CRF, the better the flooring performs, indicating slower flame spread.
Depending on building type, use, and whether sprinklers are installed, NZBC requirements typically range from 1.2 to 4.5 kW/m². This ensures flooring contributes minimally to fire development, particularly in transitional spaces and evacuation routes.
Why this matters at specification stage
For designers, understanding the distinction is essential. When choosing flooring for any project — commercial, public, or multi-unit residential — compliance depends on verifying its CRF/CHF rating, not a Group Number. Meanwhile, wall and ceiling choices must align with the appropriate Group Number requirements.
Accurate specification not only supports Building Code compliance but also ensures a safe built environment without compromising design intent.