On 29th February MBIE announced the Building Performance Guide: Fire Performance Of External Wall Cladding Systems. They have developed the Guide which is generally about how external cladding systems should be tested for fire spread performance. The Guide is a temporary solution until MBIE amend the appropriate Acceptable Solutions and Verification Methods.
The Guides requirements seem to:
- exclude buildings coming under NZBC C/AS1 (Risk Group SH)
- have limited effect on buildings that are single level or ≤10m and up to 2 levels
- have a greater effect on all other buildings covered by NZBC C/AS2 to C/AS7 and C/VM2
For anybody involved in the design of building exteriors, other than NZBC C/AS1 buildings, it is recommended that you read the guide as it is too complex to cover comprehensively in this short article.
The Guide includes:
- what constitutes a wall cladding
- testing requirements and protocols (including new options)
- design considerations for external vertical fire spread
- a risk assessment approach
- individual cladding components verses cladding systems
Additional new test Standards
Besides the existing cladding component/system testing Standards ISO 5660 and NFPA 285, the guide adds BS 8414.1 and BS 8414.2, updates BS 135 (plus Annex A & B) and adds options AS 5113 and EN 13501.
It is significant that BS 135 covers external wall insulation for multi storey buildings.
It is worth noting that the new systems Standards BS 8414 and BS135 do not seem to cover concrete framed or timber framed construction. This seems like a bit of an issue in the New Zealand market.
Standards ANSI/ASTM E2307 and BS EN 1364.4 are mentioned as being "potentially acceptable" for curtain wall systems.
Risk assessment approach
Probably the most helpful introduction to risk assessment and testing requirements is the Guides Table 1.
Table 1: External wall cladding system - risk matrix for fire testing protocols
* For building height ≤ 10m, cladding systems used for important level four buildings or multi-floor buildings incorporating staged evacuation, phased evacuation or evacuation to a place of relative safety within the building should meet the requirements for risk levels Medium or High given below.
| Building Height
Risk Group SM, SI
Risk Group CA, WB, WS, VP
| Single level
| ≤10m & ≤2 levels
| >10m & ≤25m
| >25m & ≤60m
† Where a NZ 4515 residential sprinkler system is installed then the non-sprinkler risk level in column 3 should be used instead (i.e. risk level High given below)
Ω Automatic fire sprinkler system to the requirements of NZS 4541 (as modified by the NZBC)
Risk Levels are Low, Medium and High and have related test protocol requirements P1 to P5 as follows:
- Low - No requirement for building height ≤ 10 m (NZ Building Code Clause C3.5)
- Medium - Testing protocol P1, which is mostly based on ISO 5660.1, some limits on combustibility and conditions on some cavity formation (refer Guide)
- High - Testing protocol options P2 to P4, each of which use different testing Standards, and protocol P5 which uses various level of limited combustibility or non-combustible materials (refer Guide)
For High Risk Level there is an allowance for the use of timber framing (or combustible insulation products within a framed wall assembly), but there are strict conditions put on this.
What is in the cavity?
With items in the cavity (particularly for protocols P1 & P5), you need to be aware of when some combustible items are allowed and when they must be non-combustible, or can fire stopping at each floor overcome it? Refer to the Guide for details.
Exclusions from cladding compliance
The Guide lists some substantive exclusions from the compliance with C3.5 and C3.7 (testing requirements). However conditions do apply (refer to Guide). Exclusions include but are not limited to:
- top storey external wall cladding
- signs, billboards, video screens
- green walls
- sunscreens, sun shades and louvres
- doorsets and window frames
- sealants and tapes
Expert or not?
As well as NZBC C/AS1, which is not involved in this Guide, some Low Risk Level buildings to Table 1 above may not be overly complex with regards to cladding. For the non-expert, if a Fire Engineer is not involved, consider the need for expert advice carefully, particularly for Medium Risk and without a doubt for High Risk Level buildings.
Most of the issues covered by the Guide relate to design stage issues combined with material/system testing issues, and the Guide provides some assistance in resolving these. However the real crunch comes when the Acceptable Solutions and Verification Methods are finally updated. Once the dust has settled and MBIE have done the updates, Masterspec will look at what we need to include in the appropriate Masterspec sections.
In the meantime, virtually all Masterspec cladding sections (subject to separation etc) are suitable for use with NZBC C/AS1 or Low Risk Level buildings in most cases. Medium Risk Level buildings are similar to what seems to be applied now, so this does not change the existing procedures much.
However with High Risk Level buildings the Guide varies from some existing requirements, so until MBIE update the AS’s and VM’s these are difficult to deal with definitively. During the intervening period existing Masterspec sections will not cover all of these High Risk requirements (particularly the new ones). So for High Risk Level buildings in particular, besides expert advice being necessary, consulting with the BCA will be essential in the interim (and probably thereafter).
Although the Guide is an MBIE document, Auckland Council and BRANZ were involved in its development. But if you want to use the Guide it may still pay to check with your BCA first.
Read the full article on the MBIE website