The heat release rate of an external wall cladding close to boundary is required to be limited in order to prevent the cladding materials from igniting and contributing to fire.
Table 7.5 of C/AS1 specifies the acceptable peak heat release rate and total heat release rate for external wall claddings and the limitation depends upon building height, distance to relevant boundary and purpose groups.
The smaller the heat release value, the more stringent the requirement is, and the smallest ‘Peak Heat Release Rate specified in the Table is 100kW/m2 and ‘Total Heat Released’ is 25MJ/m2 and the materials which meet this requirement are given a classification ‘A’.
Specify James Hardie cladding materials confidently in a fire rated wall application as all James Hardie cladding materials have an ‘A’ classification and make them suitable for use in this application. James Hardie cladding materials are also classified as non-combustible materials and tested to AS1530-Part 1 test method.
James Hardie has a range of fire rated wall systems tested at BRANZ varying between 30 to 90 minute fire rating. Refer to http://www.jameshardie.co.nz for more information regarding these systems or to download the Fire and Acoustic Design Manual.
Fibre cement close to boundary
August 11, 2010 at 9:51 am
Hi Singh
Just a query about timber weatherboards. We have a project where we have specified a rusticated weatherboard – because of supply issues the specified boards are not available for 3-4 weeks. As the project can not afford this delay, both the builder and wb suppliers have suggested that we use a shiplap board of similar proportions horizontally. The shiplap board has weather grooves and the same amount of lap and cover as the original rusticated board – the only difference is the chamfered edge to what would be the base of the board. We have asked the builder to seek approval from the building inspector, however do you see any issues with this proposed change.
The boards pre-primed pine for paint finish.
[Reply]
August 31, 2010 at 9:39 am
Singh
I have a query related to external, two way fire rated walls where Linea weatherboards are being used as the cladding over a drained cavity to a timber framed wall. There is a max 45 minute rating required (30 min in some areas). The wall is within 1.0m from the adjacent boundary.
As the wall is two way and therefore must provide protection from outside face – is there a concern with the base of the cavity which is required to be open for drainage ( to the extent of the openings within the cavity closure moulding) ? Is the standard pvc type cavity closure suitable or should a steel closure be used?
The Fire Engineer has suggested the use of an intumescent type foam ( weather resistant) to fill two thirds of the cavity closure.
Or is the best solution to use Rab Board and therefore ignore the cavity in regards to fire protection.
Your technical literature for both Linea & Rab Board as well as the Fire & Acoustic manual does not seem to deal with this level of detailing.
regards
Sharron
[Reply]
September 2, 2010 at 11:04 am
Sharron, with cavities the ideal system would have been to use RAB board rather than building paper. This would have provided a fire barrier should fire manage to get into the cavity. This also provide bracing and acoustic values.
Then at the top of the cavity had solid closer so that fire didnt penetrate the eases. System to a interstorey design for apartments. Cavity needs to be sealed off between apartments and started again.
[Reply]
Singh Kamboj:
September 2nd, 2010 at 12:34 pm
Hi
RAB Board can be used to achieve fire rating and it is an ideal product for walls close to boundaries where a combustible cladding material is being used such as timber weatherboard or EIFS etc.
You certainly need to block the cavity at the floor level where you have a different tenancy one above the other. In these scenarios the cavity can be blocked off by a continuous horizontal timber batten in conjunction with a z flashing.
[Reply]
September 2, 2010 at 12:28 pm
Hi Sharon
The performance of external fire rated walls using the cavity construction method have been evaluated and assessed at Branz. The performance of a fire rated wall as specified in our fire and acoustic manual doesn’t get affected whether the cladding is direct fixed or fixed over a cavity batten.
The normal uPVc vent strip is suitable for use as a vermin proofing at the base of a cavity.
You surely can use RAB Board in lieu of a building wrap, if you want to, and you achieve the extra benefits such as extra robustness from a weathertightness perspective, acoustic and bracing etc. in addition to fire rating.
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