Sorting Out Leaky Builidngs 2005

December 16, 2013

By Philip O’Sullivan
The Building Industry Authority has recently published the new Acceptable Solution E2/AS1. I had the privilege of being on the Working Group. The BIA has also recently published documents requiring treatment of timber framing (B2/AS1) and a test or Verification Method for lightweight wall claddings (E2/VM1)The Building Code tells us what a building must achieve. It’s the boney skeleton hanging off the backbone of the Building Act. More detailed and specific documents such as Acceptable Solutions and Verification Methods give us the final shape and form.
A New Benchmark
E2/AS1 has set a new benchmark. It is based upon testing, research, experience and consultation. If E2/AS1 is specified with a building consent, then it becomes mandatory. In reality a designer can rely on E2/AS1 to provide the system and many of the details.The designer may need to provide details for certain junctions, just like an engineer designs a beam for a house.

Councils have become strict over weathertightness, but not without good reason. Standards had slipped, certain cladding systems weren’t robust and the loss of treated timber was the last straw. Councils will develop their understanding over time so hopefully a realistic approach will supercede the current levels of paranoia.

What we all have to realise is that with the move back to treated timber and the use of cavities, our buildings will be much more robust. The key is to do the basics right, and then to focus attention onto risk areas such as balconies.

Some architects will no longer design balconies over enclosed areas as they will always be prone to leaks.

One interesting thing is that with greater prescription we are seeing much more innovation. Both are driven by need, but the innovators now have something to aim for.

Risk Matrix
I originated the risk matrix concept in 2001. A bar graph from British Columbia had clearly shown the relationship between eave width and leaks. There were other risk factors, but little information. I analysed 50 sites comprising 300 households that had experienced varying degrees of leakage. Ken McGunnigle from Prendos helped with some acoustic sound advice and Winstone Wallboards gave financial assistance.The most startling thing was the wind zone was less important than first thought. Building complexity, which is difficult to define, is the greatest risk, but that’s obvious. Most leaks occur at junctions; the greater the number and more complicated, the more failures. (for more info – 2002 CINZ Science Forum).

I thought some sort of table or matrix could be a useful tool, so I adopted a numerical approach that mimics Winstone’s simple method for calculating wind load for bracing.

Subsequent studies by BRANZ and the BIA using WHRS data supported my findings. The approach was then further refined by the BIA to allow different claddings at different risk levels. The benefits of the risk matrix approach in BIA Determinations, is an early sign this approach is sensible and I’m sure it will improve over time.

Once you get use to cavities, they’re easy. They’re also cheap ? it’s amazing the difference a few battens can make.The key points are:

  • Wind pressure is generally the same front and back of the cladding ? there’s no wind pressure difference to drive water across the cavity. The only forces present are gravity and surface tension, so water that penetrates just runs down the back of the cladding and out at the bottom.
  • The internal wall lining carries the wind pressure. With gable ends, as there is no internal lining, rigid sheathing is required.
  • All penetrations need to be ‘airsealed’ to maintain the cavity pressure. Some air leakage will occur, but as long as this is minimal, there is no loss of performance.
  • Battens to be vertical only to ensure drainage and ventilation. Short sloping blocks are permitted, but there must be no horizontal or raking battens.
  • With battens, “less is best”. Don’t crowd junctions. Keep battens back from corners to provide a corner cavity ? that’s where the leak is most likely to be.
  • At the bottom of a wall, drain water back to the outside.
  • Bottom clearances are essential, so keep gardens and paving clear.

E2/AS1 is not the last word. It is a vast improvement and it will improve over time. Every builder, roofer and cladding installer needs a copy. At $6 for a downloaded PDF or $35 for a nicely printed version, there’s no excuse.

The Victoria University Book Centre in Wellington sells the Approved Documents. These documents are available in three formats: hard copy, CD-ROM, and PDF files downloaded via the internet.

Orders can be placed through the Book Centre website: Alternatively, phone 0800 370 370 or email [email protected] e-mail address is being protected from spam bots, you need JavaScript enabled to view it .

Philip Philip O’Sullivan’s experience and knowledge in weathertightness and timber decay have put him to the forefront in this difficult and vexing area. Philip is currently President of the Claddings Institute. He regularly writes articles for the industry, the most well known being Dr Rot in Progressive Building. He has served on the Building Industry Authority’s committee for the revision of E2/AS1.

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