SciBlogs

Archive May 2010

Leaky Buildings — Part 1 — How did we get here. Ken Collins May 20

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With the government announcing it’s IMG_7549-web(our) package to help solve the leaky homes crisis this week, it has brought the spotlight back onto what is now a highly emotive subject. While the emphasis is rightly on getting peoples homes safe and fit to live in, it should be remembered that the leaky buildings problem is wider spread than just domestic buildings. Recent reports have shown that it includes schools, commercial and community buildings.

The politics of it is complex and controversial with blame-storming rampant. The reality is that there are so many aspects to obtaining a completed building, from design to move in, that you can’t just point your finger a one person or organisation.  Additionally the physical causes, effects and remedies are only now becoming well known and well understood.

So how did we get here?  In effect it was a combination of a number of issues, coming together all at once.

Ever since humans have built structures and shelters on this land, they have leaked, for one reason or another. In the early 20th Century buildings leaked, however the timber that was used was good strong native timber, which could withstand being wet and then drying out again. The gaps and construction technologies of the day meant that there was airflow through and inside the building structure, which allowed it to dry out. These days everything is sealed up like a chilly bin and any water that does get inside the structure can’t get out again. The timber stays wet, fungus grows and timber rots.

The use of un-treated timber was approved when the kiln drying of timber had become commercially available. Up until then all timber was naturally air dried and would normally be stood up as framing while it was still well above 20% moisture. It would then dry out as the house was completed.  The testing of the day showed that ‘dry’ timber (at it’s moisture equilibrium of about 12-15%) didn’t need treating, assuming it stayed dry. It also meant significantly less energy, chemicals and heavy metals were used in the building industry.

However history has proved that some of this timber didn’t stay dry.Imported-Photos-00026-web

What people also didn’t realise was that the old Boric treatment applied to timber being used internally (to stop borer attack) actually provided some protection against fungal attack when it did get wet.

At the same time the design fashion of the day changed to the use of parapets and low pitch roofs, monolithic plaster wall systems, and the mixing of different cladding materials on the one building.

New cladding materials and cladding systems relied to heavily on thin top coats where the base materials are not inherently water proof, or where jointing systems have proven over time to be ineffective or to be difficult to install and maintain.

The use of sealants to provide flashing and waterproofing barriers increased exponentially, at the expense of mechanical flashing systems. People relied on these chemicals to stop water getting into all sorts of little (an not so little) openings. So while sealants work very well when they are installed properly, they do need maintenance and replacement, especially where they are exposed to UV light.  However all to often they weren’t used or applied in ideal conditions and they failed prematurely as a result.

Added to this there was a lack of continuity across all the disciplines in the building industry, where traditional roles and responsibilities were fragmented.

Despite all of this, it must be pointed out that at the time the majority of people involved in the building industry thought they were doing the right things. Products were researched and tested, assessments and decisions made on the information available. Yes there were (and still are) some dodgy developers, builders and designers out there, but in no way can they account for all of the problems we are now observing.

One of the biggest realisations has been that despite the knowledge obtained from testing IMG_5557-webconstruction and cladding systems to assess their suitability for New Zealand conditions, the true test has been their actual performance in the real world over 5, 10, 20, 40 years. It is particularly hard to assess likely in-use performance by doing accelerated weathering experiments and the like. Often people relied on overseas testing and research, which wasn’t always totally applicable to NZ conditions.

The result is that in the last 10 years many methods that were thought to be ok, have proven to not be. Manufacturers have changed their installation, fixing and jointing instructions. A number of products that were tested as being suitable for NZ buildings have been withdrawn after they were found to fail. This includes products that were assessed by the Building Research Association of New Zealand (BRANZ) and given a BRANZ Appraisal Certificate, only for that certificate to be withdrawn later when problems arose.

What was thought to be best practise 10 years ago, now isn’t and so things have changed, and will continue to do so.

The fact is that you can never know with 100% accuracy how a material or a system will perform until it has actually been in use, in the environment, for a period of time. There are so many variables of, exposure, wind loads, quality of workmanship, movement inherent to timber framed buildings, not to mention maintenance (or the lack of it). After all, how many people wash their houses down every six months as is recommended by paint and roofing manufacturers.

Lessons have been learnt and things are now done differently. Will they prove to be successful over the medium to long term, the industry won’t know until we get there.

In the second part of this blog, I will look at some of the science behind the issues and what is currently thought to be the best solutions.

Timber treatment: what are the best options? Ken Collins May 11

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timber treatment 2Some time ago there was a lot of media coverage on the use of CCA (Copper Chrome Arsenic) timber preservative. This caused a number of our clients to ask for alternatives, so we did a little research into what the implications are.

Pine needs to have large amounts of preservative to stop it rotting when in contact with water, and especially in ground.

There are two alternatives to CCA commercially available: Copper Azole based (CuAz) and Alkaline Copper Quaternary (ACQ). Both rely on very high concentrations of copper to act as an agent against fungal and insect attack, and as such both are strongly alkaline.

Unfortunately this also means that these products are very aggressive to mild steel and even galvanised steel. The Building Research Association of New Zealand (BRANZ) did some testing and found that in comparison to CCA treated timber:

  • In timber treated to H3.2 (the treatment level for timber in occasional water contact, often used in exterior wall framing, in wet areas, and outdoors above ground contact like decks), mild steel corrodes at rates of up to 5 times in CuAz and ACQ preservative.
  • In timber treated to H5 (the treatment for timber in contact with the ground, eg floor piles, structural posts, and fence posts), mild steel corrodes at rates of up to 12 times in ACQ preservative. (you can’t get H5 treatment in CuAz).
  • Galvanised steel corroded at a slower rate than mild steel, however the corrosion rate of galv steel in CuAz and especially in ACQ was still significantly higher than in CCA treated timber.
  • 316 Stainless Steel performed well in all the preservatives with minimal differences in corrosion rates.

A copy of their Conference Paper is available here and the more in depth Study Report can be found here.

timber treatmentIn short this means that if you use the alternatives to CCA then all the fixings (nails and screws), bolts and brackets (in-fact anything metal) that touches the timber must be 316 stainless steel or powdercoated. Consideration also needs to be given to the use of stainless steel flashings as well, because even water runoff from ACQ and CuAz treated timber will corrode mild steel, aluminium, and galvanised steel..

What ever you do, there is no current solution to preserving timber without using large amounts of heavy metals and toxic chemicals. But in saying this, you would have to suck on a lot of logs (or wood chips) to leach enough of the chemicals out of the timber for it to have a measureable effect on you. This includes CCA treatment.

So, unless you are building a children’s play area and have particular concerns about children eating the wood, CCA treated timber is still the best to use for the construction of buildings in NZ, especially where you need to treat to H3.2 or above. Not only for cost, but also because of serious durability concerns raised by the BRANZ testing.

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