Dialogue: Treated timber safer option for framing

Rotting timber framing is an alarming development
for home-owners. Steps can be taken to reduce the problem, writes GARRY SHUTTLEWORTH.

Some buildings have always leaked. That is a generalisation, but true. From the first corrugated iron lean-to built by our pioneering forebears to the latest million-dollar mansions, intruding moisture has plagued owners and frustrated tradespeople.

The difference now is that some buildings are also suffering major structural failure from rot caused by moisture from leaks. With repair costs in such cases up to six times the bill for remedial work where no rot is involved, the situation is undeniably serious.

So why has this happened? Are more buildings leaking than previously? Why are they rotting? And what can home-owners and those planning new homes do about it?

Common sites of leaks are roofs, windows, cladding junctions, and penetrations through the cladding surface. The irony is that modern cladding on the outside of a building is the least likely of all components to leak.

The causes of leaks can be traced to changes in house-design fashions.

Houses built from the 1950s to the 1970s had many common features, some because of a limited choice of materials. All had eaves and pitched roofs, and they were often clad in timber weatherboard or stucco on an open, ventilated, timber frame.

Houses did leak in those days but it was more of a nuisance than a major concern. Owners complained more about the wind whistling through their homes.

Houses were often constructed of native timber, in which borer infestation was a problem, and by the late 1960s the use of pine had become widespread.

This had to be treated against insect attack (borer). The preferred method, using boron salts (H1 Boric), was not new. Fused boron rods were used in early sailing ships to protect hull timbers from rotting.

Manufacturers focused on developing new building materials, driven by consumer demand for innovation. As these became available, creative architects answered the call for different designs.

We builders loved it. Lighter, easier-to-use materials improved our productivity and we took pride in creating homes for those who were prepared to build something a little different.

Long may innovation in design and building techniques continue. Can you imagine new subdivisions graced only with single-storey brick or weatherboard homes, all with the same pitched roofs? Sunday driving just wouldn't be the same.

But unfortunately, innovation brought greater risks. Some new designs removed rain-shedding devices which had proven effective in the past. Flat roofs replaced pitched, eaves were removed, and internal balconies became common.

Industry research shows houses with such design features are at greater risk of invasion by the building industry's cancerous disease - moisture.

Coupled with this, the building industry has been unable to maintain the skill base to keep up with the pace of change.

The early 1980s saw the mandatory introduction of insulation into homes. Before that, the exterior wall cavity had been empty. Holes were often drilled in the top and bottom plates to help air movement, and a cladding of timber weatherboards or stucco built on a timber substrate also allowed air movement. The timber framing was also treated.

If water got in - and sometimes it did - air movement dried it out.

Many argue that the introduction of insulation alone contributed significantly to the situation we have today.

As the desire for innovation continued and more time-saving products were developed, sheet cladding became popular. Such cladding made the wall cavity an airtight box, now full of absorbent insulation. If moisture did get in, it could not escape or dry out.

Even then, leaking buildings were a manageable but unacceptable problem.

Framing timber was not rotting to any significant extent because it was treated.

In the mid-1990s science found a way to make it less attractive to insects (borer). The New Zealand Standard governing the use of framing, NZS 3602, was amended to allow the use of untreated kiln-dried timber.

Such timber has no protection against fungal attack and appears to be more susceptible to rot than treated timber if it remains damp for any length of time.

So what can we do?

Let's acknowledge that leaks cannot be eliminated from buildings. But we can identify those buildings which are more likely to leak and pay a lot more attention to them.

We can also add another insurance factor by insisting that timber framing in at-risk situations is treated using H1 Boric (not H1 LOSP, which protects only against insect attack).

The framing component is only about 6 per cent of the cost of a house. Paying an extra 10 per cent on that 6 per cent is a very small price to pay for insuring the long-term structural integrity of a house.

If we can restore the status quo of the early 1990s and work on mechanisms to improve industry training, design features and product performance, the industry and the consumer will be better served.

* Garry Shuttleworth is the chief executive of the Certified Builders Association.