Moisture Testing Benchmarks Under Review 2008

December 16, 2013
By Mark Williams

“Change” – a relatively short word but one that evokes many images, feelings and a deep sense of reality generally and more specifically in the science that surrounds us in our everyday lives, at work, at play, in the environment and all the way back to our homes again.  Personally, we are change.  We are constantly changing from the day we were conceived and will remain a unique part of the universe forever throughout its continuum.  As a natural response to change, circumstances and situations need review.

Looking at the “leaky building” crisis or “leaky house syndrome” and the numerous excellent investigative techniques that Prendos has pioneered and refined, we have gained experience and established data for the most part which remains within the established boundaries—providing us with benchmarks.  We generally have tried and tested techniques to identify and analyse the causes and effects of leaking—some more effective than others, but few that can be solely relied on, without robust corroboration.

Importantly, we have recognized that there are certain glaring exceptions that could trip up the average building surveyor in the course of their investigations and reporting.  The time has come to seriously review whether moisture level benchmarks and a couple of techniques new to the market actually provide a true reflection of the state of the framing timbers visibly hidden behind the cladding.  Is moisture testing enough?  What does a thermal image actually show?  How effective are static moisture probes?

In the normal course of taking moisture content readings in framing timbers two 10mm diameter holes are drilled approximately 25mm apart and probes are inserted that are connected to a resistance moisture meter which gives a moisture content reading albeit unadjusted for timber treatments.  The most common framing timber used in New Zealand for house and light framed structures is Radiata Pine and its in-service “normal” moisture content range has been understood and accepted at and around 12% to 14%.  Framing timbers are required to be at or below 20% at the time of installing interior linings.  The New Zealand Standard 3602 since 2005 requires that H1.2 treated framing timbers not be allowed to exceed 20% moisture content.

In reviewing the above base line acceptable moisture contents, one gets a clear picture that alarm bells should start to ring if moisture contents are found at 18% and certainly above 20% moisture content with increasing concern at significantly higher moisture content levels.  Through extensive testing underpinned by well known and respected timber scientists such as Dr Robin Wakeling from Beagle Consultancy Limited, benchmarks starting at approximately 20% moisture content for likely serious problems through to 25%-30% moisture contents at which decay and serious damage is common, have helped to set the upper threshold or trigger, that assists many of us in our investigations.

The generally accepted process of investigation, again pioneered by Prendos, requires a full survey of key locations on a building followed by the physical removal of sections of the cladding at elevated moisture points  to visually inspect the framing timbers, remove samples and attempt to confirm the cause of the leaking and mechanism of failure.  Panels are therefore not removed at every location that moisture contents are taken.  Reliance is placed on the correlation between the moisture contents taken at locations where panels are indeed removed.

Up until recently, and exacerbated by our exceptionally warm and dry summer this year, we at Prendos have noted a distinct change in the moisture content data that we have gathered from investigations over the past 12 months.  If “Mr Surveyor” goes out to a house with his moisture meter slung in its holster on his hip and a ten foot ladder under his arm and subsequently confirms moisture levels in the range of say 8% (only just registering on the moisture meter scale) to say 16%, he may well be inclined to quickly retreat to his car boot, prime up his sealant gun and promptly plug up the holes he has made, dusting off his hands as he proudly informs the previously concerned owner that he has found absolutely no high moisture readings and they are one of the very few fortunate people to escape the personal and financial cost of having to reclad their home.

The likelihood of this scenario occurring is high and of grave concern to us at Prendos and should be a concern to all Surveyors and home/building owners who commission and receive leaky building reports.

Similar to the adage “have ute – can build” the fact that a building surveyor knows how to use a moisture meter does not mean that he has a full understanding of the results that he is receiving under certain circumstances.  A great deal of science and experience are needed to identify exceptions to the rule when they come along, often in a disguised fashion.

It has become clearly evident that one must more often than not, open up and visually inspect the framing and take samples for expert timber analysis.  Using a thermal imaging infrared camera would only provide surface temperatures and no insight into the state of the underlying wood.  These cameras do have reportedly limited uses, but require a 10°c temperature difference to show up areas of interest.  Similarly, static moisture probes would be of little use if retro-fitted as the elevated moisture has come and gone. These probes will only offer some indication if they were placed at leak points.

Prendos has, on not one but a number of occasions, come across exceptions where the moisture content of framing timbers falls well below the “accepted threshold” noted above.  On a recent investigation I was dealing with a solid plaster cladding where the aluminium window and door frames were embedded and obviously would cause water to drain directly into the plaster where it could then soak through to the framing timbers.  This particular house presented a dichotomy with glaringly suspicious details that in my experience would most definitely leak.  I had to balance this against willful destruction of the owners’ property and specifically their cladding, without apparent due cause.

A high moisture content reading in the framing would have provided me due cause.  However, the range of readings in numerous areas was constrained between 9% and 16%.  After taking moisture content readings in numerous places, I could not help but feel I was missing something – conspicuous by its absence.  I almost knew that this house was leaking but I could not find a moisture level elevated sufficiently to warrant cutting through the plaster and making sure that the framing timbers were indeed intact.  One of the last clues that I needed was the sensation felt when I drove the moisture meter probes into the framing and sensed that I was not pushing into  normal framing.

This particular house was renovated with windows being previously added and removed without standard framing layouts being adhered to.  Therefore, when drilling through the plaster cladding to measure the moisture content in the framing, a number of places appeared to have no framing in locations where one would expect framing to be.  This fact tended to mask the two holes where I initially felt there was framing but once I put pressure on the probes they sank through into the wall cavity.  This situation could have been mistaken as yet another location with no framing.

Following my on site analysis, coupled with confidence in my experience and faced with facts that appeared to be misaligned, I cut through the plaster to visually see what I was dealing with.  Originally, as you can see in Photograph 1, the wall appeared to be intact with no staining or obvious cracking in the location marked.  When I probed the framing timbers through the plaster, they gave me a reading of 9%.  But when I cut in and visually inspected the framing, I found it significantly decayed as you can appreciate in Photograph 2.


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To double check my original reading, I probed the framing timbers again as can be seen in Photograph 3 that clearly shows a moisture level of 12% – well below the 18% benchmark that would normally ring alarm bells.

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Leaking had obviously occurred and caused these framing timbers to become significantly decayed.  Through the decay process and drying out during the summer as noted above, the timber has likely shrunk back allowing any leak water to possibly pass down the back surface of the plaster and not affect the framing.  A further example of this phenomenon from one of our many other sites recently investigated, shows a similar finding at a slightly higher moisture level (refer Photographs 4-6).

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This type of information prompts all of us at Prendos to always retain a healthy dollop of skepticism through any investigation we undertake until the truth is revealed and becomes plainly visible.

So, although leaky building problems have been around for a number of years now, we at Prendos have not forgotten how to learn; how to test information; and  ensure that we obtain the truth for ourselves – to properly advise our clients.

Mark Mark Williams joined Prendos in July 2002. He obtained a B.Sc in Building Science from the University of Witwatersrand in Johannesburg in 1987. Ten years later, in August 1997, Mark and his family immigrated to New Zealand and have lived and worked in Auckland for the past six years.

Mark has been actively involved in the New Zealand construction industry for the past five years and his experience spans a multitude of projects from commercial subdivisions through commercial buildings, roading, warehouses, a retirement facility, pharmaceutical and biotech facilities. He is experienced in the design, cost control, budgeting, procurement and construction delivery of all of these projects and associated on-site experience.