Timber Moisture Stuff

While it’s a particularly damp time of year, especially here in Victoria, the subject of timber moisture and installation has reared its wet head.

If you don’t already own a moisture meter, you should get one. The most common are “electrical resistance” meters, that is, using two pins which need to be inserted into the timber.

However, moisture meters aren’t the beginning and end of the whole thing, as handy as they are. All they do is give you a starting point from which to make the best possible prediction about the timber’s movement in the future.

The key here is to learn about Equilibrium Moisture Content (EMC). That is, the standard moisture content at which the timber will be happy, so to speak, in its given environment (the “expected service conditions”). Some regard EMC as a figure which applies to the timber itself,  while others regard EMC as a theoretical figure, representing the building environment, but which timber will approach from its own moisture condition.

Either way, it’s basic science, and how that information is used produces the same result- namely, whether the timber is ready to lay, and what we can expect it to do once it is.

As we will detail a little further down, it’s not hard to work out the EMC once you know the “expected service conditions”.

The challenge is to determine what the expected service conditions will be, and to what level of regularity these conditions will be maintained throughout the life of the floor. When you turn your heaters off at bedtime, you may have noticed that, in terms of temperature and humidity- what’s outside gets in! Or perhaps your customer is an Eskimo, who will have their house constantly running at a soothing 5 deg C and 80% RH while it’s a billion degrees outside. Our apologies to Eskimos.

Here’s some salient points;

1. Timber is generally released from the Mills at a moisture content of between 9-14% and by the time it arrives to the contractor, it’s generally around 10-12.5%. The key word here is “generally”

2.  Floor boards expand with extra moisture, contract with less, and different density boards will expand at different rates. Therefore, if you install boards with a moisture content of 11%, but the EMC is 13%, they will most probably expand.

Tramex have a good pin resistance meter.

The moisture meter is an essential tool, but it only gets you started. The contractor’s biggest challenge is how to interpret the meter’s findings, and use the information to best advise the customer.

Your first stop should be the Bureau of Meteorology  where you can search the weather data on the property’s region, and calculate a yearly mean temperature and RH (RH is very important!).

You must also take into account the kind of air conditioning the customer’s property will utilize, bearing in mind that this, and the outside weather conditions, will influence each other.

Fortunately, the ATFA have published a very useful paper on this subject, Acclimatisation of solid T&G Flooring . Make sure you real the WHOLE thing, but note that it has a useful EMC chart which will tell you what the timber is most likely to do, once you know the property’s “expected service conditions”. We’ve reproduced it here:

(Note that the timber’s EMC is far more influenced by RH than by temperature). Here you will see that, if the property’s expected service conditions are to be 60% RH and 20 degrees, the timber’s EMC will be 11%. If your tests produce readings of around 9-10%, the timber will most likely expand. The ATFA’s resident guru on timber moisture (and lots of other stuff), David Hayward, advises that the timber may never fully reach the EMC figure. However, it is very important to calculate an average temperature and RH scenario for the building- because it changes all the time, therefore so does the EMC.

The ATFA paper will give you an idea of how much, in terms of mm, the boards are likely to expand (what they can’t tell you is how quickly- that will depend on the species).

So, when you hear talk of “acclimatisation”, know that the devil is in the details. So the timber flooring was stacked, with spacers, in the property itself for a whole four weeks prior to install- does this mean it has been properly acclimatised? Not necessarily! The question is, were those four weeks subject to expected service conditions? Equally, timber only left on site for a few days, but moisture tests to within the calculated EMC, may require no acclimatisation at all.

This is, of course, not taking into account other moisture sources, such as the concrete subfloor-that’s another story.

It’s important to make sure your customer know that their timber floor WILL move, that is as inevitable as death, taxes, and Hollywood cliches. But being armed with this information will ensure that you install timber flooring within an acceptable moisture range, and you can all but eliminate the worst timber movement.

2 responses to “Timber Moisture Stuff

  1. Moisture meter’s are very handy tools, takes the guess work out and allows you to assess what needs to be done and by when. Excellent, time saving piece of kit.

  2. Thank you for making your post easy to read and grasp. I respect all the work you did here and I’m glad I read this.

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