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Unit: Inspecting and testing subfloors

LMFFL2004A: Moisture test timber and concrete floors
LMFFL3101A: Inspect sub-floors

Section 2: Moisture in subfloors

Drying timber to EMC

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All timber starts off green, that is, full of sappy moisture from the growing tree.

Once the tree is harvested and processed into timber, it will begin to dry naturally - quickly at first, and then more and more slowly as the moisture content drops down below fibre saturation point.

This point is around 30% MC in most species.

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Eventually, the moisture in the timber will reach a level where it's in balance - or 'equilibrium' - with the humidity in the surrounding air.

This is called equilibrium moisture content (EMC).

In most indoor coastal situations, EMC is between 10% and 15%.

In dry inland areas, or inside air conditioned buildings, it may be as low as 6 or 7%.

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Seasoning is the process of drying timber down to EMC.

It can be done naturally by simply exposing the timber to the air, although this can take some time depending on the species being dried.

For example, many hardwoods take about one year per 25 mm of thickness to achieve EMC from a green state.

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Alternatively, the timber can be kiln dried.

Some kilns have solid concrete or block walls, while others have plastic membranes stretched over a frame.

Either way, they all use mechanical ventilation systems and heat and humidity controls to allow the timber to dry much faster under controlled conditions.

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Advantages of seasoning

Traditionally, building timbers were generally used in an unseasoned state, particularly in the days when house frames were built 'stick-by-stick' on-site.

Even cypress pine floor boards were typically green, because cypress has a very low shrinkage rate when it dries, and the flooring was often covered with carpet or linoleum anyway.

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However, most species of timber perform better when they're dry, particularly the plantation pines and high grade hardwoods.

So it's common practice these days to use kiln dried timber in just about all building applications.

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The changing nature of EMC

Once the timber reaches EMC, that doesn't mean its moisture content will remain stable.

As the humidity in the atmosphere continues to rise and fall, the EMC in the timber will change accordingly.

This is why in wet weather you sometimes find that timber doors and drawers tend to stick.

What's happening is that the timber swells as it takes up moisture from the atmosphere and then shrinks back again when the dry weather comes.

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This graph shows the relationship

between EMC, relative humidity and air temperature.

You can see that as the RH rises, the EMC will also rise. This is why the general definition of seasoned timber under the Australian Standards is timber with a moisture content of between 10% and 15%.

However, the definition goes on to allow for EMC levels outside this range when they've been specified by the client. This caters for situations when the EMC is normally lower than 10% (such as in air conditioned buildings or dry inland areas) or above 15% (such as in very humid areas).

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In practice, the timber's moisture content won't change immediately as the humidity in the surrounding air rises and falls.

The speed of change will depend on various factors, including the cross-sectional size of the piece, whether there are any chemical treatments in it, and most of all, how exposed the surface of the wood is to the atmosphere.

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So if the timber is properly sealed with a surface finish and protected from the weather, there will be hardly any movement at all in the short term.

However, if the humidity stays very high or low, and the surface finish starts to break down and allow direct contact with the wood fibres, then the EMC will change more quickly in response to the humidity levels.

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Learning activity

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We said earlier that seasoned timber generally has a moisture content of 10 to 15%, unless otherwise specified.

If the air temperature was 30° C, what would this equate to in terms of relative humidity?

Click on the graph at right to see it in a larger window. This will help you to work out an approximate percentage range.

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