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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

Moisture in wood

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Living trees use an enormous amount of water to manufacture food and convert it into wood fibre.

As a rough average, for every one tonne of wood produced, a tree will take about 1000 tonnes of water from the soil and transport it up to the leaves, where the process of photosynthesis takes place.

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Hardwoods, such as the eucalypts, have pores (also called 'vessels') which join together to form pipes for transporting the sap.

Strength is provided by the fibres, which have thick cell walls and make up the bulk of the wood.

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Softwoods, such as the pines, don't have pores in their cell structure.

Instead, most of the woody tissue is made up of long narrow cells called tracheids, which transport the sap and also provide strength to the stem.

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How wood dries

When a log is freshly cut, the fibres contain a great deal of water, both in the cell cavities and the cell walls.

Since the moisture in the cavities is free to evaporate once a cell has been cut, it is termed free water.

As the wood dries, the free moisture continues to evaporate until the cavity nearly dries out.

Once that happens, the bound water, or moisture bound up in the cell walls, begins to evaporate.

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This point is called fibre saturation point, because the fibres are still saturated, even though the cell cavities are now almost dry.

Until fibre saturation point is reached, the biggest change in the timber is that it gets lighter as the free moisture is lost.

After fibre saturation point, however, the cell walls begin to shrink and get stiffer. This is the process of seasoning.

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Calculating moisture content in wood

The moisture content (MC) of wood is usually expressed as a percentage.

This simply represents the weight of the water contained in the piece, compared with the weight of the woody substance itself.

That is:

   Moisture Content (MC) % =           weight of water           x 100
                                             weight of woody substance

The fraction part gives you the proportion of water to wood fibre, and multiplying it by 100 converts the fraction into a percentage.

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For example, let's say a piece of tongue and groove baltic pine flooring weighs 6 kg.

Now let's say that the woody substance itself makes up 5 kg of the total weight.

What is the moisture content of the piece?

   MC   =   6 - 5   x 100

           =     1  x   100

           =    20 %

The equation shows that if the woody fibres weigh 5 kg and the water weighs 1 kg, then the moisture being held in the wood will add 20% to the weight of the fibres.

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Learning activity

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Here's a moisture content question for you to work out. Let's say a piece of hardwood flooring weighs 4.4 kg, and the woody fibres in the board weighs 4.0 kg.

What is the weight of the moisture in the board?

What is the moisture content of the board expressed as a percentage?

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