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Unit: Inspecting and testing subfloors

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

Section 1: Subfloor systems

Underlay board products

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Strictly speaking, the underlay is not part of the subfloor, because it doesn't perform a structural function.

However, it does form the substrate underneath certain types of floor covering, and there may be times when you need to assess it as part of your subfloor inspection.

The main board products used as underlay are hardboard, plywood, MDF and fibre cement.

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Hardboard is made from wood fibres compressed into a sheet in a high temperature press.

In principle, the bonding is achieved entirely through the 'felting' process of the fibres and the natural glue, or lignin, already present in the wood.

However, other chemicals can be added to improve the board's strength and resistance to moisture, insect attack and decay.

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Hardboard is often referred to by the trade name 'Masonite', which comes from the Mason gun used to produce the wood fibres.

In this process, the wood chips are steam heated in a high-pressure cylinder and then released into a cyclone, which causes the chips to explode into fibres.

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Medium density fibreboard (MDF)

Medium density fibreboard, or MDF, is manufactured in a similar way to particleboard, using the same sorts of glues and additives.

The big difference is in the way the wood fibres are treated before they're pressed.

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The wood is heated under pressure until the fibres and natural glues that bond them together soften, and then the fibres are rubbed apart to produce a much finer material for pressing.

The end result is a sheet with a higher density and more even consistency than particleboard.

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MDF underlay uses melamine and urea formaldehyde glue to make it more resistant to moisture than standard grade MDF.

However, it still needs to be protected from moisture to stop it from rotting or swelling over time.

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Fibre cement

Fibre cement is made by bonding cellulose fibres with cement, sand and water.

Traditionally asbestos was used as the fibre reinforcement in 'fibro' products because of its excellent durability and low cost.

However, asbestos fibres were found to cause serious health problems in people who breathed in the dust, and it is no longer used in Australia.

Fibre cement underlay is highly resistant to swelling, shrinkage, fungal decay and insect attack.

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Plywood underlay is made in the same way as other types of plywood, using rotary cut veneers.

The top face is free from knots and other defects that may cause problems with the floor covering.

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Learning activity

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When you're inspecting subfloors in older homes, it's likely that you'll come across asbestos fibro sheeting from time to time. Some homes also used asbestos-backed vinyl tiles and carpet underlay. These products are safe while they're left undisturbed, but you need to take special precautions if you plan to pull them up or do any work that might damage them.

We'll look in more detail at how to work safely around asbestos-based products in the unit: Safety at work. But for now, see if you can answer the following questions:

What types of safety precautions are needed to remove asbestos-based products?

Where should these products go once they have been removed?

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