In the last lesson we mentioned that structural sheet flooring is usually made from particleboard or plywood.

Here are some more details on these two products.

Particleboard

Particleboard is also called chipboard, because it's made from small chips of wood glued together and compressed in a high-temperature press.

Different types of formaldehyde glue are used in different grades of particleboard, depending on the end use of the product and the performance that's expected of it.

In the case of flooring, the glue is high strength phenol formaldehyde.

There are also various additives available, such as fungicides, termiticides and fire retardants, which can be specified if there's a higher than normal risk of attack or damage.

Some manufacturers also seal the edges of the sheet to protect it further from moisture.

The sheets have a groove along the two long sides, with a plastic tongue in one of the grooves.

Manufacturers tend to colour-code the tongue to identify the thickness of the sheet - so you'll often hear people refer to particular products as 'yellow tongue', 'red tongue' and so on.

Particleboard flooring is designed to withstand exposure to the weather for three months.

This gives the builder time to put down a platform floor and then install the wall frames, roof trusses, cladding and roof covering.

The flooring is glued to the joists and nailed to help secure the sheets while the glue sets.

In some cases the sheets are used right across the entire floor, including in wet areas such as bathrooms and laundries.

However, for the flooring to be successful in wet areas it needs to be well protected from moisture with the use of flashing, surface sealants and careful building techniques.

An alternative product in wet areas is compressed fibre cement, which is much more resistant to water.

Plywood

Plywood is made up of several 'plies', or layers of wood veneer, glued together and bonded in a high temperature press.

The direction of the grain in each layer runs at right angles to the layer above and below it.

This makes it stronger than particleboard, because timber fibres are very strong in the direction of their grain, and by alternating the layers the strength is spread in all directions.

The veneer is rotary cut on a lathe, and comes away in a long continuous sheet. It is then re-cut into sizes suitable for laying up into plywood and coated with a phenol formaldehyde glue.

Once it has been pressed and allowed to cool, the sheets are trimmed to size.

Like particleboard flooring, plywood flooring has a groove machined into both long sides, with a plastic tongue inserted into one side.

Plywood is more durable than particleboard, and it can be treated with a light organic solvent preservative (LOSP) for extra resistance to insect attack and fungal decay.

However, it still needs to be protected from constant exposure to moisture, so normal 'wet area' building practices still apply.

The veneer grade of plywood flooring is 'CD' - meaning a C grade top face and D grade back.

C grade means that any knot holes or splits must be filled, and D grade means that knot holes, splits and other imperfections may be left open.