If the underlay is being delivered to the jobsite prior to installation, make sure it's put under cover, in a clean and dry area.

It should also be stored away from other trade workers and accessways, because the sheets are quite thin and can be easily damaged from bumps or impacts.

The sheets (or pack) should be placed on a smooth level surface, clear of the ground, free from exposure to moisture or abnormal temperature changes.

Below are some more preparation requirements relating to the different types of underlays.

Wood-based underlays

Wood-based underlays are more dimensionally stable than solid timber, because the fibres don't all run in the same direction.

Nonetheless, the boards may still need to be acclimatised to the equilibrium moisture content of the surrounding atmosphere in the room where they will be installed, depending on the manufacturer's requirements.

Some manufacturers recommend that their underlay sheets be spread out in the room to allow the air to circulate around them for at least 24 hours prior to installation.

This preparation is also specified in AS 1884 (Resilient sheet and tiles installation).

Fibre cement underlays

Fibre cement sheets don't need to be acclimatised to the surrounding atmospheric conditions.

Note that fibre cement underlay is not recommended for use over a concrete subfloor; it's designed for wood-based subfloors only.

If you're laying resilient coverings or tiles that require a mortar bed onto a concrete subfloor, you should prepare the substrate with a cement-based levelling compound.