The two most common types of subfloors are:

• concrete slab - which may be on-ground or suspended above the ground
  
  
• timber or steel framed - which use floor joists to support structural sheet flooring, such as particleboard or plywood.

In some cases, the surface of the subfloor is suitable for a floor covering to be placed directly on top.

However, where the surface is rough or has other imperfections in it, an underlay or underlayment is placed in between the subfloor and the covering.

Underlay is used to provide a smooth even surface for the floor covering to lay on.

It sits on top of the subfloor and can be made from either a soft cushioning material or a board product.

When the covering is carpet or other textile materials, a soft underlay is used, such as sponge rubber or foam.

For vinyl and other resilient floor coverings, a hard underlay is generally better, made from hardboard, plywood, MDF or fibre cement.

Underlayment performs the same job as underlay, but consists of a smoothing compound or cement-based levelling compound that's spread on top of a concrete subfloor.

The substrate is the layer immediately under the floor covering.

If the covering is being installed directly onto the subfloor, the substrate is the subfloor.

If an underlay is going in between them, the substrate is the underlay.