Suspended Timber Floors

Suspended timber floors with tassel walls

When fill depth is less than 900mm it is acceptable to build a suspended timber floor supported by tassel walls built off the oversite concrete.

Diagram B47 - Typical detail of timber floor suspended by tassel walls

If the fill depth is greater than 900mm, joists should span from perimeter to perimeter, if required an intermediate support can be provided (i.e. a steel beam). If steel is to be used to reduce the span, steel must be designed by an engineer. The engineer appointed must be qualified by examination, be in private practice, and possess professional indemnity insurance.

Diagram B48 - Typical detail for a fully spanning suspended timber floor

Leaving virgin soil beneath the floor is unacceptable: this would allow moisture to migrate up to the floor structure, resulting in damage floor timber and coverings damage.

A situation where the subfloor is lower than ground level should never be allowed to happen. The result is a sump effect where water builds up under the floor as detailed in the diagram below.

Diagram B49 - Subfloor below ground level - Sump effect

To avoid ‘sump’ effect, ensure the top surface of concrete is above the highest level of the surface of the ground or paving at the external wall.

Suspended floors without tassel walls

Where tassel walls are not used, joists span from external walls to internal block partitions. In situations where the ends of the floor joists are built into masonry walls, the ends of the joists should be treated with 2 coats of preservative. When applying the preservative it should be brushed on liberally to ensure it protects the joists fully.

Diagram B50 - Suspended floors without tassel walls

Where a dwelling consists of a mixture of suspended timber floors and solid floors, it may be necessary to lay pipes beneath the concrete subfloor. These pipes which would be connected to the external ventilation openings would serve to allow through-ventilation of the subfloors.

To avoid the ‘sump’ effect, ensure the level of the top surface of concrete subfloor is not lower than the ground level at the external wall. Preservative should be applied to the end of joists where built into masonry.