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Different types of ground floor suspended flooring

Suspended timber floors need to have spaces underneath ventilated via air 'bricks' through the outer walls and gaps in any internal walls so that the air can move across the building underneath the floors to prevent the build up of moisture in the timber which could lead to fungal attack.

A typical suspended floor is shown below in Fig 1 with the joists resting on sleeper walls at right angles. Air bricks through the outer walls, facilitate air movement.

Suspended timber floor using sleeper walls

In old properties (1910 to 1980 ish), air bricks were often fitted just in the outer brick skin with a brick left out in the inner skin, thus the air brick vented both under the floor and the wall cavity, but modern regulations require the air brick to bridge the cavity and just ventilate under the floor.

In older properties (pre 1910), air bricks were not always fitted - in such properties, they should be retro fitted where possible.

Fig 2 shows the side view of the sleeper walls with the bricks arranged to allow air movement.

Sleeper walls and joist arrangements for suspended timber floors

The floor of any size of room can, in theory, be constructed using sleeper walls as the joists are supported along their length.

However, where the space between the concrete base and the floor is greater than 45cm, Fig 3 shows an alternative arrangement where the joists are supported in the inner wall, the size of joists is dependant on the span of the floor and there will be a limit on the span that can be accommodated but this is unlikely to cause an issue in a domestic property.

A stepped air brick is shown just to illustrate an alternative to the straight style illustrated in Fig 1 - there's no advantage in either type although the stepped style may be required if there is a high external ground level.

Suspended timber floor supported by wall

Any timber suspended floor should have insulation between the joists, in England and Wales the 2014 requirement was a U value of 0.11W/m2K for new build, 0.25W/m2K for refurbishments.