Timber floors: installation and ventilation

Kept in suspense

17 September 2018

Although suspended timber ground floors are a simple detail, ensuring they are properly ventilated makes installation more complex – as Amy Allen explains in the first of a series of articles on common site issues and how to resolve them

Suspended timber ground floors are a relatively straightforward construction detail, but one that, if installed incorrectly, can be disastrous for the client. There are thus multiple key points to note.

Commonly used in domestic projects, suspended timber ground floors are usually installed to match an existing floor construction and ground levels.

The first thing to appreciate is that a floor of this nature is suspended over a ventilated void with a concrete layer, or oversite, to the ground surface. It is essential that the void is ventilated with airbricks to opposing walls, ensuring good air flow to prevent condensation, and that there is also a damp-proof course beneath the timber structure in connection with any masonry.

Working from the ground upwards, the following should be ensured:

  • there is a concrete oversite covering the ground surface; to prevent vegetation growth and moisture ingress, this should comprise either 100mm-thick concrete or 50mm-thick concrete laid on a 1,200-gauge damp-proof membrane;
  • the surface of the oversite layer is not lower than surrounding ground levels, to avoid natural water ingress; if the sub-floor is lower than surrounding ground, then the void should be tanked, though a drainage solution may be feasible depending on the extent;
  • the sub-floor void must be at least 75mm deep from oversite to wall plate and 150mm deep from oversite to underside of floor joists;
  • the sub-floor void must be ventilated with perimeter airbricks at a minimum of 150mm above ground level, to reduce the likelihood of water ingress or blockage, and these should be fitted in opposing walls at not less than 1,500 sq. mm of ventilation per metre run of wall or 500 sq. mm per square metre of floor area, whichever is greater; this should prevent condensation and, in due course, wet or dry rot;
  • ventilation must have a free, unobstructed path to all areas, so any supporting walls in the void need to be of honeycomb construction; consideration should also be given to any areas that may not receive sufficient air circulation, such as back corners of extensions, which may require ducting;
  • damp-proof course must be incorporated to the walls beneath the timber, to provide separation from moisture from the ground;
  • the ground-floor structure must be insulated in accordance with Approved Document L, typically to a U-value of 0.22 Wm-1K-1; and
  • for domestic extensions, insulation is critical to reduce thermal bridging and condensation, demonstrating a direct link between Parts L and C.

Figure 1, taken from Approved Document C, shows the basic requirements described above.

Approved Document C

Figure 1: Approved Document C, diagram 5 © Crown


If mould on the surface of the timber is only a cosmetic issue, then it may be possible to treat it to prevent further spread. However, to remedy the issue effectively and prevent the mould returning following a treatment, you would also need to increase ventilation, and install a ground oversite and damp-proof course.

The complexity of installing a damp-proof course in an existing floor would likely require the floor’s replacement anyway, to allow for all the above points to be properly addressed.


This construction detail could typically be included as evidence against the following competencies:

  • Building control inspections: having the skills to carry out inspections of building work to ensure that it meets relevant performance requirements
  • Construction technology and environmental services: having clear understanding of the design and construction process and detailed knowledge of construction solutions.

In carrying out a building control inspection, you should be able to record evidence of inspecting a property at an appropriate stage, that is, at damp-proof course level, to demonstrate that you have achieved Level 1 of the competency. In inspecting at that stage, you would then draw on your knowledge to identify the shortfalls in construction to give evidence of having reached Level 2.

Also when carrying out a site inspection, you could demonstrate how you have applied your knowledge to carry this out and identify shortfalls in construction, again to demonstrate you have reached Level 2. Advising on courses of action to remedy the issues, providing options where feasible, would meanwhile show you had attained Level 3.

Case study


In image 1, timber is in direct contact with the masonry wall, there is no evidence of a damp-proof course and the ground covering is rubble fill, which isn’t suitable. Timber would likely be exposed to damp conditions.


In image 2, the limited number of airbricks are blocked, and mid-support walls are largely solid; air circulation will be poor.


In image 3, the result of these failings is extensive mould spread across the whole underside of the floor. If this continued, it would eventually lead to wet or dry rot and structural damage, and the floor would become inadequate, needing full replacement.

Image 1

Image 1

Image 2

Image 2

Image 3

Image 3

Amy Allen is a director at Assent Building Control

Further information