Early Timber Floors

The majority of houses built from the latter part of the nineteenth century until the mid-twentieth century had ground floors constructed from timber. Exceptions to this would be basement rooms, where damp could damage the timber, and rooms which were likely to be wet due to their function, e.g. kitchens and sculleries.

Timber ground floors – often referred to as suspended timber floors – consist of a series of joists supported by loadbearing walls and covered with floorboards. The size of a joist depends largely on its span; as its length or span increases, so must its depth in order to safely support the load imposed upon it. Deep joists are expensive and to reduce joist size intermediate supports known as sleeper walls are introduced. These are shallow masonry walls built directly onto the ground or on small foundations. In practice, ground floor joists are significantly less deep than those used in upper floors, where such intermediate support is not possible because of the habitable rooms beneath.

The plan (previous page) shows the ground floor layout of a typical late-Victorian terraced house. The joists run from side to side (front to back is another option) and are supported on the party walls, internal loadbearing partitions and intermediate sleeper walls. The joists are usually at 350–400mm centres – this offers the most economic arrangement.

In practice, such floors can often give rise to maintenance problems due to poor design and varying standards of workmanship. The following should be read with reference to the drawing above, which shows part of the floor in detail.

- It was formerly common to build the joists into the external wall to provide end support. This practice carries the danger that moisture from rising or penetrating damp can penetrate the end of the joists, causing rot. Some properties have slightly thicker walls below floor level and this gives improved protection against penetrating damp.
- Until the turn of the twentieth century damp proof courses were rare. Consequently, rising damp can attack not only the joist ends, but also the middle of the joists where they sit on the intermediate sleeper walls.
- The level of the earth or rubble infill under the floor was often below external ground level and in wet conditions, or areas with high water tables, the underfloor space was often permanently damp.
- It is important to provide ventilation to these floors in order to try to keep the underfloor space dry. In the past this was usually achieved by a series of cast iron or terracotta vents positioned just below floor level. Over the years the vents can become blocked, either through changes in external ground level or just general accumulation of debris. The reduced ventilation can lead to excessive levels of damp, which ultimately can cause outbreaks of rot.
- The sleeper walls could also impede ventilation through the floor, although in better quality construction these walls are fitted with vents or built as honeycombed walls.
- Due to the limitations of woodworking machinery, square-edged boards were common until the mid-twentieth century. Over time the boards would shrink slightly as their moisture content stabilised, resulting in small gaps appearing between the boards and draughts in the downstairs rooms.

During the early part of the twentieth century the construction of raised timber floors improved. The drawing below shows a typical example of good-quality construction typical of the 1930s to 1950s. The defects mentioned previously are less likely to occur.

In particular note that:

- the entire floor is separated from the substructure by the DPCs
- the bare earth is covered with a concrete slab (often referred to as ‘oversite’) which is at, or above, external ground level to prevent the build-up of water
- the floor joists are usually supported by honeycombed sleeper walls, enabling cross- ventilation; additionally, the joists do not touch the external wall
- vents are provided, clear of ground level, and sleeved to prevent cold air entering the cavity.