Extensions

A floor will need to provide for one or more of the following:

• Structural support of the room’s contents and users and the weight of the floor itself; and
• If the floor is a ground floor, provide resistance to:
  • Ground moisture; and
  • Heat loss (thermal insulation)

There are three general types of ground floor construction:

Solid floor

A typical way of constructing a solid floor would be to provide a base of hardcore with sand blinding, with a layer of concrete over that. To ensure a level finish to the floor, a layer of screed is added over the top of the concrete, which consists of sand and cement. A suitable gauge damp proof membrane (DPM) and thermal insulation must be provided. These can be laid over the sand blinding or on top of the concrete.

The DPM should be lapped on to the damp proof course in the external walls and, if relevant, internal walls around the floor. Thicknesses of the various parts of the floor will depend on ground conditions and the order in which they are laid. If the existing house has air bricks, ventilating existing floor voids for example, then ducting should be provided to allow air through the solid floor and into the void under the existing house. Air bricks are then placed in the new wall.

Suspended timber floor

As a requirement of the Building Regulations the structure should be protected against the growth of weeds and other plant-life. The ground should have a layer of concrete poured across and there should be a ventilated gap of at least 150mm between the underside of the timbers and the concrete, to prevent moisture gathering and affecting the condition of the joists. The timber floor joists should be sized correctly depending on their span (length between supports) and are normally laid across the shortest span from wall to wall with a gap underneath. 

An intermediate wall with a small foundation may be needed to reduce the span and keep the thickness of the floor joists to a minimum. A damp proof course (DPC) should be placed between the timber and the wall. Insulation is then placed between the joists (thickness depends on the product used). Air vents should be placed underneath to provide ventilation to the void and the air should be able to travel from one side of the building to the other.

Suspended concrete floor

This construction is similar to the timber floor above, but uses either pre-cast concrete planks or small pre-cast concrete beams with concrete blocks laid between the beams. They can normally span greater distances than timber joists. Ventilation is required in the same way as a suspended timber floor. The manufacturers may well work out the size of the concrete beams and provide the structural calculations. Otherwise a structural engineer can also provide this service.

Contaminated ground and radon

In some areas, the ground could have a certain amount of contamination where gases form, such as from landfill sites. Radon is a naturally occurring gas which is found in certain areas of the country. These gases need to be ventilated and a gas membrane will be required to stop them from entering the building.