Suspended Timber Floors

The majority of older houses in Britain have suspended timber floors (also known as hollow floors) in the ground-floor rooms. However, during and after the Second World War, timber became scarce and restrictions were placed on its use and availability, so other forms of construction were employed. The solid floor was introduced as a suitable and economic alternative at ground level.

Suspended timber ground floors

A suspended timber ground floor consists of a number of boards sometimes tongued-and-grooved – or sheets of man-made boards such as chipboard or plywood, laid over and supported by timber joists. The joists are 400mm to 600mm apart and are supported by 100mm x 50mm timber wallplates which are in turn supported by the main walls of the building, or by sleeper walls built up from a layer of concrete beneath the house, or by a combination of the two. To prevent the timber floor from absorbing any moisture, the wallplates are bedded on top of a suitable damp-proof course.

Suspended timber upper floors

Like suspended ground floors, upper floors consist of timber boards or sheets of man-made board laid over and supported by timber joists. Suspended timber floors which are not at ground level are often called single floors because the joists bridge a single span they run from wall to wall.

As it is not possible to give additional support to upper floors by using sleeper walls, the joists of single floors are bigger than those of hollow ground floors and usually bridge the narrowest span – often across the narrowest part of the room. If the joists bridge spans greater than 4.0m, timber or steel cross-members (called binders) may be used to give intermediate support. Joists in a single floor tend to flex, so rows of struts may be laid across the floor between the joists to make the floor stiffer.

The ends of the joists may be built into the walls, or supported by joist hangers built into or fixed on to the face of the wall. If the joist end is built into the wall, this part should be treated with preservative to protect it from decay.

The ceiling of the room underneath the floor is usually lath-and-plaster or plasterboard fixed to the underside of the joists. Plasterboard may have a plaster ‘skim’ finish.

Solid ground floors

The cross-section of a solid floor consists of a number of layers of different materials.

The first layer is 100 to 150mm of consolidated hardcore com¬posed of crushed stone and clean broken bricks. This is to level out any unevenness in the ground caused by the excavation and to provide a firm and level base for the floor slab.

A 50mm-thick layer of fine ash, sand or weak mix concrete is laid over the hardcore to bind the surface. This blinding will also give a smooth even surface for laying the damp-proof membrane on. The damp-proof membrane should never be laid directly on to the hardcore – it may puncture.

The damp-proof membrane may be placed above or below the floor slab. In the latter case, the DPM will keep the floor slab free from moisture. A wide range of materials are suitable as DPMs: hot and cold poured bitumen, asphalt, epoxy pitch compounds and 1000-gauge polythene sheeting.

The floor slab (concrete) can vary in thickness from 100 to 150mm and have one or two layers of mesh reinforcement, depending on the area of the floor and its intended use. The concrete slab may have a cement screed laid over its surface, depending on what type of floor finish is to be used.

To provide extra insulation, 50mm sheets of expanded polystyrene can be added between the slab and the screed.