In older houses both the ground and upper floors are likely to be of ‘suspended’ construction, with floorboards or sheet flooring supported on a series of wooden joists. In homes built after about 1940 it’s more likely that the ground floor will be of solid construction: essentially a concrete slab built up on top of the ground.
Suspended upper floors
The main load-bearing components of any suspended floor are the joists: substantial timbers placed 400 to 600mm apart which span the space between the walls of the house.
On upper floors the joists are supported by the main walls of the house. This is achieved either by building the ends of the joists into the masonry or by using joist hangers, which are built into or fixed to the face of the wall. Where the joists meet the walls, short lengths of joist material, called noggings, are nailed between the joists to support the edges of the flooring.
If the joists span more than 3m additional stiffening should be provided to prevent them twisting. This may take the form either of further noggings or angled timbers known as herringbone strutting fixed at mid-span. If the joists span more than 4m, additional timber or steel cross-members known as binders may be required.
The ceilings of rooms on the floor below, whether lath-and-plaster or plasterboard, are generally fixed to the underside of the joists.
Suspended ground floors
The joists of suspended ground floors may be supported by the house walls, either by building the ends of the joists into the masonry or by using joist hangers. More commonly, though, ground-floor joists rely on sleeper walls around the perimeter and at intervals beneath the house. To keep the joists dry, each sleeper wall must have a damp-proof course between the masonry and the wall plate – the piece of timber on which the joists rest.
Damp is the great enemy of suspended ground floors, and the space beneath the floor must be well ventilated. Airbricks in the outer walls allow air to pass beneath the floor, and sleeper walls are often built in honeycomb construction, with spaces between the bricks which allow air to circulate freely.
Solid floors consist of a series of layers built up from ground level. In modern homes solid floors should always incorporate a damp-proof membrane which is linked to the damp-proof course in the walls. In older homes solid floors may rely on a water-resistant final layer, such as quarry tiles, to keep damp at bay.
Solid floors can be up to 400mm thick. The first layer is hardcore, well graded and firmly compacted, about 150 to 200mm thick. The hardcore is then ‘blinded’ with sand or a weak mixture of sand and cement, to provide a smooth surface for the damp-proof membrane, usually heavy-gauge polythene. The main load-bearing element – the concrete slab -comes next, between 100 and 150mm thick. Finally, a finishing screed of sand and cement, about 40mm thick, is applied to produce a smooth surface.
As an alternative to placing the damp-proof membrane under the slab, it can go between the slab and the finishing screed, though this calls for the screed to be about 25mm thicker, fn either case the damp-proof membrane must be neatly folded up behind the skirtings and, ideally, tucked into the walls to link up with the damp-proof course.