How Floors are Constructed

When the time comes to replace or change a floor covering, this is the perfect opportunity to check that your floors are structurally sound, looking for signs of localized damage, woodworm infestations and ensuring that the surface is level. Indeed, whatever floor covering you choose, whether it is soft carpeting or cushioned sheet flooring, or hard ceramic or stone tiles, you will not achieve the first-rate effect you seek without careful preparation.


Before deciding what needs to be done to your floors and the best way to treat them, it helps if you understand how floors are constructed -the different materials used and the ways they are put together. According to a commercial construction contractor, floors can be divided into two sorts: solid and suspended.

Floors Construct


Based on materials supplied by Atlanta Concrete Contractors, a  solid floor is what it says: solid. It consists of a sand and cement screed laid over a solid concrete slab, which in turn sits on a hardcore base. Hardcore is the name given to a mixture of stones, clean broken brick and gravel which evens out the ground surface after it has been excavated and provides a firm and level base for the main concrete slab. Normally, the 100 to 150 mm (4 to 6 in) hardcore layer is ‘blinded’ with a 50 mm (2 in) layer of finer material (sand and fine gravel) which gives a smooth even surface on top of which is laid a damp-proof membrane (DPMI – typically heavy-duty (1000 gauge) polyethylene sheeting. This membrane is essential to prevent damp rising up through the floor slab and it is tied in at the edges to the damp-proof course (DPC) in the walls, which prevents damp rising up through the brickwork. Sometimes, the DPM is laid under rather than over the floor slab.

The floor slab itself is 100 to 150 mm (4 to 6 in) thick and, in modern houses, a 50 mm (2 in) layer of expanded polystyrene is put between this and the cement screed to ‘provide insulation. The screed is generally 50 mm (2 in) thick or 63 mm (2V2 in) thick if the DPM is laid over the floor slab.

Older houses will not have any insulation and many will not have a damp-proof membrane – or, if they do have one, it may have failed.


A ‘suspended’ timber floor consists of two parts: the floor joists running one way and the floorboards running the other. The floor joists are supported by the outer walls of the house – and, for ground floors, by ‘sleeper’ walls – either by being put into holes left in the house walls or with joist hangers secured to the surface of the wall.

Timber wallplates are used on top of the sleeper walls, which must incorporate a damp-proof course (DPC). Joists used to support upper floors are larger than those used to support ground floors, because they have a longer unsupported span, and struts are generally fitted between the joists.

Timber floorboards are the traditional way of creating a suspended floor; in modern houses, they will often be replaced with sheets of flooring-grade chipboard (particle board). Both floorboards and chipboard sheets can have square edges or be ‘tongued and grooved’ – that is, a tongue on the side of one board (or sheet) fits into a groove on the edge of the neighbouring board or sheet. Tongued-and-grooved boards have the advantage that they fit together more securely without a gap for draughts; their disadvantage is that they are much more difficult to take up in the future.

The joists which support upper floors also provide support for the ceiling of the room below – typically sheets of plasterboard (dry wall or gypsum board) nailed to the underside of the joists.


The space underneath floorboards or flooring sheets is used in houses to run essential services – electric cables, gas pipes, water pipes and central heating pipes. In a solid floor, similar services may be embedded in the concrete or laid inside a duct – and in some houses, special warming cables may have been installed in a solid floor to provide underfloor heating.

Because of the presence of electric cable or some kind of pipe below a timber floor surface, great care needs to be taken when drilling into the floor, putting in floorboard nails or using a saw to take up floorboards or floor sheeting. A simple metal detector can be used to determine the exact position of pipes and cables.

Floors Construct

Underneath suspended ground floors, pipes and cables can be kept below the level of the joists, but underneath upper floor suspended floors, they must be run in the gap between the flooring and the ceiling underneath. Wherever possible, pipes and cables should be run parallel with the joists and clipped to them low down, so that they do not pose any threat to working on the floor from above. Where they have to be run across the joists (that is, parallel with the floorboards), electric cables should be run through holes cut in the centre of the joists, so that they are well away from floorboard nails or screws used to hold down flooring sheets. In a few instances, it may be possible to run water, gas and central heating pipes like this as well, but normally these will have to be fitted into notches cut out of the top of the joist. Ideally, a metal plate should then be fitted over the top of the pipe so that it is protected from nails or screws. In solid floors, pipes and cables should not be laid in direct contact with concrete and should be protected in some way.

For electric cables, the best answer is to bury a length of metal conduit in the floor screed and to run the cable inside that. For pipes, the ideal is to insert a purpose-made duct into the floor screed which has a removable cover so that access to the pipe (and especially joints in the pipe) is possible at a later date should a leak occur. The pipes themselves should be well insulated. In older houses, pipes may have been embedded directly into the concrete.

Filed Under: Home & Maintenance


About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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