Floor Construction - NEW
Peter Eade offers a general rundown on floor construction, both at ground level and upstairs.
Until the middle of the last century, there were only two types of ground floors used in residential housing: solid concrete slab floors or timber suspended floors. Solid floors were made up of crushed hardcore laid over with a concrete slab; there would probably be no damp-proofing or thermal insulation.
Timber suspended floors were constructed on an earth subfloor with brick sleeper walls. These walls were built on strip concrete foundations to which were bedded the damp-course and timber plate, and then floor joists. The floorboards were then nailed onto the joists. Underfloor ventilation was provided by airbricks built into the external walls. Floors to the upper rooms were constructed of timber joists and floorboards with no soundproofing.
Today there is a much wider choice when it comes to deciding which type of floor suits your requirements.
Solid Ground Floors
Solid concrete floors are constructed with 150mm of a crushed hardcore base laid to levels onto which there is a layer of sand blinding, this being done before receiving the damp proof membrane. The DPM is then laid over the whole of the floor and is lapped up to the external wall damp proof course. Once this preparation work is completed the over-site concrete can be laid to a minimum depth of 125mm and tamped to levels.
Diagram showing solid ground floor construction
Thermal insulation is provided by 100mm of floor insulation which is either laid before pouring the concrete or placed onto the concrete slab. If the insulation is placed below the concrete the insulation must turn up the edges of the concrete to avoid thermal bridging. If the insulation is to be placed above the concrete then this is done later, once the house is roofed and weatherproof.
Suspended Concrete Floors
Concrete suspended floors are made up of reinforced concrete beams which are dry laid on the damp-proof course on the inner skin of the external cavity walls. The beam manufacturer provides drawings showing the pattern of where the beams are to be positioned.
The subfloor under the beams is levelled subsoil and must be a minimum of 150mm below the underside of the beams. Ventilation to this void is provided by periscope-shaped air vents through the external walls. If the house is being built in an area where there could be soil contamination or possibly radon gas, the earth forming the subfloor will need to be overlaid with a gas-proof membrane.
Diagram showing beam and block ground floor construction
The concrete beams are shaped a bit like a top hat with ledges on either side to support the concrete floor blocks. These are laid between the beams to form the floor. Once the blocks are laid, a dry mix of sand and cement is generally brushed over the surface of the floor to fill any gaps between the blocks and the beams. At this point, nothing further is done until the house is roofed in and waterproof. The thermal insulation can then be laid onto a polythene vapour barrier ready to receive the cement floor screed or timber floor decking.
There is now an alternative to using concrete blocks between the beams which offers a high level of thermal insulation. Several companies produce insulated beam and block systems. Once the concrete beams are in position, preformed EPS insulation blocks are then laid between the beams ready to receive the underfloor heating circuits. The floor is then finished with a structural cement screed, usually to a depth of around 75mm. The advantages of this type of beam and block floor are the increased thermal insulation and a lower finished floor level.
Timber Suspended Floors
It is still possible to have a traditional timber suspended floor much as used in Victorian times, the general design is still the same with sleeper walls, joists and floor boarding. The only difference being the requirement to have a solid concrete subfloor over a damp proof membrane and thermal insulation laid between the joists on battens.
Timber Upper Floors
All floors above the ground floor are usually constructed with timber floor joists onto which the floor decking is laid. The decking can be either tongue and groove soft/hardwood boarding or tongue and groove flooring-grade chipboard. It is prudent to use moisture-proof chipboard. Soundproofing is achieved by placing a minimum of 100mm of mineral between the joists.
Concrete Upper Floors
Concrete upper floor construction is an alternative to timber joists, and there are several reasons why this is becoming popular. The first is that concrete offers greatly improved acoustic performance, particularly when you consider noises associated with children playing online video games in their bedrooms. Underfloor heating can be installed in much the same way as it is on the ground floor. The common problem of squeaky floors caused by the thermal movement of timber is completely eradicated. There is also a much wider choice of floor finishes. And the fire rating of concrete is far higher than that of timber.
The most popular form of heating is currently underfloor heating on the ground floor. There are also several underfloor heating systems available for suspended timber ground and upper floors, both electric and wet. Steel panel radiators are still an option and are available in a wide choice of styles and designs. And if you decide to have underfloor heating upstairs, it is well worth checking online to learn more about your options.
Timber joists are common for upstairs, seen here with underfloor heating installed.
Underfloor heating to the ground floor is straightforward, particularly if the floor insulation is laid above the oversite concrete. The heating circuits are held by clips that are easily fixed to the rigid foam insulation and, once the pipes are full of water and tested, the cement screed can be laid ready to receive the finish floor covering. Don’t forget that if the final chosen floor finish is to be either nailed or screwed into the floor, the heating pipes are just below the surface.
Metal Web Joists
Metal web joists are a popular flooring option for selfbuilders. Made from two lengths of strength-graded timber, joined together with engineered steel webs, the joists offer strength and flexibility.
Multiple joist depths are available to create a stiffer floor at shallower depths and with greater load-bearing capacity compared to conventional options. Their rigidity also means the joists can span long distances, their light weight making them relatively easy to handle. The wide timber chords mean they stay in place prior to decking out, making it easier to get a positive floor or ceiling fixing.
According to Richard Pashley, from Engineered Timber Solutions, another benefit is the ease of service runs due to the open web nature of the joists, with no additional cutting or drilling required to fit services once installed. “Once the metal web joists are installed there is minimal shrinkage, which reduces movement and therefore the potential for any squeaks,” says Pashley. Some metal web joist manufacturers offer a site measuring service and can create end details for the joists to keep them within the existing roofscape – useful for loft conversions.
This Beginner's Guide to Floor Construction is from the August 2021 issue of SelfBuild & Design magazine.