There are two chief types of floor used in modern house building; solid and suspended wooden. In their simplest forms a solid floor is a concrete slab laid to the required depth, while a suspended floor consists of wooden joists slung between two opposing walls with chipboard sheeting or floorboards laid on top. The upper stories of a dwelling will almost always be of the suspended type with a solid ground floor.
Traditionally, in older properties, the ground floor would also be suspended to avoid the dual problems of damp and cold penetration. Modern damp proof courses and insulation materials avoid this. A solid floor also obviates the need for wood-rot preventing underfloor ventilation, so reducing heat loss, and being cheaper and simpler to construct.
A solid ground floor is actually made up from a number of different layers, rather than one single slab of concrete. First the ground is dug out and infilled with a compacted layer of hardcore, the rough surface of which is blinded with a thin layer of sand. The sand fills the gaps and, when rolled flat, gives a smooth surface on which to lay the damp-proof membrane (DPM.) The DPM must be joined in to the damp proof course of the walls to provide continuous, effective protection. and is followed by the concrete which is laid in a layer of between 100mm and 150mm (4" to 6") thick. Insulation is provided by a layer of flooring grade polystyrene or other proprietary material which is topped off with the final layer of sand and cement screed set to a thickness of 44mm (1¾") to give the floor a smooth finish ready to be covered.
Wooden joists used for flooring application are different from most other timber used in construction and must be cut from structurally graded softwood which will have been checked to ensure there are no flaws or weaknesses in them that could endanger the structural integrity of the finished floor. These joists will be individually stamped to prove their suitability for the application. They are 50mm (2") wide and available in a variety of thicknesses depending on the unsupported span of the floor; the longer the span the thicker the joist.
Although accurate calculations are often used to give a definitive thickness it is possible to gauge the joist size with this rule of thumb to give the thickness in inches:
(Span in feet / 2) + 2 inches so for a 12' span: (12 / 2) + 2 = 8"
For conversions of existing buildings where top-floor room height may be at a premium, the depth of the joist may be adjusted according to the likely loading of an area and therefore allowing an extra inch or two of headroom. If the loading is likely to preclude such a reduction it may be necessary to use Rolled Steel Joists (RSJs) instead. Structural engineers will be able to provide accurate calculations as to how the end result can be achieved safely.
The ends of the joists are fitted into purpose made galvanised steel hangers which are built into the inner leaf of the cavity wall. This allows the hangers to be set as the wall is built but without the need to actually have the timbers put into position until much later. There are a variety of hangers available depending on the requirement, but the general principle is the same for most of them with the brackets placed squarely along the top of a course of bricks or blocks at regular intervals, usually 400mm (1' 4".)
Where cross joints need to be made between joists away from the wall hangers, for example around a fireplace or a staircase, galvanised framing anchors are used to avoid the need for complicated mortise and tenon joints. There are a wide range of these available which come in both left and right handed versions depending on the joint being made.
Suspended wooden floors are usually finished with a top layer of chipboard sheeting as it's cheap, quick to fit and, being man-made, will not shrink or cup. However, in recent years, the traditional floor board has made a resurgence with the increasing popularity of real wooden flooring. Made from softwood, they are usually 150mm x 25mm (6" x 1") in cross section and 2.4m to 4m (8' to 13') in length. They are simply nailed into place and can then be finished with varnish or woodstain. As an alternative to buying new it is also possible to obtain reclaimed floorboards to lend some immediate character to a new build.
Where suspended wooden floors are used for the lowest floor of a building, or over a garage, they must be insulated to meet current building regulations U-Values, currently 0.25, in order to prevent excessive heat loss through them. Sheets of slab insulation material cut to size and suspended between the joists is perhaps the easiest way of achieving this. For more information on this see the Insulation section.
Jackson Building Centres supply JJI silent floor joists from James Jones - these offer a number of advantages compared with traditional joists and are certainly worth considering for a new build. No squeaky floors, ever!
James Jones & Sons Ltd manufactures structurally engineered timber joists - the JJI-Joist - combining high-grade softwood with an engineered composite panel. These are standard building components used in commercial and residential construction as load bearing or non load bearing members for roofs, walls and floors.
JJI-Joists are available in a range of sizes familiar to the UK Construction Industry. Depths range from 145mm to 450mm and widths from 45mm to 97mm.
Due to the unique form of the JJI-Joist British Board of Agrément Approval, it is possible to design and produce large volumes of non-standard JJI-Joists to allow for particular span and depth situations that cannot be covered by the standard range.
The manufacturer has a special interactive section on their website where you can cross-refer your span and load conditions and receive a list of joists suitable for the application. Please click on the link to access this feature - Specify my JJI-Joist.
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