What do we mean by upper floors?
Upper floors, otherwise known as intermediate floors, relate to the second storey and above of a typical domestic dwelling and most will only have a second or third storey to consider. These floor structures provide the formation level at each subsequent storey and have to be designed with two specific loads in mind; first is their own weight plus any roof or wall loadings from above, otherwise known as the dead load, and the second is an anticipated imposed load which will include occupants (people) and typical furniture. Standard imposed loads are assumed to be 1.5 kN/m² (153 KG) which should cater for most occupant scenarios but extra discrete consideration should be given for excessively heavy items like water tanks, large baths, pianos, etc. Raves and parties, especially with bouncing teenagers, will definitely challenge these standard design loads and due consideration should be given if this kind of use or entertainment is anticipated. If you are building a block of flats then the common areas will have different and increased minimum standards for compliance.
Solid timber joists
Solid timber joists are probably the most well understood intermediate floor structure and generally the lowest cost option. These joists can be used with any building system with generic details for how each joist should be supported for each of the better known systems including masonry cavity wall and timber framed panels. When it comes to SIPS and ICF, each of the system manufacturers will have their own recommended solutions which may involve mechanically fixed timber ledges, hangers and straps. Span tables are available through the NHBC (and others) which provide guidance on the sectional size and grade of the timber required to achieve a clear span. Typically, the maximum clear span you would ever be able to achieve with timber joists is circa 5m unless you specify more unusual thicknesses which will come at a much higher cost. Strutting is also required in a central position for modest spans but with up to three rows of strutting equally spaced for larger spans. Diagonal, herringbone or criss-cross struts are by far the most effective in stabilising the joists but many contractors prefer to use solid blocking as this is quicker to install. Most two storey domestic dwellings using timber joists will have a single layer of plasterboard fixed to the underside of the joists to form the ceiling below, 100mm of mineral wool insulation between the joists (supported by the plasterboard) and then 18 or 22mm tongued and grooved chipboard to form the floor deck above. This would be considered a standard entry level specification.
Engineered joists
A popular option, especially with system build, is the use of engineered joists. These normally fall into two categories, being I-Beams or metal-web joists. The I-Beam incorporates a solid web using OSB boarding or similar with solid timber rails top and bottom to form the beam flange. The metal-web joist is effectively two lengths of timber acting as the top and bottom of the beam with continuous herringbone metallic bracing tying the two together with perfect dimensional control on both sides. Both types of joist are capable of spanning further distances than solid timber but there are no standard span tables and each system manufacturer will design their joists size and frequency based upon the layout drawings provided.
An additional advantage with metal web joists is the ease with which services can be channelled through the large web voids created by virtue of the metallic bracing. These joists can happily sit in hangers or be supported by ledges or, with timber frame, built into the external panelling just like solid timber. Where joists run parallel to external walls, restraining straps may also be incorporated and continuous bracing through the webs to tie all of the joists together will also be necessary. So, they span further, are lighter and easier to handle but they are a more expensive option at perhaps a 30/40% premium over solid timber joists.
Concrete beams with infill blocks
Concrete block and beam floors can make excellent intermediate floors but they are only suitable for masonry cavity wall or solid concrete panel external wall construction due to their mass. This extra weight is, however, one of their key benefits providing excellent thermal mass in the middle of the building and improved acoustic performance between storeys. They will add to your budget as the supplier costs will be higher as well as the installation costs and additional plant requirements for lifting. In addition, the first floor decking is best provided as a screed and so this can also be a popular choice for those wanting underfloor heating upstairs. You’ll need to think about how to run your services through the floor and typically the plasterboard ceiling below would be hung off brackets fixed to the concrete beams to create a useable service void but the formation of this also adds additional expense.
Cassette floors
A cassette floor is a generic description for a large piece of floor that has been factory produced. Concrete cassettes are quite common and have been available for some time. However, timber frame manufacturers in particular are also known for prefabricating large floor sections which can be craned into position and bolted together. Those with links to the volumetric market will be especially keen on this kind of floor option. So, if you’re using system built you could talk to your manufacturer about cassettes being an option.
Structural floor decking materials
For joisted floors there are several options at your disposal. Most common would be tongued and grooved chipboard at either 18 or 22mm in thickness with 22 being eminently more preferable. However, the OSB manufacturers (Orientated Strand Board) also produce floor decking in 18 and 22mm options and some believe this is preferable and thereafter there is plywood which is generally the most expensive option.
Some are treated so that they are classed as moisture resistant and others come with a factory applied PVC coating which can make them storm proof. Given that we erect our buildings in all weathers and that decking is required as the building storeys are completed (but before the building is watertight), this level of weather protection makes great sense. Those of you opting for acoustic floors will be less concerned as you will be applying additional substrate layers once the building is in the dry (see forthcoming article on improving sound insulation).
Running your services
Service voids have to be provided and your choice of floor structure will influence how this is best achieved. With solid timber joists and I-Beam engineered joists, holes will need to be cut in the webs; engineers don’t like this and there is clear guidance available, especially from the I-Beam manufacturers, on how to minimise the loss of integrity in the joists by carefully deciding on your hole diameters and locations. Some people add counter battens on to the underside of the joists to create a void in which small services can be run.
One of the great advantages of the metal web joists is the larger diameter services which can travel through their webs. For short distances, ducting from extractor fans can be positioned and parts of the installation for mechanical heat recovery can also potentially get lost in the floor. However, and as described above, a discreet service void is necessary on the underside of a concrete block and beam floor.
Incorporating underfloor heating
Underfloor heating can readily be positioned between both solid and engineered joists. Usually this involves a metal spreader plate which spans from the top of each joist to its neighbour and has a pipe recess to conceal the underfloor heating pipes. This plate should be insulated directly below and also firmly supported so that it does not sag and thus keeps the metal plate in direct contact with the floor deck above for maximum transference.
A neat alternative idea with a timber joisted floor is to put 50mm timber battens on top of your floor deck at 400mm centres, lay your underfloor heating pipes between the battens, clip them to the deck and then cover with a lean mix screed. You can then lay a thin plywood layer over the top to regulate your final floor finish. The advantage of this is the extra density for acoustic gain as well as great dispersion of heat through the screed. But the extra weight needs to be calculated at the outset as it increases the floor’s dead load and the overall floor thickness will increase which will affect your staircase and door threshold heights.
Eliminating squeaks
Squeaks arise in timber floors from a combination of factors; first, is the deflection in the joists causing movement in the floor under load and, second, could be inappropriate strutting (or blocking) allowing the joists to finely twist. Third, could be the method and type of fixings used to secure the floor deck to the top of the joists. Fourth, might be the natural drying process of the intermediate floor once heating has been applied and where moisture contents might reduce from anywhere between 12-14% (at the time of construction) down to 8-10% when fully dried. Being conscious of all of these elements will help to reduce the risks of eventual floor squeaks as it is mightily difficult to cure once the house is finished and your final floor finishes are down!
Written and published in March 2020
