The subfloor, referring to the surface over which flooring will be installed, requires preparation before the actual installation of the flooring.
Preparation procedures necessitate that prior to the installation of the flooring, the installer must ensure that the subfloor is sufficiently flat and dry for floated floors and additionally clean and sound with adhesive fixed floors.
Important note: Engineered timber flooring should not be installed over carpet tiles, carpet, and other soft floor coverings.
Concrete subfloors
Irrespective of the installation method, all concrete subfloors need to be sufficiently dry to accept the flooring system. Also noting that additional moisture vapour protection is to be provided. Slabs should be below 4% moisture content by impedance meter or below 85% in-slab relative humidity prior to considering installation.
With adhesive fixed floors, moisture vapour protection is often achieved by an applied moisture vapour barrier or with floating floors a moisture vapour plastic layer of 200µm (0.2mm) incorporated into the foam underlay. If the underlay moisture vapour barrier is not sufficiently thick or not present, then 200µm polyethylene (black builder’s plastic) may be used beneath the underlay. The plastic is overlapped about 200mm, joints taped with a moisture resistant plastic tape and the polyethylene brought up to the height of the floor at the floor perimeter.
When using applied moisture vapour barriers and leveling compounds the manufacturer’s instructions for those products need to be followed. Such products along with the adhesive need to be compatible and usually from the same manufacturer to ensure warranties are maintained.
For floors fixed with adhesive directly to the slab, they require a level of flatness which deviates less than 3mm under a 3m straight edge placed anywhere on the slab. The required flatness can be achieved by grinding off high spots and filling low spots with leveling compound. Note that if adhering timber flooring over a leveling compound then the compound must have sufficient tensile strength.
Also, when adhering to a concrete subfloor it must be clean and sound. This is often achieved by lightly grinding the surface to remove any loose or flaky layers as well as cleaning off surface contaminants such as oil, paint, grease, glue, dirt, wax and sealant. When flooring is adhered to the moisture vapour barrier, a full trowel bed of adhesive utilising the trowel recommended by the adhesive manufacturer is to be used. The trowel needs to be at the correct angle to achieve the required spread rate. Contact between the flooring and adhesive is essential to obtain a strong bond and also to minimise any hollow sounds.
For floors laid on battens or on plywood installed over a slab, 200 µm polyethylene is usually installed by the same method as outlined earlier for floating floors. Concrete slabs need to be flat to less than a 3mm deviation under a 1.5m straight edge placed anywhere on the slab (noting that the flooring is directly not fixed to the slab). Slab cleanliness and structure are not as imperative here when compared to directly fixed timber floors to concrete slab with adhesives.
Wooden subfloors
A wooden subfloor may be timber boards or sheet flooring of particleboard or plywood and can typical be between 9% and 13% moisture content. If the flooring is on the lower level with soil beneath, then greater consideration needs to be given subfloor moisture. Upper level floors with rooms beneath are usually sufficiently dry.
With soil beneath, a site assessment of the subfloor space is necessary as damp soil and inadequate ventilation can lead to moisture related problems in the floor above. If there are concerns with subfloor dampness then ventilation may need to be increased, and provided there is no ponding of water, black 200 µm plastic over the soil can reduce soil evaporation and create a substantially drier subfloor space. If seepage is occurring, then covered drainage is often needed. At times mechanical ventilation is installed if natural ventilation cannot be adequately provided.
A resistance moisture meter can be used to estimate the moisture content (readings corrected for temperature and species) of solid timber subfloor members and these should not exceed 12% – 13%. Moisture meter readings are not reliable in plywood or particleboard, and if there is potential concern that the subfloor may be high in moisture content, then testing by the oven dry method may be needed, where again the moisture content should not be above 12 – 13%.
Furthermore, additional checks are required to ensure the particleboard or plywood subfloor has been secured appropriately and there is no excessive movement or noise before installation begins. Rough sanding is often needed, noting that new particleboard has a wax layer that needs removing when adhering directly to it. Kustom Timber flooring is not to be laid over timber subfloors that do not meet the requirements set out above.
Ceramic, Marble, Slate, Terrazzo, and other tile subfloors
When the subfloor is tiled, the flooring is often installed as a floated floor, however it is still necessary to ensure that the subfloor is sufficiently flat, or that work is undertaken to achieve the required flatness. The subfloor is to be assessed to ensure that there is no evidence of high moisture even though the flooring is to be installed oven appropriate moisture vapour barrier (as described in the concrete subfloor section.)
When adhesive fixing, it is preferable that the tiles are uplifted but floors can usually be adhesive fixed over tiles provided that they are securely fixed in place. It may also be necessary to grind the tiles to create a smooth surface suitable for bonding. A combination of grinding and levelling compound is used to achieve the required flatness (as indicated for concrete subfloors) and with grout lines and any voids also filled. Again, it is necessary to check that there is no evidence on high moisture prior to applying the moisture vapour barrier to the manufacturer’s requirements.
Subfloors with underfloor heating
Kustom Timber’s products can be adhesive fixed to slabs with embedded hydronic and diffuser plate heating systems. Prior to floor installation, newly installed subfloor heating systems need to be used to dry the concrete before the floor can be installed. In order to do this, the heating system is switched on at least 2 weeks before laying the floor and then switched off 48 hours prior to installing the floor. At the surface of the subfloor the temperature can be a maximum of 27°C.
A week after the floor is installed the heating system is turned back on with the temperature gradually increased by no more than 2°C per day. This should be implemented until the subfloor’s surface reaches a maximum temperature of 26 degrees Celsius. And then, the same in reverse when turning off each time the heating system is utilised. Floorboards are installed over a full trowel bed of adhesive directly to the slab. Prior to this a moisture barrier is applied across the entire slab. While hydronic heating is in use, whether increasing or decreasing the temperature it should be by only 1- 2°C per day.
Expansion Joints
Timber is a natural product that has natural variations in its properties. And just like any natural material, it can be affected by a variety of factors which include relative humidity changes, daily temperature, and windy conditions. As these change, the timber is expected to contract and expand.
Even though our engineered timber floors have less movement in board width when compared to solid timber floors, it is important to note that these floors still expand and contract a little. Accordingly, perimeter expansion allowance and control joints are required. Some wider floors will also require intermediate expansion allowance to be provided. This lets the floor expand and contract as a number of individual rafts.
With floated floors, at the perimeter of the floor and to any other vertical surfaces (e.g. kitchen island bench or meeting another floor type), an allowance for floor movement of at least 10 mm is to be provided. Floors wider than 8m or longer than 10m, also require intermediate expansion joints to be included. Note that floor width is measured across the board width direction and floor length over the board length direction. Compartmentalisation is used to break the floor areas up into individual ‘rafts’.
To achieve this, control joints are provided at all doorways, the end of hallways, where floor areas can move in opposing directions, and where one floor area may move laterally past another. Floated floors are not to be installed under very heavy objects such as kitchen island benches. At the perimeter of the floor and around benches, the movement gap is to be covered by skirting/scotia and noting the fixing is not into the floor as this would prevent the free movement of the floor. For the same reason caulking between the floor and any adjoining vertical surface (e.g. skirting) should not be done.If a subfloor has a joint in it, then the joint should be checked for possible moisture and this corrected if need be. A joint is to be provided in the floor above a joint in the subfloor, at the same location.
With adhesive fixed floors there is generally less need for control and expansion joints throughout the floor than with floated floors. This is because the boards are fixed to the subfloor and move individually with seasonal movement rather than as floating rafts. Also, the adhesive provides much greater restraint to board movement. Hence compartmentalisation is not needed, but it must be considered that due to some expansion in the length of a floor with higher humidity conditions, this can cause greater pressure at board ends leading to end-peaking in longer floors.
It is therefore often necessary to provide control joints in locations such as the end of hallways leading into a living area. As such, control joints or intermediate expansion allowance is still required in floor areas that are wider than 8m or longer than 10m, but floors from say hallways to bedrooms leading off that hallway may not need a control joint at the doorway as would be necessary with compartmentalising a floated floor. Similar to floating floors, perimeter expansion allowance needs to be provided and this includes to all vertical surfaces and the same 10mm allowance.