Flooring Site Preparation Guide

Wood flooring should only be installed into a room with normal living conditions. This means that it will need to be sealed, dry, free from damp, heated and with temperature and humidity levels within the following scope:

• Temperature – 15-24°C
• Relative Humidity – 40-65%

A wood floor should not be installed anywhere that suffers from damp. We advise the following site preparation checks to rule out obvious causes, but please bear in mind that damp is seasonal and may not be evident at the time of inspection. If you have any concerns about damp you should consult an expert before you order your wood flooring, and ensure that any issues are resolved before it’s delivered.

• Check that any solid ground floor subfloors have an effective damp-proof membrane. If this is not the case, do not proceed with installation until this is rectified. Broadleaf Ultra Dry is a paint on moisture barrier that can be easily applied to concrete sub-floors. It is compatible with Broadleaf Ultra Dry adhesive for glue fixing and be used under a floated floor. You must not breach it with floor fixings.
• Check that air bricks below suspended floors have not been removed or blocked up and that air circulation is sufficient to prevent moisture build up and that existing joists show no signs of moisture damage.
• Make sure that drains next to the house are free of leaves and debris so that they do not block and cause water to build up against walls or under a suspended floor.
• Check that walls do not show signs of water ingress or damp.
• Check the relative humidity of the room. Readings outside the normal range for a heated environment (40-65%) could indicate the presence of damp.

All plastering in and near the room where wood flooring is to be fitted must be complete and fully dry before it’s delivered.

This will take at least 2 weeks from change of colour, which is generally about 2 weeks from application of plaster to plasterboard, and 4 weeks from plaster to render. Damp weather and other environmental conditions can extend this.

Underfloor pipes (excluding underfloor heating which is dealt with separately) should be properly insulated to prevent hotspots causing localised shrinkage in your wood flooring.

Check the floor with the heating on to identify any unusually warm areas and resolve these by lagging pipes where these are accessible. Be aware that if it isn’t possible to deal with this some localised shrinkage of boards may occur after fitting. This will normally only be an aesthetic issue though and is unlikely to affect the structural integrity of the wood floor.

The condition of the subfloor will be integral to the stability and performance of your finished wood floor, and as such is one of the most crucial aspects of site preparation. All subfloors should be dry, level and free from dust.

Concrete subfloors must contain a damp-proof membrane and have a moisture content of no more than 4% (by weight) as measured with a Tramex concrete encounter meter. These meters should not be confused with standard damp meters, they are specially designed for testing subfloors and if your fitter does not have one or you are planning to fit your own wood floor they can be borrowed from Broadleaf showrooms.

Generally, existing screeds should be below this level unless they are damp. New concrete up to 50mm thick will normally take approximately 1 month per inch (1 day per mm) to reach it and thicker screeds can take proportionally longer.

A good preliminary moisture test is to fix a small area of plastic sheet to the surface of the concrete with airtight tape. After a couple of days, check the underside of the sheet. If there is no condensation present, then it is likely that the 4% level has been reached, but be sure to check with a meter before proceeding to installation.

If you are going to glue-fix wood flooring directly to a concrete screed, or float engineered wood flooring over it, it must be level to within 3mm across 2 metres. Check this at the site preparation stage and resolve any more significant imperfections with self-levelling compound or ply prior to delivery of the timber.

Before glue-fixing, we also recommend the application of a primer to aid bonding and offer an additional moisture barrier. With Broadleaf Ultra Adhesive, use Broadleaf Ultra Dry Moisture Barrier & Primer.

To nail-fix wood flooring above concrete, you will first need to install softwood battens. These must be dry (request them as such from builders’ merchants) and a minimum of 20mm x 50mm. They must be screw-fixed at a maximum of 400mm centres at 90° to the intended direction of the floor. Insulation can be added between the battens if desired. Battens should be packed as necessary to ensure that the surface is level to within 3mm across 2 metres.

Timber subfloors should be structural flooring-grade plywood or chipboard (T&G) or existing floorboards. Plank flooring can be nail-fixed directly to chipboard/plywood in whichever direction is preferred. For added stability and sound insulation in this scenario we recommend applying glue in strips every 300mm as a supplement to the nails, particularly when fixing to chipboard which does not hold nails as firmly as plywood over the long term. Engineered wood flooring can be floated over this style of timber subfloor and solid and engineered parquet flooring can be glued.

In the case of existing floorboards, these must be dry, reasonably level and firmly fixed. If they are not reasonably level we recommend that you remove them at the site preparation stage and either fix the new wood floor directly to joists (see below), or put down a structural chipboard/plywood subfloor and fit to this instead (see above). depending on what best suits your chosen product.

If existing floorboards are on the ground floor, check that air bricks are present, that these haven’t been blocked up, and that there is no moisture build up beneath the existing boards. Bear in mind that this must be the case all year round. Sound, rot-free joists are a good indicator that this is the case and cupping in the surface of the existing boards could highlight a problem. If there are moisture issues, do not proceed with the installation until these have been resolved.

All being well, screw down any loose boards – check beneath them for services first – punch back any face nails to just below the surface of the boards and sand out any significant high points. Once subfloor preparation is complete, plank floors can then be nail fixed over the existing floor at 90°, packing it if necessary to eliminate any residual unevenness in the surface of the old boards.

If you want to fix the new wood flooring in the same direction as the old wood floor, you will first need to overlay it with flooring grade plywood or chipboard at least 6mm thick.  Screw fix this to the old boards at regular intervals and then nail fix the new wood floor through this into the existing one.  Again, in this instance it may be easier simply to remove the existing boards during site preparation and fix the new floor directly to the joists.

If you want to fit parquet flooring over an existing plank floor rather than repalcing this with structural flooring grade chipboard/plywood and fitting to that, you will need to proceed as for a new plank floor laid in the same direction as the old one (see above) so that you have a level, unified subfloor to glue to.

Joists need to be structurally sound, dry, rot-free, relatively level and at no more than 400mm centres to successfully support a plank floor. The planks will also need to be of structural thickness (normally 20mm).

For ground floor installations the space beneath the joists must be free of moisture and you must be confident that this is the case all year round. During site preparation, always check that air bricks are present and have not been blocked up, there is no moisture in the space beneath the joists, and the joists themselves are sound and rot free. If you are in any doubt, consult a damp expert and resolve any issues before proceeding with the installation.

Nail fix the floor at 90°to the joists, packing where necessary to ensure that the floor is level.

Tiled or stone floors are not recommended as subfloors. Ideally, these floor coverings should be removed, and any wood floor fixed directly over the concrete subfloor beneath, once this meets the required criteria. If this is not possible, or practical, then a suitable damp-proof membrane must be installed over the tiles/stone followed by flooring grade tongue and grooved plywood (18mm min). The flooring can then be fixed over this as recommended for the product. Remember that any nail fixing will need to remain within the thickness of the ply.

Once any new concrete has reached the required moisture level and plaster is fully dry, the heating must then be run to drive out any latent moisture before the wood flooring is delivered.

Conventional heating must be be run for a minimum of 2 weeks, and if the ambient temperature in the room (temperature without the heating on) is too high for the heating to activate at its normal level, you will need to raise the thermostat temperature to ensure that it does.

For underfloor heating recommendations see our Underfloor Heating guidelines.

All Broadleaf wood flooring must be acclimatised for 7-14 days before installation. It should not be delivered to acclimatise until all concrete and plaster are fully dry (see section 1) and the heating has been run for a minimum of 2 weeks after this (see above).

Keep the wood flooring in the room where it will be fitted, or one with similar environmental conditions. It should be unwrapped and lattice stacked (criss cross) so that the air can circulate (even parquet). In particularly dry or damp weather, or if using wide boards, err towards the upper end of the recommended acclimatisation period.