Preparation of New & Existing Subfloors

Subfloors and the issues surrounding them

A subfloor is what is underneath your present floor covering. Subfloors can become a nightmare if you do not follow the correct procedures because whatever goes on top of the subfloor will be affected by it.

Damp, damaged membranes etc can allow moisture to rise upwards, moisture will always go to the driest part. Our fitters will check the floor for moisture and the surrounding area for humidity to ensure that there will be no problems once the floor is laid. The moisture content of the floor screed should be checked along with the relative humidity.

If the following procedures are followed then you will be providing the right environment for a beautiful natural floor that should last a lifetime and you will not get “cupping” or warping problems which only occur where there is excessive moisture. Cupping is where the edges of the boards curl up.

Types of Subfloor

There are 5 basic types of sub floors upon which we fix hard wood floors.

  • Concrete screeds, power floated floors
  • Joists
  • Ground floor joists with open air below which are found in old houses that have air bricks and probably earth underneath
  • Old floor boards
  • Chipboard or plywood structure
  • Underfloor Heating Screed


Concrete: Concrete screeded floors can vary in condition and levels. If it is flat and smooth with a moisture content of 2% or less then we can glue the boards directly to the floor using our advance SW-890 flooring adhesive, which is generally the best method of fitting rather than floating the wood floor with underlay.
If your subfloor is not flat and smooth you can use self leveling compounds but be aware that they can lift if you do not apply the compound correctly.

If you have your own builder or fitter please make sure they are experienced in fitting wood floors and understand the consequences of not following the correct procedures for concrete screeds.

To be extra cautious you should always put a Damp Proof Membrane (DPM) like the Bona R580 on any concrete screed. Some concrete takes 10 years to dry out completely and if the membrane underneath cracks for any reason such as subsidence (which can happen in long dry spells) then moisture can work through the screed and affect the floor.

Joists: If you are fixing floor boards directly to joist you can either use secret nails or screws. These are applied using a purpose-made nail gun or screw fixer which is set at a predetermined angle so that the groove of the next board can be fitted up without the screw or nail showing.

The joists must be in good condition and free of any wood worm, damp decay, dry rot or any other fungal type disease that can affect their integrity.

Older homes in the Victorian and Edwardian era had joists over open ground with air bricks if the circulation is blocked then it can lead to serious damp problems. If you have enough height then a plywood base using at least 18mm marine plywood. The ply can be screwed to the joists and the floor boards glued directly to the plywood. This will cut out drafts and prevent the damp air affecting the boards. If you do not have the height due to the existing fixtures in the house then you must always use an engineered board and ensure that all the ventilation openings and air bricks are clear

Old Floor Boards: Generally old floor boards will be pine or cheap softwood. If these boards are sound, free of dry rot and fungus, then a new hardwood floor can either be secret nailed, screwed or glued directly on to these boards.

If the boards are loose then it would be best to screw them firmly into the joist. If necessary an acoustic can be placed between the two layers of wood when nailing or screwing down, this will reduce noise levels including creaking and helps to reduce the transmission of cold air from below.

If you are unsure then our fitters would be happy to make a site visit to ascertain the best way of fixing your floor

Chipboard or Plywood Floors: Chipboard and ply wood sub bases are the ideal surface to fit a hardwood floor. You will be able to choose any of our wooden floors and on plywood they can be nailed, screwed or glued, but on chipboard due to it being particles of wood and glue nails and screws would not hold properly over time so it is always best to glue the floor boards down.

Underfloor Heating: This is a specialist area and should be treated with care. Whichever system you have, the manufacturer’s instructions should always be followed. If it is a screeded system then always glue the boards direct to the screed this ensure not heat loss You can find out more in our underfloor heating section.

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