A wood floor is only as sound as its subfloor. Spending time and care to ensure a subfloor is prepared correctly will go a long way to ensuring a successful wood floor fit.
In this post we share our guidelines on what’s important, with advice on critical factors such as moisture, stability and levelling.
Managing moisture is key to any wood floor fit. A damp subfloor can ruin a wood floor by causing it to over-expand, distort, discolour or delaminate.
These don’t necessarily have a built-in damp proof membrane – even up to 1970, this wasn’t obligatory. The floor may not necessarily look or feel damp, and if it has previously been overlaid with carpet then moisture has been able to dissipate into the air. Yet once a new wood floor is laid, the moisture can no longer escape and will collect in the underside of the boards, inevitably leading to problems.
A new solid screeded floor can take a long time to dry out, even in warm conditions.
Our rule of thumb is 1 day per mm for the first 50mm thickness, plus 2.5 days per mm for extra thickness.
Yet, in a moist or cold atmosphere the drying time could be longer. Even when a screeded floor appears to be dry on the surface, it will often still contain moisture deep down.
There’s no safer way to test the moisture levels of a subfloor than by using professional equipment such as the Protimeter MMS. Timber subfloors can be tested using the twin-prong WME tool and screeded subfloors require the use of the Hygrometer which is either inserted into a hole drilled into the screed, or used in conjunction with a humidity box.
Ideally, moisture levels should not exceed:
- 70% Relative Humidity (RH) for cement-based concrete
- 11% Wood Moisture Equivalent (WME) for wooden subfloors or joists
Where time does not allow for complete drying, or if there is a fundamental damp issue, various damp-proofing products are available in our range. Yet, it’s worth bearing in mind that the correct use and application of these is critical to ensure the flooring is protected from escaping moisture.
A liquid DPM will hold back a fairly high level of residual moisture in a new floor screed, allowing it to dissipate slowly over time. Make sure the screed is fully cured and thoroughly cleaned before application to ensure a strong bond. And bear in mind that liquid DPMs aren’t usually suitable over underfloor heating or anhydrite screeds.
With floating installations where there are low levels of residual moisture, the use of a good quality 1000g polythene DPM with carefully taped joints can provide a simple and cost-effective barrier. In old properties where moisture levels are higher or likely to continue, our damp proof barrier offers a combined waterproof layer and underlay.
Where a new timber floor is being nailed down to an old timber structure or new timber joists that haven’t been dried, we recommend using a membrane such as Moistop to prevent the transfer of moisture.
Don’t forget to consider the atmospheric conditions in a property as these too can effect a wood floor. In new properties or extensions with trades that are still drying out, the air humidity can be high, and if the flooring is acclimatised in these conditions, it will shrink over time. Likewise, old properties sometimes suffer from damp walls which can create a higher moisture level in the air during wet seasons.
Heating and plenty of ventilation can help with drying out a new property over a period of time while dehumidifiers can help speed up the process. It’s important to carry out tests using a professional moisture meter to confirm when conditions are suitable for a wood floor.
Resolving moisture issues in older properties can sometimes be more costly and time-consuming. However, the solutions often benefit the property as a whole, as well as ensuring the new wood floor is a success.
A common issue in older properties is that the subfloors may have deteriorated with age and may no longer provide a sturdy base for a new wood floor.
Old Timber Subfloors
Any defects, infestations or rot in timber floors need to be dealt with and any loose or distorted joints firmly fixed or re-levelled. Uneven boards can be overlaid with good quality ply to provide a smooth, stable base. Yet it’s worth considering the effects of covering a traditional timber floor – you may need to check that the airbricks below are still clear and allow airflow.
Old floor screeds often become soft and friable, and sometimes even the structural concrete below may need to be repaired or replaced. Any old adhesives or contamination will also need to be removed, especially if wood flooring is being stuck directly to the surface.
Even a new floor screed should be checked for soundness, as a weak cement ratio or badly mixed batch can sometimes result in a loose surface.
Something else to consider is whether an Anhydrite or Gypsum-based ‘flow’ screed has been used. These are often very smooth yet have a soft and weak surface. Where a wood floor is to be installed, the surface laitance should be removed by grinding and replaced with a reinforced levelling compound.
A word of warning: many liquid damp proof membranes do not work well with Anhydrite and Gypsum screeds so full drying-out of the subfloor is essential.
Existing floors such as plastic or ceramic tiles and parquet blocks should always be treated with caution as they could well be disguising another issue. It’s also not possible to be totally sure how well they’re bonded to the structural substrate.
As a general rule, it’s wise to remove these and prepare the subfloor from a sound base. Bear in mind that an older finish may have been installed as a form of damp proofing, so once removed a new membrane may need to be installed.
There are a variety of other historical subfloor materials. Asphalt is usually stable and can often be overlaid with a fibre-reinforced levelling compound, but other composite screeds can be very unstable and need to be removed. If you’re in doubt, we recommend doing some further research.
Our technical experts are on-hand for advice so please feel free to give us a call on 02920 888 223
It’s vital to ensure the subfloor is flat. Wood floors are intolerant to unevenness with common problems including bounding, squeaking, joint failure or adhesive de-bonding.
We recommend that bumps or dips should not exceed 3mm over any 2 metre area.
If the subfloor requires levelling, screeded floors can usually be corrected with a levelling compound. We advise using fibre-reinforced mixes over underfloor heating or where a new floor is to be bonded to a subfloor. These are also suitable over many timber subfloors.
Underfloor heating adds a whole additional dimension to subfloor preparation, and that’s why we have a whole other post for it. Head here to read-up on our guidelines. And just a tip – underfloor heating will accentuate the effects or any dampness or defects.
Although every effort has been made to ensure this information is to the best of our knowledge, we cannot accept responsibility for any issues arising from the application of these guidelines.