If you’re working on a large-scale commercial or a multi-residential building project, the stakes are high, and you’ll know that there’s no room for errors that could lead to callbacks or delays. That’s why taking the time to perfect subfloor preparation isn’t just another box to tick; it’s a critical step to guarantee the longevity of your wood floor installations.
In this guide, we’ll walk you through a clear, step-by-step approach to subfloor preparation. From identifying potential issues to tackling moisture and levelling challenges, we’ll show you how the right process and using the right products can save you time, reduce any risks, and set you up for success on site.
Factors to Consider When Preparing a Subfloor
Step 1. Assess the Subfloor
Before proceeding, inspect and secure the working area; skipping this stage could lead to hidden issues that may resurface later.
Safety First
Be sure to remove all loose debris, nails or tripping hazards. For older buildings, we recommend checking for asbestos or lead paint, and if present, following safety precautions by calling a specialist to remove and dispose of it correctly. Always wear the correct PPE, especially if you’re sanding or grinding down concrete.
Identify the Subfloor Type
Your entire subfloor preparation process will always depend on the substrate material you’re working with.
- Concrete – Common in commercial or new-build residential projects.
- Timber/Joists – Found in older homes and multi-story buildings.
- Tiles or Existing Floor Coverings – May require full removal or surface treatment.
- Mixed Substrates – Sometimes different materials meet in the same room, requiring special handling.
Check for Structural Integrity
The subfloor must be solid and stable before you continue.
- Look for cracks, soft spots, or deflection
- Timber joists must not flex beyond BS8201 recommendations
- Loose or damaged boards should be repaired or replaced.
If the base isn’t sound, stop and fix it. No amount of adhesive used or smoothing compound can compensate for structural failure.
Step 2. Moisture Management
Moisture in the subfloor is one of the main causes of wood flooring failures, and if ignored, it can lead to many issues such as cupping of the board edges, boards buckling or lifting completely, breakdown of the adhesive used due to the weakening of the bond between the floor and subfloor, and mould growth – an unseen health risk.
How to Test for Moisture in a Subfloor
One way to test for moisture in a concrete subfloor is a Hygrometer Test, which measures relative humidity (RH) deep within the slab by using a Hygrometer for reliable results and is typically used for larger, commercial projects to be covered for compliance and warranties.
When is a Damp Proof Membrane (DPM) Required?
If the readings are above safe limits, you must install a DPM to prevent moisture migration.
- For concrete, products like Marldon MXS140 Rapid DPM provide a fast, solvent-free solution.
- For timber, address the moisture source first, as ventilation or other repairs may be needed before continuing.
Step 3. Create a Flat and Smooth Surface
Even the highest-quality flooring will fail on an uneven base. High spots, dips, or rough patches lead to visible gaps or uneven board edges, creaks, movement and stress on adhesives.
How to Level a Subfloor
- Check the subfloor with a long straightedge or laser level
- Mark high and low areas clearly for correction
- Prime the subfloor as required to ensure the smoothing compound bonds properly
- Apply a smoothing compound such as Marldon Fibre Reinforced MXS110
Step 4. Acclimatise Your Wood Flooring
Wood is a natural material that reacts to its environment. If you skip acclimatisation, the flooring may shrink or swell, causing gaps or warping.
Best Practices for Acclimatisation
Store the wood in the room it’s being installed for 48-72 hours (or however specified by the manufacturer). Keep the room temperature and humidity stable, ideally between 18–22°C and 45–60% RH and open the boxes of flooring to allow for airflow.
Keep rooms at a consistent temperature and humidity before, during, and after installation. Advise your clients and customers accordingly so they can make the most out of their wood flooring.
Step 5. Prepare for a Clean and Professional Finish
Before you start laying the floor, run through this final checklist:
- Check the subfloor is clean, dry, level, and structurally sound.
- Moisture readings are within safe limits
- DPM, primer, or levelling compounds are fully cured
- Expansion gaps are planned for
- The work area is dust-free
A Solid Foundation for Installation Success
Subfloor preparation is the key to a beautiful, long-lasting wood floor. By carefully managing moisture, levelling the surface, and acclimatising materials, you’re saving yourself from any call-backs and setting your customers up for wood flooring that will last for years to come.
Follow this link to browse all of our Marldon subfloor preparation products. Alternatively, to discuss specific requirements or for a quote, call us on 01772 696600 or send an email to [email protected]