Yes, you can use self-levelling compound on a timber subfloor — but it requires careful preparation and the right product. Timber moves with moisture and temperature changes, which means a standard concrete-grade compound can crack if applied incorrectly. Follow the steps below and you'll end up with a flat, stable base ready for your new flooring.
Why Timber Subfloors Need Special Attention
Unlike concrete, timber is a flexible, living material. It expands and contracts with humidity and temperature — something New Zealand's varied climate makes especially relevant, from humid Northland summers to cold Otago winters. A rigid compound that bonds well to concrete can crack or delaminate on a moving timber substrate if the wrong product or method is used.
The key is choosing a flexible or polymer-modified self-levelling compound specifically rated for timber subfloors, and making sure the subfloor itself is solid before you pour anything.
Step 1 — Assess and Repair the Subfloor
Before reaching for the compound, inspect your subfloor thoroughly:
- Check for movement. Walk across every area and listen for squeaks or feel for flex. Any bounce or movement must be fixed first — screw down loose sheets or boards at 150–200 mm intervals using appropriate screws.
- Check for damage. Rot, delamination (in sheet subfloors), or water damage must be cut out and replaced. Do not level over damaged timber.
- Check moisture levels. Use a moisture meter. Timber subfloors should generally read below 15% moisture content before applying compound. High moisture can cause adhesion failure and mould growth — relevant given Healthy Homes standards for rental properties.
- Ensure structural integrity. If joists or bearers look suspect, consult a qualified builder before proceeding.
Step 2 — Choose the Right Product
Not all self-levelling compounds are suitable for timber. Look for products labelled as suitable for flexible or timber substrates. In New Zealand, well-known options include products from Ardex and Mapei, both of which offer compounds specifically formulated for timber subfloors. Always read the technical data sheet for the product you choose — maximum depth per pour, cure times, and primer requirements all vary.
| Product Type | Suitable for Timber? | Notes |
|---|---|---|
| Standard concrete compound | Generally no | Can crack on flexible substrates |
| Polymer-modified compound | Yes (check label) | Flexible, better adhesion |
| Fibre-reinforced compound | Often yes | Added crack resistance |
Step 3 — Prime the Surface
Priming is not optional on timber. Timber is porous and will rapidly absorb moisture from the compound, causing it to dry unevenly and potentially crack or shrink. Use the primer recommended by your compound manufacturer — using a different brand's primer can void product warranties and cause adhesion failure.
Apply primer with a brush or roller, working it into any gaps between boards. Allow it to dry fully (check the manufacturer's instructions — typically 30–60 minutes, but this varies).
Filling Gaps First
If there are gaps between floorboards wider than about 3–4 mm, fill these with a flexible gap filler or backing rod before priming. Large gaps can cause compound to sink or crack.
Step 4 — Mix and Pour the Compound
- Mix compound strictly to the manufacturer's water ratio — too much water weakens the final product.
- Work quickly; most compounds have a pot life of 10–20 minutes.
- Pour and spread with a gauge rake set to your desired depth. Most compounds have a minimum and maximum pour depth (commonly 3–50 mm depending on the product).
- Use a spiked roller to release air bubbles.
- Keep the area free of foot traffic during the cure period (often 2–24 hours before light foot traffic, longer before heavy flooring installation).
Step 5 — Allow Proper Cure Time Before Laying Flooring
Don't rush this stage. Laying tiles, vinyl, or timber flooring over compound that hasn't fully cured can cause adhesion failure or cracking. Check the technical data sheet for cure times relative to your specific flooring type.
For tile installations, Ardex and Mapei both publish detailed guides on their NZ websites. BRANZ also provides good general guidance on subfloor preparation standards in New Zealand.
Depth Limitations and Building Code Considerations
If you're raising floor levels significantly, be aware this can affect door clearances, thresholds, and transitions to adjoining rooms. Major structural changes to floors may require a building consent — check with your local council. For guidance on what work is exempt, visit MBIE Building Performance. Minor levelling work for flooring replacement is generally considered maintenance, but when in doubt, ask your council.
When to Call a Professional
Call a qualified flooring installer if:
- The subfloor has significant structural damage, rot, or moisture problems
- You need to level more than 20–25 mm across a large area
- The existing subfloor doesn't meet the flatness tolerance required for your chosen flooring (e.g., large-format tiles typically require no more than 3 mm variation over 1.8 m)
- You're unsure which product is right for your situation
A professional flooring installer will have the tools, experience, and product knowledge to get the job done right the first time. You can find qualified local flooring installers on tradedirectory.co.nz.
Safety note: Self-levelling compounds are alkaline and can irritate skin and eyes. Wear gloves, safety glasses, and ensure good ventilation when mixing and pouring.
