Choosing the Right Substrate for Timber Flooring
Movement, hollow sound, poor adhesion and moisture problems are usually blamed on the flooring. In practice they are usually caused by what sits underneath it.
The substrate is the structural surface beneath the floor, and its job is to provide a stable, level, dry base. Get it right and the floor is stable, quiet, well bonded and long lived. Get it wrong and no board on the market will save the installation.
Concrete slabs
The most common substrate in New Zealand new builds, and an excellent base when properly prepared. Before installation the slab should be:
- Structurally sound
- Clean and free of contaminants
- Level to within 2mm over 2m
- Fully cured
- Tested for moisture
Moisture is the one that catches projects out. New Zealand conditions can create significant moisture pressure inside a slab, particularly in a new build where the concrete is still releasing water. A professional moisture barrier system before installation protects both the adhesive and the timber from migration.
Direct-stick onto prepared concrete gives exceptional stability and the solid underfoot feel most people prefer.
Existing timber floors
Common in villa and bungalow renovation, and often the most practical answer. The existing floor needs to be structurally stable, securely fixed, free of excessive movement, reasonably level, and dry.
Older floors usually need sanding, levelling or repair before the new floor goes down. Retaining and upgrading an existing timber substrate is frequently both cheaper and better than removing it.
Direct to joists
Our boards can be installed directly onto joists, or onto strand board over joists, which is not true of thinner engineered products. The cross-laminated construction and the 20mm thickness are what make direct fixing viable. Where there is a choice we would still use the strand board overlay, because it gives a continuous, level base for the adhesive.
In most New Zealand residential construction, standard timber joists sit at 450mm centres, which distributes load evenly and limits deflection over time.
Three variables can change that figure:
- Joist material, grade and size, which set the maximum span
- Anticipated floor load, including flooring, furniture and occupants
- Flooring thickness, which affects how far the board can bridge
Joist spacing and floor load are governed by the New Zealand Building Code, and on any project where the span is not obviously standard, a builder or structural engineer should confirm it. This is not a decision to take from a blog post, including this one.
The reason a 20mm cross-laminated board bridges well is the same reason it does everything else well, and it is set out in what is inside an engineered oak board.
What to resolve before you order
- What is the substrate, and is it structurally sound?
- Has the moisture content been tested, not estimated?
- Is it level within tolerance, and if not, what will bring it there?
- Is a moisture barrier required, and who is supplying it?
- If it is a renovation, is the old covering coming up or staying?
That last one has its own answer in laying timber over existing vinyl.
Full substrate requirements, tolerances and adhesive selection are in the Installation Instructions, and substrate compliance is a condition of the Warranty. To have a specific substrate reviewed before you commit, book a design consultation.
