Checking your subfloor for flatness before laying new flooring is one of the most important steps you can take to avoid costly problems later. The good news is that the basic check is straightforward — all you need is a long straight edge, a tape measure or feeler gauge, and a pencil to mark problem areas. Most NZ flooring manufacturers specify a flatness tolerance, and if your subfloor doesn't meet it, your warranty could be void.
What You'll Need
Understanding Flatness Tolerances
Before you start, it helps to know what you're measuring against. Flatness tolerance is the maximum allowable gap between the straight edge and the subfloor surface over a given distance. In New Zealand, the most commonly referenced standard is:
| Flooring Type | Typical Tolerance |
|---|---|
| Rigid vinyl planks / LVT | 3 mm over 1.8 m |
| Laminate flooring | 3 mm over 1.8 m |
| Ceramic or porcelain tiles | 3 mm over 3 m (check tile manufacturer specs) |
| Solid or engineered timber | 3 mm over 1.8 m |
| Sheet vinyl / vinyl tiles | 3 mm over 1.8 m |
Always check your specific product's installation guide, as some manufacturers specify tighter tolerances — particularly for rigid core vinyl and large-format tiles. Refer to BRANZ for general NZ construction guidance on floor preparation.
Step-by-Step: How to Check Subfloor Flatness
1. Clear and Clean the Subfloor
Remove all furniture, old floor coverings, and debris. Sweep or vacuum thoroughly. Any grit or lumps under your straight edge will give you a false reading.
2. Lay the Straight Edge Across the Floor
Place your straight edge flat on the subfloor surface. Work in a grid pattern — run it parallel to one wall, then at 90 degrees, then diagonally. This ensures you catch humps, dips, and twists in all directions.
3. Look for Gaps and Rock
Crouching down to floor level, shine a torch along the underside of the straight edge. Light gaps between the straight edge and the floor indicate low spots (hollows). If the straight edge rocks, there's a high spot (hump) underneath.
4. Measure the Gap
Slide a feeler gauge or a thin ruler into any gap you find. Record the measurement and mark the location with pencil. A gap greater than 3 mm over 1.8 m in most cases means the area needs attention before you lay your new floor.
5. Check Around Joins and Edges
Pay special attention to:
- Sheet joins in plywood or particleboard subfloors — these can ridge or dip over time
- Doorways and transitions between rooms
- Areas around pipes or old fixings where the subfloor may have been disturbed
6. Map Your Problem Areas
Draw a rough sketch of the room and mark all the high and low spots. This helps you plan your remediation work efficiently.
Fixing High and Low Spots
High spots (humps) on timber subfloors can often be sanded or planed down. On concrete, they may need grinding — this can generate silica dust, so always wear a P2 respirator and check WorkSafe NZ guidelines for working safely with concrete dust.
Low spots (hollows) are typically filled with a floor levelling compound or self-levelling underlayment. Products from Ardex and Mapei are widely used in NZ and are available through flooring and trade suppliers. Always follow the manufacturer's mixing and application instructions, and allow full cure time before laying your floor.
For particleboard or plywood subfloors, raised joins can be sanded back, and any flex or movement should be fixed by screwing down loose sheets before you fill.
Common Subfloor Types in NZ Homes
- Concrete slab — common in modern NZ homes and slab-on-ground construction. Can have surface variation from the pour.
- Particleboard (often called chipboard) — very common in NZ homes built from the 1970s onwards. Can swell, delaminate, or ridge at joins if it has been wet.
- Plywood — generally more stable than particleboard; check for soft spots that may indicate moisture damage.
- Tongue-and-groove timber boards — older NZ homes often have these. They can cup, bow, or have gaps between boards that need addressing.
When to Call a Professional
If you find widespread unevenness, soft or spongy areas, signs of moisture or mould, or structural movement, stop and get a professional assessment before proceeding. These issues can indicate problems beyond simple surface preparation — such as subfloor framing damage, moisture ingress, or ground movement — which may require a Licensed Building Practitioner (LBP) to assess and repair.
Some remediation work on subfloors — particularly if it involves structural framing — may require a building consent. Check with your local council, as requirements vary. The MBIE Building Performance website has useful guidance on what work requires consent under Schedule 1 exemptions.
For a thorough job and peace of mind, consider hiring a qualified flooring installer. You can find experienced local professionals on tradedirectory.co.nz.
