Tiling over a bowed or humped floor is one of the most common subfloor challenges NZ homeowners face — and skipping the prep is the fastest way to end up with cracked tiles, hollow spots, and a job that needs redoing. The good news is that a slight bow can usually be corrected with the right approach before a single tile goes down.
Why a Flat Subfloor Matters
Tile is a rigid material. If it bridges a high spot or spans a hollow, it has no flex to absorb movement — so it cracks, or the grout fails prematurely. The industry standard for floor flatness under tiles is no more than 3 mm variation over a 1.8 m straightedge (sometimes written as 3 mm in 1800 mm). This aligns with guidance from BRANZ and is consistent with good trade practice across NZ.
For large-format tiles (generally 600 mm or longer on any side), the tolerance tightens further — aim for no more than 2 mm over 1.8 m, as bigger tiles are less forgiving of any deviation.
Step 1 — Assess the Hump
Before reaching for any products, understand what you're dealing with:
- Lay a long spirit level or straightedge (at least 1.8 m) across the floor in multiple directions.
- Mark the high point with a pencil.
- Measure the peak height above the surrounding floor — even 5–6 mm can be manageable, but anything beyond 10–12 mm may need professional assessment.
- Check whether the hump is in a concrete slab or a timber subfloor, as the fix differs.
Step 2 — Identify the Cause
A hump in a concrete slab is usually a pour or finishing imperfection. A hump in a timber floor can indicate:
- A joist that is slightly higher than its neighbours
- Swelling from moisture
- A structural issue (rare, but important to rule out)
If you suspect a structural problem — for example, the floor has changed shape recently, or there are cracks in the surrounding structure — consult a Licensed Building Practitioner or structural engineer before proceeding. The MBIE Building Performance website has useful guidance on when building consent may be required for structural alterations.
Step 3 — Fixing a Hump in a Concrete Slab
Grinding Down the High Spot
For a concrete slab, grinding is usually the most reliable fix. A concrete angle grinder or floor grinder (available from hire shops) can bring a high spot down flush with the surrounding floor.
- Wear a P2 dust mask, safety glasses, and hearing protection — concrete dust is a health hazard. Check WorkSafe NZ guidance on silica dust if you're doing significant grinding.
- Work gradually, checking with your straightedge frequently.
- Feather the edges so there's no sharp transition.
- Vacuum thoroughly and prime before applying any levelling compound.
Using a Levelling or Smoothing Compound
If the hump is very slight (under 3 mm) and the surrounding floor has low spots too, a self-levelling compound can sometimes average things out — but be aware that self-levellers flow to the lowest point and won't automatically flatten a peak. You may still need to grind the high spot first, then use a leveller to bring the rest of the floor up.
Products from Ardex and Mapei are widely used by NZ tilers for floor preparation and levelling compounds. Always follow the manufacturer's mixing and coverage instructions.
Step 4 — Fixing a Hump in a Timber Subfloor
Timber floors require a different approach:
- Plane or sand the high joist if the hump is caused by one joist sitting proud — this is straightforward carpentry but requires access from below or lifting boards.
- Sand the subfloor surface with a belt sander if the hump is in the sheet flooring itself (e.g., particleboard or plywood).
- Apply a fibre-reinforced levelling compound rated for timber subfloors — not all levellers are suitable for flexible substrates, so check the product datasheet carefully.
- Ensure the subfloor is structurally sound, dry, and well-fastened (screw down any squeaky or loose sheets) before tiling.
| Subfloor Type | Preferred Fix | Key Consideration |
|---|---|---|
| Concrete slab | Grind high spot, then level | Silica dust — use PPE |
| Timber/plywood | Plane joist or sand, then level | Use flexible-rated compound |
| Particleboard | Sand + level or replace sheet | Particleboard can swell; check moisture |
Step 5 — Prime and Check Before Tiling
Once the floor is flat:
- Prime the surface with a product recommended by your adhesive manufacturer — this improves adhesion and reduces suction.
- Re-check flatness with your straightedge in multiple directions.
- Allow all compounds to cure fully (check the product datasheet — typically 24 hours minimum, longer in cold or humid NZ conditions).
- Use a notched trowel and quality tile adhesive appropriate for your tile size and subfloor type. Large-format tiles generally need a larger notch and back-buttering.
When to Call a Professional
Call a qualified tiler or builder if:
- The hump is more than 10 mm high or covers a large area
- You suspect a structural cause (subsidence, joist failure, moisture damage)
- You're unsure whether your subfloor is suitable for tiling at all
- The floor is in a wet area (bathroom, laundry) where waterproofing under the tiles is required — this is critical and must comply with NZ Building Code clause E3
A skilled tiler will have the tools and experience to assess and fix subfloor issues efficiently. You can find qualified local tilers on tradedirectory.co.nz.
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