Checking your timber subfloor before laying new flooring is one of the most important steps you can take to ensure a long-lasting, professional result. Most flooring failures — from squeaks and bouncing to cracked tiles and lifting planks — trace back to a subfloor that wasn't properly assessed or prepared beforehand. The good news is that with a few basic tools and some patience, you can do a thorough check yourself.
Understanding the Difference Between Level and Flat
These two terms are often confused, but they mean different things in flooring:
- Level means the floor sits on a true horizontal plane (no slope from one side of the room to the other).
- Flat means the surface has no humps, dips, or ridges over a given span.
Most flooring products — particularly rigid options like tiles, engineered timber, and laminate — require the subfloor to be flat, not necessarily perfectly level. A gentle, consistent slope across a whole room is generally less problematic than a localised dip or hump. Always check the manufacturer's specification for the product you're installing.
What Tolerance Do You Need?
The industry-standard tolerance used in New Zealand (and referenced by most flooring manufacturers) is 3 mm over a 1.8 m straightedge. Some products, particularly ceramic and porcelain tiles, may require tighter tolerances — as little as 3 mm over 3 m. Always read the installation guide for your specific product before you start.
| Flooring Type | Typical Flatness Tolerance |
|---|---|
| Carpet | 6–8 mm over 1.8 m (more forgiving) |
| Laminate / Floating engineered timber | 3 mm over 1.8 m |
| Glue-down timber or vinyl plank | 3 mm over 1.8 m |
| Ceramic / porcelain tiles | 3 mm over 3 m |
These are general guides — always defer to your flooring product's technical data sheet.
Tools You'll Need
- A 1.8 m (or longer) aluminium straightedge or spirit level
- A digital level or bubble spirit level
- A tape measure
- A pencil or chalk to mark problem areas
- A torch to spot gaps under the straightedge
You can pick up straightedges and spirit levels at Bunnings or Mitre 10.
Step-by-Step: How to Check Your Subfloor
1. Clear and Clean the Area
Remove all furniture, floor coverings, and debris. Sweep or vacuum thoroughly — even small pieces of grit can throw off your readings.
2. Do a Visual Inspection First
Get down low with a torch and look across the floor at a raking angle. You'll quickly spot obvious humps, dips, or areas where boards have cupped or sprung up. Mark anything suspicious with chalk.
3. Walk the Floor
Walk slowly across the entire subfloor, paying attention to any bounce, flex, or movement. Bouncy areas may indicate undersized joists, damaged noggins, or rot. Squeaks often point to loose fixings or boards rubbing against joists.
4. Use the Straightedge
Lay your straightedge or spirit level flat on the floor and slide it in multiple directions — along the length, across the width, and diagonally. Shine a torch along the gap between the straightedge and the floor. Any gap greater than 3 mm is a problem area that needs attention before you lay your new flooring. Mark all high and low spots clearly.
5. Check for Level (Slope)
Place your spirit level or digital level on the floor in several locations. A slight, consistent slope across a large room may be acceptable for some floating floors, but check your product's guidelines. Significant slope — particularly in wet areas — may need to be addressed to comply with NZS 4121 (accessibility) or the NZ Building Code clause E3 (internal moisture).
6. Check Subfloor Thickness and Joist Spacing
For timber subfloors, the sheet thickness and joist spacing matter. Standard NZ practice under NZS 3604 specifies minimum sheet thicknesses depending on joist centres. If your subfloor feels soft or flexible, it may be undersized for the new flooring load — especially relevant for tiles.
7. Look for Moisture and Rot
Timber subfloors in New Zealand are particularly vulnerable to moisture, especially in older homes with poor underfloor ventilation. Press firmly on boards across the floor — any softness or sponginess warrants closer investigation. Use a moisture meter if you have one; most flooring manufacturers specify a maximum moisture content (commonly around 14–16% for timber subfloors). BRANZ has useful guidance on managing subfloor moisture in NZ conditions.
Fixing Problems Before You Lay
- High spots: Sand or plane down raised boards or nail heads. A belt sander works well for larger areas.
- Low spots and dips: Fill with a suitable floor levelling compound — products from Ardex or Mapei are widely used by NZ flooring professionals.
- Loose or squeaky boards: Re-nail or screw boards back down to the joists. Use appropriate-length screws to avoid penetrating services below.
- Rot or structural damage: This must be assessed and repaired before any new flooring goes down.
When to Call a Professional
If you find significant structural movement, rot, or damage to joists, stop and call a qualified tradesperson. Structural repairs to a timber subfloor may require a building consent depending on the scope of work — check with your local council, as Schedule 1 exemptions under the Building Act have specific limits. You can find experienced local flooring installers and builders on tradedirectory.co.nz.
If your subfloor issues are in a wet area (bathroom or laundry), be aware that waterproofing work must comply with NZ Building Code clause E3, and depending on the scope, may involve restricted or consent-required work. Check with MBIE Building Performance if you're unsure.
