Most solid timber floorboards can be sanded several times over their lifetime — but there is a limit. Once a board gets too thin, sanding it further risks structural weakness, squeaking, or even boards that flex and crack underfoot. Knowing when you've reached that limit can save you from a costly mistake.
Why Thickness Matters
Every time a floor is sanded, a small amount of timber is removed — typically 1–3 mm per sand, depending on how much levelling and finishing is needed. Solid tongue-and-groove floorboards installed in NZ homes are commonly 19 mm thick when new (a common size in NZ homes), though older villas and bungalows may have thicker boards of 25 mm or more. Engineered timber floors have a much thinner wear layer — sometimes only 2–6 mm — and have far fewer sanding cycles available.
The key concern is the tongue-and-groove joint. As the board face is sanded down, you eventually approach the top of the groove cut into the board's edge. Once you sand into or past that groove, the board loses its structural connection to its neighbour, and the floor can become unstable.
The Minimum Safe Thickness Rule
As a general guide used by experienced floor sanders, a solid timber floorboard should have at least 12 mm of thickness remaining above the subfloor (or above the top of the tongue) before another full sand is considered safe. Some tradespeople use a slightly more conservative figure of 15 mm as the minimum, particularly for floors with heavy joist spans or where the boards are already showing signs of flex.
For engineered timber floors, always check the manufacturer's specifications — the wear layer may only allow one or two light sands, and going beyond that will expose the core material beneath.
How to Measure Your Floorboard Thickness
Here are the most practical ways to check:
1. Lift a Floor Vent or Register
The easiest method is to remove a floor heating vent, cold air return, or access cover. This exposes the edge of the boards, where you can measure the full thickness with a steel ruler or vernier caliper.
2. Check at a Doorway or Threshold
Where flooring meets a doorway or transitions to another surface, the board edge is sometimes visible. A careful look — or a thin ruler slid underneath a threshold strip — can give you a reading.
3. Drill a Small Test Hole
In a hidden spot (inside a wardrobe, under a fixed appliance), you can carefully drill a small hole through the board and use a depth gauge or a bent wire to measure from the surface to the subfloor. Fill the hole with a matching timber plug or wood filler afterwards.
4. Look for Warning Signs on the Surface
Even without measuring, certain visual clues suggest a board is getting dangerously thin:
- Visible groove lines appearing at the board edges (the tongue-and-groove joint is close to the surface)
- Cupping or warping that won't flatten — thin boards are more prone to movement
- Springiness or flex when you walk across the floor
- Boards that sound hollow when tapped
- Previous nail or screw heads sitting very close to the surface, or already exposed
Checking Engineered Timber Floors
If you have an engineered timber floor, the rules are stricter. The wear layer is the only part that can be sanded, and it is often only 2–6 mm thick. Check your floor's documentation or contact the manufacturer directly. BRANZ has useful guidance on timber floor types and their maintenance limits. When in doubt, have a professional assess the floor before proceeding.
What Happens If You Sand Too Far?
Sanding through the groove weakens the mechanical connection between boards. You may end up with:
- Boards that lift, shift, or squeak
- Gaps opening between boards
- Boards that crack or split under foot traffic
- A floor that needs full replacement far sooner than expected
In some cases, boards that are borderline thin can be face-nailed or screwed and plugged to stabilise them before a light sand — but this is a job for an experienced floor sander, not a DIY fix.
When to Replace Rather Than Sand
If your boards are below the safe minimum thickness, or if they show significant rot, structural damage, or deep cupping that can't be flattened, replacement is the better option. Replacing individual boards is possible if matching timber can be sourced — recycled native timbers like rimu, matai, and kauri are available from salvage yards around NZ and can often be matched reasonably well. Full replacement may also be an opportunity to upgrade to a product that better suits your home's conditions.
Check with your local council whether any work triggers a building consent requirement — most like-for-like floor repairs are exempt under Schedule 1 of the Building Act, but it pays to confirm. See MBIE Building Performance for guidance on consent exemptions.
When to Call a Professional
If you're unsure about your floor's thickness, or if the boards are showing signs of structural weakness, it's well worth getting a qualified floor sander to assess the job before you commit. An experienced professional will have the tools and eye to judge whether your floor has another sand in it — and can advise on the best approach to get the most life out of your boards. You can find licensed floor sanding tradespeople in your area at tradedirectory.co.nz.
