Sanding and polishing old timber floorboards can completely transform a room, bringing warmth and character back to a tired floor. But not every old floor is a good candidate — and getting it wrong can mean costly repairs or a floor that simply can't be saved. Here's how to assess your floorboards before you commit to the project.
Check the Timber Thickness First
This is the single most important factor. Every time a floor is sanded, a small amount of timber is removed — typically a few millimetres per sand. Solid timber floorboards need enough thickness remaining above the tongue-and-groove joint to survive the process safely.
As a general guide, you need at least 6–8 mm of timber above the top of the tongue before sanding is viable. If your boards are already thin from previous sands, you may be close to — or past — that limit. A qualified floor sander can use a depth gauge or probe to measure this accurately. If you're unsure, this alone is worth getting a professional assessment for.
Engineered Timber Floors
Engineered timber has a real wood veneer over a plywood base. Whether it can be sanded depends entirely on the thickness of that veneer layer — some can be lightly sanded once or twice, others cannot be sanded at all. Always check the manufacturer's specifications before proceeding.
Look for Structural Problems
Before worrying about the surface, check what's underneath. Walk slowly across the floor and note:
- Springy or bouncy sections — could indicate rotted or damaged joists beneath
- Squeaking boards — often caused by loose fixings or boards rubbing together
- Soft spots — a sign of moisture damage or rot
- Significant unevenness or cupping — boards that have warped due to moisture
Minor squeaks and small gaps can often be addressed as part of a sanding job, but structural issues need to be fixed first. BRANZ has useful guidance on diagnosing timber floor problems, including moisture-related damage common in New Zealand homes.
Assess the Surface Condition
Take a close look at the boards themselves:
- Deep gouges or burns — shallow ones sand out fine; very deep damage may remain visible or require board replacement
- Cupping or crowning — moisture-related warping that may need to dry out before sanding
- Previous filler or paint — old paint (especially on pre-1980s floors) may contain lead, which requires careful handling (see safety note below)
- Gaps between boards — small gaps are normal and can be filled; large gaps may need specialist attention
| Condition | Sandable? | Notes |
|---|---|---|
| Light surface scratches | Yes | Sands out easily |
| Old varnish or polyurethane | Yes | Will be removed during sanding |
| Shallow gouges | Usually | May still show faintly |
| Deep gouges or burns | Sometimes | May need board replacement |
| Cupping/warping | Sometimes | Fix moisture source first |
| Rot or structural damage | No | Repair joists/boards first |
| Very thin boards | No | Risk of sanding through |
Check for Nails and Fixings
Exposed nail heads will tear sanding belts and damage equipment. Before sanding, all nail heads must be punched below the surface — typically 2–3 mm. Walk the floor and mark any raised nails with chalk. This is something you can do yourself to save time (and money) if hiring a professional sander.
Identify Your Timber Species
New Zealand homes commonly feature rimu, matai, heart kauri, pine, and macrocarpa floors. Knowing your species matters because:
- Hardness varies significantly — softer timbers like pine scratch more easily after finishing
- Some species absorb stains and oils differently
- Native timbers like kauri are increasingly rare and valuable — a specialist floor sander familiar with NZ native timbers is worth seeking out
The Lead Paint Safety Check
If your home was built before 1980, there's a real chance old paint or coatings on your floors contain lead. Sanding lead paint creates hazardous dust. Before sanding any painted floor in an older home, use a lead test kit (available from hardware stores like Bunnings or Mitre 10) or have the surface professionally tested. WorkSafe NZ provides guidance on working safely with lead-based materials.
Do a Simple Moisture Test
Timber floors and moisture don't mix well. If your floor has been exposed to leaks, flooding, or high humidity, the boards may have swollen, cupped, or started to rot. A moisture meter (available to hire or buy) can give you a reading — timber generally needs to be at or below around 14–16% moisture content before sanding. Fix any leaks or subfloor ventilation issues first, and allow the floor to dry out fully.
Poor subfloor ventilation is a common issue in New Zealand, particularly in older homes. The Energywise website has helpful information on managing moisture in NZ homes.
When to Call a Professional
While a keen DIYer can tackle some floor preparation tasks, the actual sanding and finishing is best left to a qualified floor sander — especially for large areas, valuable native timber, or floors with complex issues. A professional will:
- Accurately assess board thickness and viability
- Use commercial-grade equipment that produces a far better result than hired consumer machines
- Know which finish (polyurethane, oil, hardwax) suits your timber and lifestyle
- Handle lead paint and dust safely
You can find experienced, local floor sanding professionals on tradedirectory.co.nz — it's always worth getting two or three quotes and asking to see examples of previous work.