Vienna Woods · For builders & specifiers
Installed to code, still out of spec: the subfloor gap nobody owns
A subfloor can pass every building-code check and still miss what engineered timber flooring needs. Here is where the numbers part company, and how to specify the gap so it does not land on the builder.
Most flooring disputes are not about the flooring. They start at the subfloor, in the gap between what the building code accepts and what an engineered timber floor actually needs. The code is written around level; the floor is sold on flat. The two are measured differently, to different numbers, and the tighter one is enforced by the manufacturer’s warranty rather than the building consent. If nobody writes it into the contract, the cost of closing that gap tends to land on the builder.
First, the distinctionThey sound interchangeable and they are not. Level is about slope: how far the floor tips away from horizontal across the room. Flat is about local deviation: the bumps and hollows you find under a straightedge, whether or not the whole plane is sloping. A floor can be dead level and still be lumpy, or run slightly downhill and be perfectly flat. Engineered timber flooring cares about flat. A board bridges a hollow and rocks underfoot; it rides a hump and the joint peaks. Gentle slope it can live with; local deviation it cannot.
Flat is not level
Where the numbers partThe building code mostly governs level, and where it does touch flatness it is looser than any engineered-flooring spec. NZS 3604, the timber-framing standard, carries a single floor figure and it is for level, not flat: about 5mm of deviation from level in any 10m. MBIE’s Guide to Tolerances, Materials and Workmanship allows an individual board or sheet to sit out of flat by around 6mm over 3m. NZS 3124 allows a depression of up to 8mm under a 3m line on timber bearing surfaces. Engineered timber flooring is tighter than all three: the common manufacturer spec, and the warranty behind it, want the subfloor flat to within 3mm under a 3m straightedge. So a subfloor can clear every code figure and still miss the flooring spec by a clear margin.
The standards figures above come from NZS 3604:2011 (Table 2.1), the MBIE Guide to Tolerances (section 2.1) and NZS 3124:1987 (Table 2). The 3mm figure is not itself a New Zealand standard; it is the manufacturer and installer benchmark discussed below. Confirm the exact target for your floor to the manufacturer’s installation spec.
Code-compliant and flooring-ready are different targets
| Source | What it sets | Figure |
|---|---|---|
| NZS 3604:2011 | Floor level (deviation from horizontal) | 5mm in 10m, max 10mm over longer runs |
| MBIE Guide to Tolerances | Flatness of an individual board or sheet | Within about ±6mm over 3m |
| NZS 3124:1987 | Depression on a timber bearing surface | Up to 8mm under a 3m line |
| Engineered flooring spec | Subfloor flatness before laying | Within 3mm under a 3m straightedge |
The ownership gapHere is the awkward part: the 3mm-under-3m figure does not come from the Building Code or a cited New Zealand standard. It is a convergence. Flooring-association guidance (the ATFA specification is the common reference across the Tasman and here), individual manufacturer installation specs, and ordinary installer practice have settled on roughly the same number, and it is enforced through the flooring warranty rather than a consent. The association guidance itself tightens for some subfloors: ATFA puts the limit at 3mm under a 3m straightedge for a slab, but 3mm under a 1.5m straightedge for a sheet, batten or joist subfloor. So no single document a builder signs up to at consent actually carries the figure. The inspector signs off level; the manufacturer’s warranty quietly demands flat. When the floor later rocks or gaps and a warranty claim is declined for an out-of-tolerance subfloor, the cost of putting it right lands on whoever the contract left holding it. Absent a clause, that is usually the builder.
Nobody owns the 3-in-3 figure
How to specify itThe fix is cheap and it is contractual: name the number, name who hits it, and name who records it. Three lines in the specification close the gap.
Write the figure into the subfloor spec
State the tolerance
Put the flooring figure in the subfloor spec in plain terms: subfloor to be flat to within 3mm under a 3m straightedge, confirmed against the selected manufacturer’s installation spec, and tightened to 3mm under 1.5m for sheet, batten or joist subfloors if that spec requires it. Do not leave it at “code-compliant”, because code-compliant is the looser number.Name who grinds and fills
Flatness is achieved by grinding the highs and filling the hollows with a suitable levelling compound before the floor arrives. Say whose scope that is, the builder’s or the flooring contractor’s, and whose budget carries it. Left unsaid, it becomes a variation argued on site after the boards are already in the room.Name who records the readings
Require subfloor flatness and moisture to be checked and the readings recorded before laying, and say who signs that off. A dated record, straightedge and moisture both, is what protects the warranty and settles a dispute before it starts. It is fair for any party to ask to see the readings for the job.Before you sign off
None of this is exotic. It is one number, one scope line and one record, agreed before the subfloor is signed off rather than argued after the floor is down.
Subfloor sign-off checklist
| Check | What good looks like |
|---|---|
| Flatness figure specified | 3mm under a 3m straightedge written into the subfloor spec, not left to code |
| Scope assigned | Grinding and filling sits in a named party’s scope and budget |
| Flatness verified | Straightedge checked across the floor and the result recorded before laying |
| Moisture verified | Slab or timber subfloor read and recorded; barrier chosen off the reading |
| Readings on file | Dated flatness and moisture records held, available on request |
| Manufacturer spec confirmed | Target figures checked against the selected floor’s installation spec |
Good to know
Not necessarily. The building code mainly sets level, not flatness. NZS 3604 allows about 5mm off level in 10m, the MBIE tolerances guide allows around 6mm out of flat over 3m, and NZS 3124 allows up to 8mm under a 3m line. Engineered timber flooring typically wants the subfloor flat to within 3mm under a 3m straightedge, so a floor can be fully code-compliant and still fall short of the flooring spec.
The common manufacturer and installer benchmark is a subfloor flat to within 3mm under a 3m straightedge before laying, tightening to 3mm under 1.5m for some sheet, batten or joist subfloors. It is set by the flooring manufacturer’s installation spec rather than a NZ standard, so confirm the exact figure for your floor.
Level is slope, how far the whole floor tips from horizontal. Flat is local deviation, the bumps and hollows under a straightedge. Engineered timber flooring cares about flat: a board bridges a hollow and rocks, or rides a hump and the joint peaks. A floor can be level and still be lumpy.
It depends on the contract. If the flooring figure is not written into the subfloor spec, the cost of grinding, filling and re-laying usually lands on the builder when a warranty claim is later declined. Naming the tolerance, the scope and the record in the specification is what moves that risk off the builder.
Common questions
Does a code-compliant subfloor meet the engineered flooring spec?
Not necessarily. The building code mainly sets level, not flatness. NZS 3604 allows about 5mm off level in 10m, the MBIE tolerances guide allows around 6mm out of flat over 3m, and NZS 3124 allows up to 8mm under a 3m line. Engineered timber flooring typically wants the subfloor flat to within 3mm under a 3m straightedge, so a floor can be fully code-compliant and still fall short of the flooring spec.What is the subfloor tolerance for timber flooring in NZ?
The common manufacturer and installer benchmark is a subfloor flat to within 3mm under a 3m straightedge before laying, tightening to 3mm under 1.5m for some sheet, batten or joist subfloors. It is set by the flooring manufacturer’s installation spec rather than a NZ standard, so confirm the exact figure for your floor.What is the difference between flat and level?
Level is slope, how far the whole floor tips from horizontal. Flat is local deviation, the bumps and hollows under a straightedge. Engineered timber flooring cares about flat: a board bridges a hollow and rocks, or rides a hump and the joint peaks. A floor can be level and still be lumpy.Who is responsible if the subfloor is out of tolerance?
It depends on the contract. If the flooring figure is not written into the subfloor spec, the cost of grinding, filling and re-laying usually lands on the builder when a warranty claim is later declined. Naming the tolerance, the scope and the record in the specification is what moves that risk off the builder.Read next
Go deeper
The three tolerances that decide a floor
Flatness, moisture and the expansion gap, with the numbers a good installer works to. See understanding tolerances for timber flooring in New Zealand.How a floor is laid
The full method, subfloor to finished edge. Read our timber flooring installation guide.Specifying Vienna Woods
Samples, specifications and support for a specified project: for architects and specifiers.Vienna Woods · For builders & specifiers
Specify it once, and it stops being your problem
We work with builders, architects and specifiers to get the subfloor figure into the spec and the floor laid to it. Order free samples, or talk to us about a specified project.
