Building tolerances: defect or acceptable?

When a client points at a gap, a crack or a slope and calls it a defect, the MBIE Guide to Tolerances, Materials and Workmanship 2015 is how the industry decides who’s right. It helps you work out whether an imperfection is an actual defect under s362Q of the Building Act, or just normal building that a homeowner has to live with.

One rule up front: imperfections that only show up under critical lighting — low-angle sun at 15° or less — are not defects. If you have to stand in one spot at dawn to see it, it doesn’t count.

The 6-step process for deciding if it’s a defect

Before you reach for the Guide, the Guide itself says to work through the paperwork in order. Each step can settle the question before you get to the next one.

1. Check the contract, drawings, specifications and schedule of quantities.
2. Check the building consent and the supporting documents supplied to council.
3. Check the manufacturer’s specifications and installation instructions.
4. Check the builder’s defect tolerance schedule, if one was agreed in the contract.
5. Check any relevant NZ Standard — for example NZS 3604, NZS 3114 or NZS 3631.
6. Refer to this Guide to Tolerances.

Timber decks (§1.5)

Decks move with the weather, so a lot of what looks like a fault is just timber doing its thing. What matters is whether the boards stay put and stay reasonably level.

Counts as a defect:

• Slope steeper than 1:200 — that’s more than 10mm of fall over 2m.
• Floor out of level by more than 8mm in any 3m.
• Inconsistent gaps between boards.
• Height difference greater than 3mm between boards at butt joints.
• Fixings driven more than 2mm below the surface, unless that was specified.
• Cupping greater than 3mm per 100mm of board width.
• Rusting fixings, or fixings not aligned within ±3mm.

Counts as acceptable:

• Average gaps of 2–12mm as timber expands and contracts through the seasons.
• Average butt-joint gaps up to 6mm in dry weather.
• Surface checking, knots and knot dislodging in standard or merchantable grade — as long as the boards stay fastened.
• Fading, weathering and silvering of unfinished timber over time.
• Natural mould growth on unfinished timber.

Floors & concrete

Concrete floors (§2.2)

Some cracking in concrete is common and isn’t necessarily poor workmanship. The line is drawn on width, movement and how the pour was finished.

Counts as a defect: cracks wider than 3mm or with vertical displacement; visible reinforcing or ‘bony’ (poorly vibrated) concrete; hollows or mounds exceeding NZS 3114.

Counts as acceptable: defined crack-control joints (saw cuts), which are accepted practice; and some cracking, which is common and not on its own a sign of bad work.

Timber-framed floors (§2.4)

All timber floors move to some degree, so a bit of give is expected. The problems are the ones that come from how it was installed.

Counts as a defect:

• Slope steeper than 1:200.
• Floor out of level by more than 5mm in any 10m, or more than 10mm total over 10m-plus (NZS 3604).
• Sheets or boards not flat within ±6mm per 3m.
• Squeaks caused by incorrect installation, such as insufficient fastening.
• Damp subfloor or blocked vents.

Counts as acceptable: some squeaking and springiness that shows up over time, since all timber floors move to some degree.

Driveways & hard surfaces

Concrete driveways (§1.4)

Cracking is common in concrete, and a slight cross-fall is deliberate so water drains away. Judge the finish from a normal viewing distance of 3m, not with your nose to it.

Counts as a defect: unrepaired gaps wider than 3mm; variations in surface texture visible from 3m; and colour loss from recurring efflorescence.

Counts as acceptable: cracks up to 3mm wide; a slight cross-fall for drainage; discolouring from vehicle oil after handover; and any variation that isn’t visible from 3m.

Asphalt driveways (§1.3)

Fresh patch repairs look darker at first and lighten over time, and a bit of grit variation is normal.

Counts as a defect: cracks wider than 2mm; depressions deeper than 3mm; mounds higher than 6mm; and a surface plane outside ±6mm per 3m.

Counts as acceptable: patch repairs that are darker than the existing surface (they lighten over time); and some grit or texture variation.

Retaining walls (§1.2)

A retaining wall is doing its job when it stays plumb, drains properly and holds the fill back. Movement and drainage failures are the tells here.

Counts as a defect: outward bowing or buckling; water collecting behind the wall without drainage; coarse fill passing through the wall; and a wall that leans into the slope it’s retaining.

Counts as acceptable: specified gaps left for water drainage.