NZ TIMBER EDITORIAL GUIDE
Structural Timber Sizes NZ
New Zealand merchants commonly group structural timber around 90, 140, 190 and 240 mm depths with a 45 mm thickness. Those are product families, not a ladder of automatic spans. A designer selects the member after considering grade, spacing, loads, supports and exposure.
What the familiar size families are used to organise
| Catalogue family | Often encountered in | What still controls selection |
|---|---|---|
| 90 x 45 | wall framing, blocking, small members | design, stud height/spacing, grade and load |
| 140 x 45 | joists, rafters and deeper framing | span, spacing, load, restraint and treatment |
| 190 x 45 | floor/roof members | engineered or compliance-pathway design |
| 240 x 45 | deeper floor/roof members | full structural schedule and handling/availability |
Uses overlap. A 140 x 45 piece can be suitable in one joist layout and unsuitable in another. Never promote the next depth merely because it looks stronger without checking the design consequences.
Nominal description versus supplied section
A catalogue description is a buying convention. Sawing, drying and dressing can produce an actual finished section that differs from a rough-sawn or nominal label. Confirm the producer's declared dimensions and tolerances, especially when members must fit hangers, existing framing or proprietary connectors.
- State whether the schedule expects rough-sawn or machine-gauged timber.
- Match connector widths to the documented supplied section.
- Check whether treatment is applied wet and how that affects handling or movement.
- Do not use imperial nicknames such as 4x2 as the only order description.
Treatment families prevent a common ordering error
The Timber Dealers catalogue separates H1.2 protected framing and H3.2 exterior structural families. That reflects a genuine specification boundary. A size shared by both categories does not make the products interchangeable. Ground-contact work may require another product family entirely.
| Application clue | Treatment question | Structural question |
|---|---|---|
| enclosed interior framing | is H1.2 or another documented pathway required? | which grade, section and spacing? |
| exterior deck frame above ground | is H3.2 documented for the detail? | which joist/bearer design? |
| post embedded in soil | which H4/H5 product is specified? | post size, embedment and loads? |
Turn drawings into an order
- List every member mark from the drawing.
- Copy grade, treatment, section and length without abbreviation.
- Group identical specifications, then calculate quantity and waste.
- Keep connector, fixing and delivery requirements on linked lines.
- Ask the supplier to flag unavailable lengths or substitutions before pricing.
Length optimisation without redesign
Optimising stock length is a commercial exercise only after the structural cut list is fixed. A 6.0 m length may yield two 3.0 m members, but only if the drawing allows the cuts, end quality is acceptable and grade/treatment identification remains traceable. Splicing two shorter lengths is not an equivalent optimisation unless the design contains that joint.
| Cut requirement | Possible stock comparison | Check |
|---|---|---|
| two 2.4 m pieces | 4.8 m versus two 2.4 m | saw allowance and end defects |
| 3.6 m plus 1.2 m blocking | 4.8 m | whether offcut grade/treatment marks remain identifiable |
| single 5.4 m joist | 5.4 or 6.0 m | availability, delivery and waste |
| paired members | matched lengths | crown, moisture and connection schedule |
Give the supplier a cut schedule when optimisation is requested. A quantity of total linear metres hides which uninterrupted lengths are essential.
Depth affects the whole assembly
Moving from 140 to 190 mm changes more than timber cost. It can alter finished floor level, stair geometry, hanger type, blocking, service routes and truck handling. Width or thickness changes can affect connector fit and bearing. Any size substitution therefore goes back to the designer, even when the proposed member seems larger.
- Coordinate member depth with architectural levels.
- Match hangers and bolts to actual supplied section.
- Check long-length access before approving optimisation.
- Keep engineered timber products separate from solid-timber size families.
Sources and technical guidance
- MBIE - structural timber technical information (official)
- CodeHub - NZS 3631 grading rules (official)
- Timber Dealers - structural catalogue families (commercial)