NZ TIMBER EDITORIAL GUIDE
How to Read Timber Dimensions
A listing such as 100 x 50 x 4.8 m normally describes two cross-section dimensions followed by length. But a useful purchase check goes further: are those dimensions nominal or finished, is the unit each or linear metre, and which face provides installed cover?
Read from cross-section to length
| Listing | Plain-language reading | Common trap |
|---|---|---|
| 100 x 50 x 4.8 m | 100 mm by 50 mm section, 4.8 m long | assuming the first number is always installed width |
| 90 x 19 decking - $/LM | board section, price per linear metre | treating $/LM as $/m2 |
| 150 x 19 x 1.8 paling | one 1.8 m paling | forgetting overlap or gap changes cover |
| 100 x 100 x 2.4 post | square post, 2.4 m long | assuming size establishes embedment or spacing |
Some sectors write depth x thickness while others prioritise width x thickness. Use the product drawing/profile and do not rely on number order when orientation affects the build.
Nominal, rough-sawn and finished
Nominal names help group products, but actual dimensions can change through sawing, drying, gauging or profiling. A dressed board may finish smaller than its nominal source size. Structural connectors and renovations around existing framing need the declared supplied section, not a remembered nickname.
Cover is not always face width
Decking coverage includes the installed gap. Rusticated or shiplap profiles may overlap, so effective cover is narrower than board width. Palings may be gapped, butted or overlapped. Quantity calculations must use effective cover for the chosen layout.
| Layout | Coverage input |
|---|---|
| 90 mm decking with documented 5 mm gap | 95 mm installed module |
| 150 mm paling with 20 mm overlap | 130 mm effective cover |
| Board-and-batten fence | board cover plus batten sequence |
| Profiled cladding | producer-declared effective cover |
The example gaps and overlaps illustrate arithmetic only; obtain the installation requirement for the actual product.
Units change the comparison
- Each: multiply unit price by pieces.
- LM: multiply rate by total linear metres, including waste.
- m2: confirm whether the rate describes sheet area, installed cover or a service.
- Sheet: compare dimensions and thickness as well as the price.
- Pack: confirm piece count, length mix and whether broken packs are available.
For 24 lengths of 4.8 m at $5.20/LM, total linear length is 115.2 m and the reference is $599.04 before tax/freight assumptions. For 24 pieces priced at $5.20 each, it is $124.80. The unit is commercially decisive.
Profile drawings beat listing shorthand
Decking, weatherboards, mouldings and tongue-and-groove sheets can have grooves, rebates or rounded edges. The headline width may be the widest physical dimension while effective cover is shown only on a profile drawing. Ask for that drawing when quantities or junctions depend on millimetres.
Timber movement also means an installed module is not guessed from today's tape measurement. The supplier may specify a gap according to species, moisture condition and season. Use documented cover/gap rules for the selected batch and system.
- Face width: visible or widest surface.
- Thickness: maximum body depth, excluding some profile details unless stated.
- Effective cover: increment added by one installed piece.
- Length: stock or set length; random length means a distribution, not an average promise.
A unit-conversion desk check
For 35 boards at 4.8 m, the order contains 168 LM. If each board has 95 mm installed module, the theoretical covered area is 15.96 m2 before end waste: 168 x 0.095. If a quote instead prices by piece, compare 35 units. Write every conversion with the unit beside the number.
| Conversion | Formula |
|---|---|
| pieces to LM | pieces x length in metres |
| LM to theoretical area | LM x installed cover in metres |
| area to LM | area / installed cover in metres |
| sheet area | length x width in metres |
| net to order quantity | net x (1 + justified waste rate) |