If you want a wide decking board, Sydney Timber Supply sells two widths that sit 5 mm apart: 135x19 mm and 140x19 mm. They look almost identical on paper. In practice the choice is not really about the 5 mm at all. It is about which timber you want, because at our yard the width follows the species.
The short answer
Our 135x19 mm boards are Australian hardwoods with supplier-published Class 1 above-ground durability. Our 140x19 mm boards are imported: Merbau and Peninsula Blonde, both listed by the supplier as Class 2 above ground. So choosing between the two widths usually means choosing between an Australian Class 1 species and an imported Class 2 one, and the width difference comes along for the ride.
One Australian board sits between them. Blackbutt is supplied at 135x21 mm rather than 135x19 mm, so it shares the 135 mm width and the 139 mm cover width, with a slightly thicker section.
What we supply at each width
| Width | Species | Origin | Supplier durability |
|---|---|---|---|
| 135x19 mm | Ironbark, Spotted Gum, Forest Red Gum, Gympie Messmate | Australian grown | Class 1 above ground |
| 135x21 mm | Blackbutt | Australian grown | Class 1 above ground |
| 140x19 mm | Merbau, Peninsula Blonde | Imported | Class 2 above ground |
Durability classes are taken from our supplier's specification sheets, which are linked on each product page. A durability class describes natural resistance in the heartwood. It is not a warranty, and it does not remove the need for correct gaps, ventilation, sealed cut ends and a maintained finish.
The two 140x19 boards are not interchangeable in character. Merbau is a well-known reddish-brown decking hardwood; Peninsula Blonde is our Feature Grade line, with pronounced natural variation from pale blonde and honey through tan to deeper brown across boards and packs. Read the product pages before choosing between them.
What the 5 mm actually changes
Board width plus the gap between boards gives the cover width, and cover width is what turns a deck area into a linear metre quantity. At the 4 mm gap our calculator assumes as a default:
- a 135 mm board covers 139 mm, so a 20 m2 deck needs about 144 linear metres before wastage;
- a 140 mm board covers 144 mm, so the same deck needs about 139 linear metres.
That is a difference of about 3.5 percent in linear metres. It also means about 3.5 percent fewer fixing rows, since decking is fixed with two screws per joist per our supplier's installation guidance. On a small deck this is noise. On a large one it is worth putting into the estimate rather than assuming the two widths are equivalent.
Peninsula Blonde is the exception to the 4 mm default: its supplier specifies a minimum 7 mm gap, which lifts its cover width to 147 mm. Check the gap on the product page before you calculate.
What the 5 mm does not change is the look. Once laid, a 135 mm deck and a 140 mm deck both read as a wide-board deck. Nobody standing on the finished surface will identify the difference.
When the width genuinely decides it
- You are extending or repairing an existing deck. This is the one case where the width is the whole decision. Measure the existing boards and match the actual face width. A 135 mm board will not line up in a 140 mm deck, and the mismatch compounds across every row.
- You are working to a fixed deck width with no rip. If you want whole boards edge to edge with no cut board at the last row, the cover width has to divide into your deck width. Sometimes one of the two widths lands closer than the other.
- A specification names the size. If a drawing calls for 140x19, supply 140x19.
Outside those cases, choose the species first and accept the width that comes with it.
Availability is part of the decision
Stock position matters as much as specification. Our Merbau lines move in and out of stock, and when a line is sold out it can still be ordered ahead against the next supply, with the lead time confirmed at the time. Peninsula Blonde is a genuine limited run rather than a standing line, so check the current state before planning around it. The Australian species at 135 mm are more often supplied to order than carried as continuous shelf stock, particularly the denser and less common ones. Check the current state on the product page and ask before you commit to a date.
Working out your quantity
Measure the deck area, decide the gap, and convert to linear metres with the timber calculator. Add an allowance for cuts and wastage. If the difference between nominal, actual and cover width is not yet clear, start with how decking board sizes are measured, then come back to this comparison.
For the thicker sections at these widths, see 135x35, 140x35 and 140x45 compared, and the 135x19 vs 135x35 guide for the step up in thickness at the same width. The full decision is walked through in how to choose hardwood decking board sizes in Sydney, and the range is in the decking collection.