A board sold as 90x19 mm and a board sold as 86x19 mm can end up covering exactly the same width of deck. That is not a contradiction, and it is not a supplier being loose with the numbers. It is the difference between the size a board is called, the size it actually measures, and the width it covers once it is laid with a gap. Getting those three apart is the single most useful thing you can do before ordering decking.
The three numbers that matter
Every decking board carries up to three widths, and only one of them is the number on the label.
- Nominal width. The name the size is sold under. At Sydney Timber Supply the size in the product title is the size we hold the board to, so our nominal and actual widths are the same number.
- Actual width. What a tape measure reads across the face of the board. This is the number that decides whether a replacement board will sit in an existing deck.
- Cover width. The actual width plus the ventilation gap between boards. This is the number that decides how many linear metres your deck needs.
Confusion almost always comes from the gap between the first two. In some parts of the timber trade a board is named for the size it was sawn at and then sold after it has been dressed down, so a board called 90 mm can measure less than 90 mm across the face. That practice is real, but it is not universal, and it is not how our range is described.
What our boards actually measure
Our decking sizes are actual sizes. An 86x19 mm board measures 86 mm across the face. A 90x19 mm board measures 90 mm. We do not list a board at one width and supply another.
Here is what that means once the gap is added. The figures below use the 4 mm gap our timber calculator assumes as its default.
| Board | Actual width | Cover width at a 4 mm gap | Linear metres for 20 m2 |
|---|---|---|---|
| 86x19 mm | 86 mm | 90 mm | about 222 lm |
| 90x19 mm | 90 mm | 94 mm | about 213 lm |
| 135x19 mm | 135 mm | 139 mm | about 144 lm |
| 140x19 mm | 140 mm | 144 mm | about 139 lm |
Those linear metre figures are the deck area divided by the cover width in metres, before any allowance for cuts and wastage. They are an illustration at one area and one gap, not a quote.
Notice the coincidence in the first row. An 86 mm board laid at a 4 mm gap covers 90 mm. That is why an 86x19 deck and a 90 mm cover width can describe the same surface, and it is also why the two boards are not interchangeable: a genuine 90 mm board laid the same way covers 94 mm, so it needs fewer boards for the same area.
Why the gap is not a detail
The gap between boards is there for drainage and airflow, and it changes your order more than most people expect. On the 86 mm board, moving from a 4 mm gap to a 6 mm gap lifts the cover width from 90 mm to 92 mm and takes roughly 5 linear metres off a 20 m2 deck. Our supplier documentation sets a minimum gap for each profile, and it is not the same figure on every board, which is exactly the sort of detail worth reading before you calculate.
Work your own numbers with the timber calculator, which takes the board width and the gap and returns the linear metres. The arithmetic behind it is set out in the decking timber calculator guide.
How to check what you are actually buying
- Ask for the actual face width in millimetres, not the trade name of the size. If a supplier quotes a nominal size, ask what a board measures.
- Ask what gap the quantity assumes. A linear metre figure without a stated gap cannot be checked.
- Measure an existing board before you order a repair. Matching the cover width matters more than matching the name of the size.
- Check the specification sheet on the product page. Ours publish the supplier data for each profile, including minimum gaps and fixing distances.
- Confirm the length mix. Most of our decking is supplied in random lengths, so the total linear metres is what you order, not a set of chosen lengths.
Which of our boards is which
Our narrow Australian hardwood boards are 86x19 mm in Ironbark, Spotted Gum, Forest Red Gum, Gympie Messmate and Blackbutt. Our genuine 90x19 mm boards are the imported Merbau and Jambu Mixed. Which species comes in which width, and why, is set out in our 86x19 vs 90x19 decking guide.
For the wide boards, the same measurement logic applies at 135 mm and 140 mm, and that comparison has its own guide. For the heavier sections, see 135x35, 140x35 and 140x45 compared. To work through the whole decision from the start, read how to choose hardwood decking board sizes in Sydney.