Most people choosing decking start with the species and treat the board size as a detail to sort out later. It is worth doing in the other order at least once, because the size decides your quantity, your fixing count, your budget and, at our yard, which species are even available to you. This is the short version of everything we know about picking a board size, with links to the detail where it matters.
Start here: the four questions
- Narrow board or wide board? Narrow boards give a finer, more closely striped surface and more board lines. Wide boards give a cleaner, more open deck with fewer joins and fewer fixings. This is mostly an aesthetic call.
- Standard thickness or thick section? 19 mm is the standard decking thickness and the default for a residential deck on a conventional joist layout. 35 mm and 45 mm are heavier sections chosen for visual mass or a more substantial walking surface.
- Which species? This is usually the decision that matters most, and at Sydney Timber Supply it constrains the size, not the other way around.
- Are you matching an existing deck? If so, this overrides everything above. Measure what is there.
The sizes we supply
| Size | Species available | Cover width at a 4 mm gap |
|---|---|---|
| 86x19 mm | Ironbark, Spotted Gum, Forest Red Gum, Gympie Messmate, Blackbutt | 90 mm |
| 90x19 mm | Merbau, Jambu Mixed | 94 mm |
| 135x19 mm | Ironbark, Spotted Gum, Forest Red Gum, Gympie Messmate | 139 mm |
| 135x21 mm | Blackbutt, supplied at 21 mm rather than 19 mm at this width | 139 mm |
| 140x19 mm | Merbau, Peninsula Blonde | 144 mm, or 147 mm on Peninsula Blonde at its 7 mm minimum gap |
| 135x35 mm | Ironbark, Spotted Gum, Forest Red Gum, Gympie Messmate, each with a 140 mm width option | 139 mm, or 144 mm at 140 |
| 140x35 mm | Blackbutt | 144 mm |
| 90x42 mm | Merbau | 94 mm |
| 140x45 mm | Merbau | 144 mm |
The pattern is worth naming plainly: the Australian hardwoods with supplier-published Class 1 above-ground durability come in 86 and 135 mm widths, and the imported Merbau, Jambu and Peninsula Blonde lines come in 90 and 140 mm widths. If you have your heart set on a particular width, that will usually settle the species question for you.
Board width and thickness
The numbers on a decking board are not always what they seem, and the difference between the size a board is called, the size it measures and the width it covers is the most common source of ordering mistakes. Our sizes are actual sizes: an 86 mm board measures 86 mm. Cover width adds the gap on top. How decking board sizes are measured sets that out in full.
For the specific width comparisons, we have separate guides for 86x19 vs 90x19 at the narrow end and 135x19 vs 140x19 at the wide end. For thickness, the 135x19 vs 135x35 guide covers the step up at the same width, and 135x35, 140x35 and 140x45 compared covers the heavier sections against each other.
Random lengths and what they mean for you
Most of our hardwood decking is supplied in random lengths. You order a total number of linear metres and we supply boards that make up that quantity, rather than a chosen set of lengths. The current length mix depends on supplier availability, so if your deck depends on particular lengths, say so before you order and we will confirm what is holding. Preferred individual lengths can be requested by quote.
This is why decking is priced and ordered per linear metre rather than per board, and why your first job is converting an area into a run of board.
Estimating linear metres
The method is the same for every size:
- Measure the deck area in square metres.
- Decide the gap between boards. Check the minimum in the specification sheet on the product page, since it is not the same for every profile.
- Add the gap to the board width to get the cover width, and convert to metres.
- Divide the area by the cover width to get linear metres.
- Add an allowance for cuts and wastage.
The timber calculator does this for you, and the calculator guide shows the working so you can check it.
Before you order
- Confirm the actual face width in millimetres, not just the name of the size.
- Confirm the gap your quantity assumes.
- Check the specification sheet on the product page for minimum gaps, joist spacing and fixing distances.
- Ask about current availability and lead time, especially on the denser and less common species.
- If you are matching existing boards, measure them first and match the actual width.
Where to go next
Browse the full range in the decking collection, or the Australian species in the hardwood collection. If you would rather talk it through with quantities in hand, request a quote with your area and the species you are weighing up. We supply across Sydney and the Central Coast with pickup from West Gosford.