A Merbau deck can look straightforward on a plan, but the board width and thickness you choose affect the finished appearance, material quantity, fixing method and subframe design. Merbau decking board sizes are not simply a cosmetic choice. Selecting the right profile before ordering helps avoid mixed batches, unnecessary joins and a deck that feels out of proportion with the home.
For most residential decks around Sydney, the decision comes down to 90 mm or 140 mm wide boards, generally in a 19 mm thickness. Wider and heavier-duty options are also available for particular applications. The right choice depends on the deck area, joist layout, access for handling boards and the finish you want when the job is complete.
Common Merbau decking board sizes
Merbau is commonly supplied as a dressed hardwood decking board with eased edges and a smooth face. Actual available sizes can vary between mills and stock lines, so always confirm the finished dimensions, length range and profile before finalising quantities.
The most common board sizes for residential decking are 90 x 19 mm and 140 x 19 mm. Both are proven choices for outdoor decks, patios and entertaining areas. A 90 mm board delivers a traditional hardwood deck look with more board lines across the surface. A 140 mm board covers the same area with fewer boards and fewer fixings, creating a broader, cleaner visual finish.
For projects needing extra section depth, 140 x 25 mm Merbau decking may be specified. This is a heavier board and is often considered where a more substantial deck surface is wanted, or where the supporting structure has been designed for it. Do not treat a thicker decking board as a substitute for correctly sized joists, bearers and posts. The subframe still needs to suit the span, load and site conditions.
Merbau decking is commonly supplied in mixed or random lengths rather than one fixed length. Lengths may range from shorter boards suitable for stairs and landings through to longer lengths for main deck runs. Exact availability changes with supply, so a cut plan should work with the lengths on hand rather than assume every board will arrive at the same length.
90 x 19 mm Merbau decking
The 90 x 19 mm board is a practical option for smaller decks, balconies, verandahs, steps and detailed areas with several corners or cut-outs. It is easier to handle than a wider board and can reduce waste where the deck has short runs. Its narrower face also suits heritage-style homes and projects where a finer board pattern is preferred.
The trade-off is that a 90 mm board takes more lineal metres to cover the same square metres. It also means more fasteners and more time spent setting out and installing boards. On a large open deck, that additional labour can matter.
140 x 19 mm Merbau decking
A 140 x 19 mm board is a popular choice for larger entertaining decks and open backyard areas. The wider face can make a deck feel less busy and can reduce the number of boards, fixings and joins required across the surface. It is often a good fit where the deck runs are long and clear.
Wider boards need consistent board spacing and careful fixing to maintain an even finish. They can also be less convenient to work around posts, angled edges and tight service penetrations. If the deck has many interruptions, the apparent material saving from wider boards can be reduced by offcuts.
140 x 25 mm Merbau decking
The 140 x 25 mm option gives a more substantial board profile and can suit high-use spaces, commercial-style applications or projects designed around a heavier hardwood finish. It adds weight and cost, and its suitability comes back to the engineered or planned subframe.
For a standard residential deck, 19 mm boards are often the efficient choice when installed over a correctly built frame. Choose 25 mm because it suits the design and performance requirements, not simply because thicker sounds better.
Choosing the right Merbau decking board size
Start with the scale of the deck. A compact courtyard deck can look balanced with 90 mm boards, particularly when it includes steps, planter edges or multiple direction changes. A broad deck beside a pool or outdoor room often suits 140 mm boards because the wider lines keep the surface visually calm.
Next, consider the board direction. Decking should usually run in the direction that best suits drainage, the main sightline and the practical joist arrangement. Long, uninterrupted board runs look sharp, but they may require longer stock lengths and more planning around butt joints. Stagger joins across the deck rather than lining them up in one row.
Access is another real-world factor. A 140 mm board is still manageable for most jobs, but long hardwood lengths are heavy. For a narrow side access, elevated deck or apartment-style balcony, shorter lengths or narrower boards can make handling safer and simpler.
Finally, match the board to the rest of the build. A deck surface only performs as well as its structural timber, fixings and moisture management. H3 treated pine or suitable hardwood framing, correctly spaced joists, galvanised or stainless steel fasteners and joist protection tape all contribute to a longer-lasting result.
Board spacing, coverage and quantities
Merbau is a natural hardwood, so it responds to changes in moisture and weather. Leave a consistent gap between boards to allow drainage, airflow and seasonal movement. A typical deck gap is around 4-6 mm, but the final spacing should reflect the board moisture content, installation conditions and the fixing system being used.
That gap changes your coverage calculation. A 90 mm board with a 5 mm gap has an effective cover of approximately 95 mm. A 140 mm board with a 5 mm gap has an effective cover of approximately 145 mm. Use the effective cover, not the face width alone, when estimating lineal metres.
As a simple guide, divide the deck area in square metres by the effective board cover in metres. For example, 20 square metres divided by 0.145 gives about 138 lineal metres of 140 mm decking. Add an allowance for trimming, cuts, defects and layout changes. A straightforward rectangular deck may need around 5-10 per cent extra, while decks with angles, stairs, curved edges or lots of posts can require more.
Do not forget fascia, stair treads, picture-frame borders and screening if they are part of the design. These components are often made from the same Merbau profile but should be measured separately. Ordering enough surface decking without allowing for edges and steps is a common cause of delays.
Match joist spacing to the board and job
Joist spacing must suit the decking thickness, board width, expected use and relevant construction requirements. For many 19 mm hardwood decking applications, joists are commonly set at centres around 450 mm or closer, but the correct spacing should be verified for the exact board and project. Heavier traffic, diagonal board layouts and areas that may carry outdoor furniture or a spa can require a tighter layout or additional structural design.
Diagonal decking is worth calling out. It looks excellent, but boards cross the joists at an angle, increasing the effective span between supports. That normally means closer joist centres and more cutting. Plan this before materials are ordered, not after the frame is built.
Use fixings suitable for Merbau and external exposure. Hardwood can split if fixed without pre-drilling, especially close to board ends. Pre-drilling and using quality stainless steel screws gives a clean finish and helps prevent staining associated with incompatible metals. Hidden fastening systems may suit selected board profiles, but confirm the profile and installation requirements first.
Allow for Merbau's natural characteristics
Merbau has a rich brown colour and distinctive grain, but it is not a uniform manufactured board. Colour variation, grain movement and natural feature are part of the material. Set boards out from several packs during installation to distribute variation across the deck rather than creating patches of similar colour.
Fresh Merbau can also release natural tannins when exposed to rain. Keep this in mind near light-coloured paving, rendered walls, pool surrounds and drainage points. Protect nearby surfaces during construction, direct runoff appropriately and follow the recommended cleaning and finishing process.
A suitable decking oil can help maintain the colour and reduce weathering, but it is maintenance, not a one-off treatment. How often it is needed depends on sun exposure, foot traffic, leaf debris and how exposed the deck is to rain. A covered veranda and a full-sun western deck will not age at the same rate.
For a clean order, prepare a plan showing deck dimensions, board direction, stairs, borders and structural spans. Bring that together with your preferred Merbau decking board size, then source the boards, framing, screws, protection tape and finishing products as one project package. Green Forest Timber can help Sydney homeowners and trade customers line up those materials before the first bearer goes in.
A well-sized board makes installation easier, but the best result comes from planning the whole deck around it. Measure twice, allow for gaps and offcuts, and confirm current stock lengths before cutting your frame to suit.