How to choose decking boards: profile, span and thickness

A decking board is a small beam that you walk on. That is the sentence that should sit above every sample rack. Colour is a later conversation. Species is a serious conversation, and we have had it for Australian hardwoods and for composite in Australian heat. This essay is about the geometry: thickness, profile, the way a board spans, the groove that accepts a hidden clip, the gap that lets the deck breathe, and the fall that stops a threshold becoming a dam.

I have watched people choose a board the way they choose a sofa — by rubbing the face in an air-conditioned yard. Then the same board is asked to jump 450 millimetres between joists in a western sun, with a barbecue on one end and a crowd on the other. The sofa does not have to do that. The board does.

Spanning tables are typical, not gospel. They assume a species, a grade, a moisture content and a load that may not be your Saturday. AS 1684 and the National Construction Code will not rescue a thin board on a heroic spacing because a flyer said “suitable for decks”.

Thickness: 19, 21 and 32 millimetres

These three numbers are the ones you will actually meet in an Australian timber yard. There are others — imported profiles, composite thicknesses that do not match hardwood, structural planks that are really floorboards — but the argument on most jobs is between a slim wearing board and a thicker one.

Nineteen millimetres is the slim contemporary look. It is lighter on the frame, cheaper per metre in many species, and easier to handle up a set of stairs. It is also less forgiving. The board has less depth to share load, less meat above a hidden groove, and less patience for a joist that was set a bit wide “because the last deck was fine”. Use 19 mm when the joist spacing is tight, the species is up to it, and the deck is not a dance floor. Do not use it to stretch a quote.

Twenty-one millimetres is the common hardwood decking thickness for a reason. It is enough section to feel like timber, enough to take a screw without looking nervous, and enough to span the spacings most domestic frames already use. If you are unsure, and the species is a durable eucalypt, 21 mm is the default that still looks like a designer surface rather than a boardwalk.

Thirty-two millimetres is a different object. It reads as architecture. It can span further, take a harder life, and make a low deck feel like a platform rather than a skin. It is heavier. It costs more. It asks the frame to be designed for the mass, especially if you are going over pine joists that were sized for 19 mm. Used on stairs and on the field, it can unify a job. Used only on the treads, it can be a deliberate change of gear.

Thickness is not a moral scale. A 19 mm board on 300-millimetre centres in a quiet courtyard can outlast a 32 mm board on a soggy, wide-spaced frame. The number is a starting point for the span conversation, not a substitute for it.

Profile: reeded, smooth, pencil-round and the edges you feel

The face of the board is a maintenance decision dressed as a style decision.

Reeded — the fine grooves running along the face — was sold for years as the non-slip option. It can help when the board is new and wet. It also collects grit, tannin and the grey paste that Australian decks make out of dust and sunscreen. Cleaning a reeded face is longer work. Bare feet meet ridges. Furniture leaves a more complicated mark. I like reeded on a service run, a shady side path, a place where grip matters more than the evening of the surface. I am less romantic about it under a dining table.

Smooth is easier to live with than folklore admits. A good arris or a light pencil-round stops the edge feeling sharp. Water sheets. A broom works. The timber’s grain, which is why you paid for hardwood, is visible instead of being broken into a corrugated pattern. Smooth can be slippery when it is new, wet and oily; so can tiles, so can composite, so can reeded boards with a film of algae. Grip is a system — fall, shade, biofilm, footwear — not a texture stamp.

Many Australian hardwood boards are dressed to be used either face up. That is useful. It is also how a job ends up half-and-half when two carpenters disagree. Decide on the drawing. If you are using a profile that is only finished one face, do not discover it when the pack is half down.

Edges matter more than people think. A sharp square edge looks precise on day one and feathers as the sun works the arris. A small radius ages more kindly. A wide board with a big bevel can look plantation-heavy next to a fine house. Match the edge to the architecture, not to the first pack on the pallet.

Grooves, clips and the thickness you no longer have

Boards grooved for hidden fasteners are not the same section as a square-edge board of the same nominal thickness. The groove removes material exactly where the clip asks the board to lock. On a 19 mm profile that groove is a meaningful bite. On a 21 mm it is more comfortable. On some composites the groove is the product.

If you choose a hidden system, choose the board that was milled for it. Site-routing a groove into a random hardwood pack is how clips sit tight on Monday and rattle in March. Confirm that the species’ movement and density suit the clip. Ironbark in a clip designed for a polite imported board is a common disappointment.

Also confirm how a single board comes out when it fails. A designer deck that cannot surrender a stained or split plank without destroying a field of neighbours is not a clever fastening idea. It is a future argument.

Spanning tables, typical figures, and the load you will not admit

Yards publish spanning figures. Manufacturers publish them. They are useful. They are also typical. They assume a uniformly distributed load of a certain kind, a grade, a dry board, and a joist that is actually where the drawing said. They do not assume a spa step, a masonry planter, a crowd at a twenty-first, or a joist that wandered.

Treat a table as a conversation starter:

• If the table says a 19 mm board will span 450 millimetres in that species, that is a starting maximum, not a target.
• If the household is lively, close the spacing or thicken the board.
• If the joists are already in and a bit adventurous, the board cannot rescue them. Resize the frame or live with the bounce.
• If the board is composite, use the composite table, not the hardwood one. The materials do not share a spine.

AS 1684 is a framing document. It helps you think about the structure under the board. It does not bless a wearing surface that was chosen from a colour swatch. When a deck is a storey up, or carries a spa, or sits in a bushfire-prone setting with extra mass and extra rules, you are past typical tables and into a practitioner’s drawing.

Ventilation: the gap is not a style

The gap between boards is how the deck dries, how it moves, and how the leaf litter either falls through or becomes compost. A tight, almost-closed contemporary gap looks good in a photograph taken the morning after installation. By the third autumn it can be a series of damp knife-edges.

Leave what the species and the profile need. Hardwood wants room to swell in a wet week. Composite wants the gaps the manufacturer drew, including the ends. A coastal deck wants enough space that salt water can drain and be washed through. A leafy suburban deck wants enough space that you are not paying someone to pick out Liquidambar seeds with a screwdriver.

There is a lower bound: a gap so wide it catches a chair leg or a child’s shoe. There is an upper bound: a gap so tight the boards become a single sweating skin. Typical domestic hardwood sits in a modest millimetre range that every carpenter knows and every architect tries to shrink. Resist the shrink unless you have a species and a climate that forgive it.

The other ventilation is underneath. A deck hard against soil, wrapped in foil, boxed in with no vermin-proof but air-possible undercroft, will cook or rot regardless of how pretty the gap looks from above. Boards cannot ventilate a closed box.

Fall: drainage is a kindness to the house

A dead-flat deck is a shallow pond with furniture. You need a fall — away from the building, toward a grate, toward the lawn, toward wherever water is allowed to go. The number can be modest. The direction cannot be accidental.

Watch the threshold. Most wet-season stains I see are not on the field; they are at the sliding door, where the last board was packed up to meet a track and created a dam. Watch the step off a covered area onto an open deck: the covered boards stay fat with moisture, the open boards dry and shrink, and the join becomes a ridge if nobody planned it.

Fall and thickness meet at the stair. A 32 mm tread with a 19 mm field needs a nosing logic. A composite stair tread with a different span rule needs its own note. Do not let the field’s aesthetic dictate a tread that flexes.

A way to choose without drowning in samples

Work in this order, even if the yard wants you to start with colour.

1. Climate and use. Barefoot pool path, dining terrace, service run, storey-up platform. Heat, leaves, salt, kids.
2. Frame. Joist spacing you can actually have, not the spacing you wish the existing joists were.
3. Thickness and species together. A durable hardwood at 21 mm, a composite at its specified span, a 32 mm platform if the architecture wants weight.
4. Profile and fastener. Smooth or reeded; square edge and screws, or grooved and clips. Decide how a board is replaced.
5. Gaps and fall. Write them. Do not leave them to “what we usually do”.
6. Colour and finish. Last, because they are the only part you can change without a saw.

If two boards survive that order, take the one you can buy in length and grade without mixing packs. A designer deck is a field, not a collage of leftovers.

The board you want is the one that spans honestly, dries after rain, and still feels like a decision in five summers. Everything else is a sample.