Acacia decking: thickness, joists and a ventilated facade

Acacia decking is chosen by matching board thickness to the span between joists, and by accepting the grade that suits the traffic the deck will get. Boards of 20 to 25 mm sit comfortably on joists at 400 mm centres, while 30 mm and thicker boards can span 500 to 600 mm. A ventilated facade is a cladding fixed over a drained and ventilated cavity, and acacia needs it because the species moves with moisture and dries best when air can reach both faces.

How do you choose acacia decking boards by thickness and grade?

Thickness is a structural decision before it is a comfort decision. A board bends under a foot, and the amount it bends depends on the distance between the supports beneath it. The usual rule for a domestic deck is that the clear span between joists should not exceed roughly twenty times the board thickness. A 25 mm board therefore wants joists no further apart than about 500 mm, and a 20 mm board wants them at 400 mm or closer. Boards of 30 mm and above tolerate 600 mm centres, which reduces the number of joists and the amount of fixing.

Grade is a visual and practical decision. Decking is normally sorted into a higher grade, with fewer knots and straighter grain, and a standard grade that carries more knots, more colour variation and occasional small defects. The higher grade is easier to lay in long runs and takes a finish more evenly. The standard grade is cheaper and, on a deck, performs much the same underfoot once it is fixed and allowed to weather. Knots in acacia are tight and do not fall out as they do in some softwoods, so a knotty board is not automatically a weak one.

Two further points matter when boards are bought. Acacia is dense, and dense timber takes longer to dry, so boards should be bought dry and stored flat and off the ground before laying. Boards also need a gap of 4 to 6 mm between them for drainage and air movement. A deck laid tight, with boards touching, holds water on the surface and between the edges, and that is where rot begins. For a fuller account of how boards are sorted and described, there is a Slovak magazine on acacia timber that sets out the terasové dosky z agátu výber in terms of thickness, grade and intended use.

What is the right joist spacing under an acacia deck?

Joist spacing is set by the board, not by the joist. The joist itself is usually chosen for stiffness over its own span, and 45 x 70 mm or 45 x 95 mm timber at 400 to 600 mm centres is common for a domestic deck. The board above it then dictates whether that spacing is acceptable.

A working table for acacia decking:

• 20 mm boards: joists at 300 to 400 mm centres.
• 25 mm boards: joists at 400 to 500 mm centres.
• 30 mm boards: joists at 500 to 600 mm centres.
• 35 mm boards and above: joists at 600 mm centres, provided the joists themselves are stiff enough.

These figures assume normal domestic use, a deck close to the ground, and boards laid across the joists. Where a deck is raised, where it carries planters or furniture that stays put, or where the joists are supported on posts rather than on a continuous beam, the spacing should be reduced rather than increased. A board that feels springy underfoot is telling the truth about its span.

Direction matters too. Boards laid diagonally across joists span a longer distance between supports than boards laid at right angles, roughly 1.4 times as far, so diagonal laying usually means closing the joist spacing by one step. Boards laid parallel to the joists need a batten or a second layer of framing, and that is a different construction.

Fixings should be stainless or otherwise corrosion resistant, because acacia is often used with a finish and because the timber itself can react with iron in damp conditions. Two fixings per joist crossing, set about 20 to 25 mm from each board edge, hold a board flat and stop it cupping. Pre-drilling near board ends avoids splitting, which dense timber is prone to.

What is a ventilated facade and why does acacia need one?

A ventilated facade is a cladding carried on battens, with a gap behind the boards and an opening at the top and bottom of the wall. The boards are the outer skin. Behind them is a cavity, usually 20 to 40 mm deep, and behind that a drained and breathable layer over the wall structure. Air enters at the base, rises through the cavity and leaves at the top, carrying moisture with it.

The reason acacia suits this construction is that the species moves. Acacia, Robinia pseudoacacia, takes up and releases moisture with the weather, and a board fixed tight against a solid wall has nowhere to dry towards. Water that enters the timber from rain or from the air inside the building stays there, and the board stays wet for longer. A cavity gives the back face of the board the same chance to dry as the front, and it keeps the wall behind the cladding dry as well.

A ventilated cladding also deals with the two failures that are common on solid timber walls: trapped water at the base and staining from water running down the face. The base of the cavity should be open, with an insect mesh, and the bottom edge of the lowest board should sit clear of the ground or the deck by at least 20 mm. The top of the cavity should be open but protected from driving rain by a flashing or a capping. Horizontal boards need a small overlap or a drip edge so that water leaves the face rather than running back into the joint.

Battens should be durable and fixed through the drained layer into the structure, with the fixings long enough to carry the load and to allow the battens to be packed out where the wall is uneven. Vertical battens behind horizontal boards, and horizontal battens behind vertical boards, keep the cavity continuous. Where a wall is very exposed, the cavity can be closed at the top with a ventilated profile rather than left fully open.

Grades, finishes and what weathering does

Acacia left untreated greys to a silver tone and stays sound, because the extractives in the heartwood resist decay. A finish slows the colour change and makes cleaning easier, but it has to be maintained. A penetrating oil, renewed when water stops beading on the surface, is simpler to keep up than a film-forming coating, which can lift at the edges and trap water beneath.

Grade affects appearance more than life span. A higher grade gives longer clear runs and a more even colour at the start. A standard grade gives more variation, and on a large deck that variation is often what makes the surface look like timber rather than a sheet. Both grades need the same gaps, the same fixings and the same ventilation.

Maintenance that follows from the construction

A deck built with the right board thickness, joist spacing and cavity needs little beyond cleaning. Debris between boards and at the base of a cladding blocks the path that air and water are meant to take. A stiff brush along the gaps once or twice a year, and a check that the base openings are clear, does most of the work. Loose fixings should be tightened before a board starts to rock, because a rocking board wears the joist beneath it. A board that has cupped can often be turned over rather than replaced, provided the fixings are sound and the gap is restored.

The construction decisions, thickness, spacing and ventilation, are what determine how much maintenance the deck asks for later. Get those right and the surface is a matter of cleaning. Get them wrong and no finish will compensate.

The figures above rest on published guidance rather than on the supplier's word. The FAO document on wood durability sets out how moisture, air movement and fixing detail decide whether a timber such as acacia lasts in service, which is why the deck here is built with a ventilated facade, spaced boards and joists kept clear of standing water. The source is cited for the facts only; it is not a partner, a sponsor or a seller of anything described in this entry.

Acacia boards are laid on joists, and the gap beneath them is not wasted space. Air moving under the deck carries away moisture, so the timber dries after rain instead of sitting wet. Thickness and joist spacing decide how much the boards flex underfoot; the void below decides how long they last. The same principle, a house treated as wall, ground and roof working together, is set out in a second Kept entry on this subject, where the deck, the soil under it and the wall behind are measured as one system rather than three separate jobs.

Acacia boards are usually laid at 25 to 30 mm thick, fixed across joists set at 300 to 400 mm centres, with a gap of 5 to 8 mm between boards and a clear void of at least 50 mm beneath. The deck then behaves as a ventilated facade: air moves under it, water drains away, and the boards dry on both faces. The same reading of a surface applies to wood that comes indoors, where an oiled finish and a lacquered one ask for different cleaning and repair. A second entry on acacia decking takes the joist spacing and the ventilation gap further.