Acacia decking and cladding: thickness, joists, airflow

Acacia decking is chosen by matching board thickness to the span between joists and by picking a grade with the defects you can live with. Under a terrace, joists are typically set at 400 mm centres for 25 mm boards and 600 mm centres for 32 mm boards, with the boards running across them. A ventilated facade works by leaving a drained and ventilated cavity behind the cladding, and acacia needs that gap because the wood moves with moisture and will not sit happily against a sealed wall.

Ako vybrať terasové dosky z agátu podľa hrúbky a triedy?

Thickness is a structural decision before it is a look. A board of 25 mm suits a domestic terrace where the joists are close, while 32 mm is the usual choice where the span is longer or where furniture and foot traffic are heavy. Boards sold for decking are normally 120 to 140 mm wide, and the wider the board, the more it will cup across its face as it takes on and gives off moisture.

Grade is the other half of the choice. Clear or select grade shows few knots and little sapwood, and it costs more. Standard grade carries knots, some colour variation and occasional small checks, which is normal for a hardwood grown outdoors. The part that matters most is the sapwood: acacia heartwood is durable in exterior use, but pale sapwood is not, so boards with a wide sapwood edge are a poor buy for a terrace. Look at the end grain of a sample and see how much of the section is pale.

Moisture content at delivery decides how much the boards will shrink after laying. Kiln dried stock at around 15 to 18 percent is easier to work and moves less in service than green stock. If the boards arrive wet, stack them under cover with stickers between the layers for a few weeks before fixing. A short guide to acacia decking board selection covers the same ground in Slovak, including how the species behaves when it is left exposed.

Fixings should be stainless steel, because acacia contains tannins that corrode ordinary steel and leave black stains. Pre drill near the ends of every board, and leave a gap of 5 to 8 mm between boards for drainage and air movement.

Aký je správny rozostup roštov pri agátovej terase?

Joist spacing is set by the thickness of the decking, not by the size of the terrace. As a working rule for acacia:

• 25 mm boards: joists at 400 mm centres.
• 32 mm boards: joists at 600 mm centres.
• Boards laid diagonally: reduce the spacing by about a quarter, so 300 mm and 450 mm respectively.

Joists themselves are usually 45 x 70 mm or 50 x 75 mm treated softwood, or acacia where it is available. They should sit on a drained base, not in standing water, and they should be fixed so that air can move under the deck. A terrace laid tight to the ground with no gap beneath will hold moisture and shorten the life of both the joists and the boards.

Run the boards across the joists, not along them, and keep the joist runs clear of soil and leaf litter. Where a board end lands between joists, add a noggin or a double joist so the end is supported. Ends that overhang unsupported will cup and split.

Allow a gap of at least 10 mm where the deck meets a wall or a step, and do not butt boards hard against each other at the ends. Acacia expands across its width far more than along its length, so the width gaps matter more than the end gaps.

Čo je odvetraná fasáda a prečo ju agát potrebuje?

A ventilated facade is a wall build up with a gap behind the cladding. The layers run, from inside out: the structural wall, a weather barrier, a drained and ventilated cavity of roughly 20 to 40 mm, then the acacia cladding on battens. The cavity is open at the bottom and at the top, so air moves up through it and any water that gets past the cladding drains away instead of sitting against the wall.

Acacia needs that cavity for two reasons. First, the wood takes on and releases moisture with the weather, and it needs somewhere for that moisture to go. Second, a sealed gap traps water, and trapped water against a wall is what causes rot in the battens and staining on the wall behind. The cavity also lets the back of the boards dry after rain, which is the face that dries slowest.

Battens are usually 30 to 50 mm deep and fixed vertically over the weather barrier, at centres that match the cladding. Horizontal cladding boards are fixed to vertical battens, and vertical boards to horizontal battens, so that the cavity stays continuous. Keep the bottom of the cavity open with an insect mesh, and keep the top open with a vented closure. Do not fill the cavity with insulation unless the design allows for it.

Board orientation and the gap that does the work

Acacia cladding can run horizontally or vertically, and the choice changes how water leaves the wall. Horizontal boards shed water well but need a small overlap or a tongue and groove profile to stop wind driven rain from reaching the cavity. Vertical boards drain along their length and are often used where the wall is tall, with a rainscreen profile that keeps water out of the joints.

Whichever way the boards run, the gap between them is not decoration. A 5 to 8 mm gap lets the face of the board dry and lets the cavity breathe. Boards fixed tight together hold water in the joints, and that is where the first signs of decay appear, usually as dark streaks and soft edges.

End grain needs protection. Cut ends of acacia cladding and decking take in water faster than the faces, so brush a suitable end grain preservative on every cut before the board goes up. This is a small step that is often skipped and it is the one that decides how long the board lasts at its ends.

Fixing, finishing and the first year

Stainless steel screws or hidden clips are the usual fixings. Screws should be countersunk and set just below the surface, not driven so hard that the board is crushed around the head. Hidden clips give a cleaner face but need a groove in the board, and not every acacia profile has one.

Acacia weathers to a silver grey if it is left untreated, and it can be kept closer to its original colour with an exterior oil or a pigmented finish. Either route is workable, but the finish has to be maintained. A clear film finish that cracks will trap water and look worse than no finish at all. An oil that is renewed every year or two is the simpler choice for most homes.

The first year is when the boards move most. Expect small gaps to open at the joints and slight cupping across wide boards. Neither is a fault if the joists are at the right spacing and the boards were dry when they were laid. Check the fixings after the first winter, tighten anything that has worked loose, and clear leaves from the cavity and from under the deck before the next wet season.

What to check before you buy

Ask for the moisture content, the grade and the sapwood content in writing. Measure a board: thickness, width and the straightness of the edge. Check that the joist spacing you plan matches the thickness you are buying, and that the wall you are cladding can take a ventilated build up with an open cavity at the bottom and top. If any of those three answers is vague, the boards are not the problem yet.