Decking Calculator (South Africa)

Work out your deck's area first, then divide the width by your board's width plus the expansion gap between boards, rounded up, to get the number of rows. Multiply that by your deck's length to get the total linear metres of board, add cutting waste, typically 10%, then divide by the length you're buying boards in and round up to whole boards. As a worked example, a 4m by 3m deck, boards along the length, 102mm wide with a 5mm gap, needs 29 rows, which comes to 36 boards at 3.6m each. The calculator above does this for your own dimensions, and also works out the joists, bearers, posts and fasteners most bare board-count calculators leave out entirely.

Because a deck's substructure is directional, not just an area. Joists always run at right angles under the boards, and bearers run under the joists, parallel to the boards, resting on the posts. A calculator that only takes a bare square metreage can give you a materials cost for the boards themselves, but it cannot honestly work out how many joists, bearers or posts you need, since that depends on which way each run goes relative to your deck's length and width. The calculator above asks for your length, width and board direction for exactly this reason, then works out the whole substructure from that grid, not just the visible boards on top.

It depends on your board's thickness and material. As a South African rule of thumb, referencing SANS 1008, joist spacing (centre to centre) should be roughly 20 to 22 times your board's thickness. That works out to about 450mm centres for standard 21 to 22mm timber decking, and about 600mm for a thicker 32mm board. Composite decking needs tighter spacing, commonly around 350mm centres, since composite boards are less stiff than solid timber of the same thickness, manufacturers such as Eva-Last specify this for their own boards. The calculator above sets the right default automatically from your board material and thickness, lets you override it if your supplier specifies something different, and rounds the joist count up so the actual built centres never exceed that maximum.

As general South African rules of thumb for a low-level residential deck, bearers are commonly spaced 1.8 to 2.4m apart, and posts along each bearer are commonly spaced 1.8 to 3.0m apart. Both depend on your timber size, the deck's height and any load it carries, so a taller deck, a wider span, or a deck built to carry a hot tub or a lot of entertaining load should be checked against a proper span table or signed off by an engineer, this calculator's defaults are a reasonable starting point for a typical low-level deck, not an engineered design. The calculator above uses 1.8m and 2m as its own defaults, both editable, and treats each as a maximum, rounding the member count up so the built spacing never exceeds the figure you enter.

It depends on your bearer line count and post spacing along each line, both of which the calculator above works out from your deck's dimensions. A freestanding deck needs posts under every bearer line, all the way round. An attached deck, bolted to the house with a ledger board on one edge, needs one fewer bearer line (and its posts) since the house structure carries that edge instead. As a worked example, the 4m by 3m deck above needs 9 posts freestanding, or 6 posts attached to a house, since one whole bearer line and its posts are replaced by the ledger.

Common South African sizes are 50x152mm for joists and 50x228mm (or a heavier 76x228mm for longer spans or a taller deck) for bearers, both CCA treated pine, sold in standard stock lengths such as 6.0m or 6.6m. Posts are commonly 114x114mm for a typical residential deck, or 152x152mm for a heavier structure. Treatment class matters: H3 treatment is the standard for timber above ground and exposed to weather, which covers your joists, bearers and the above-ground part of your posts, while H4 treatment is required for anything embedded in the ground or set in concrete, since that timber faces sustained soil and moisture contact. CCA treatment alone does not protect timber from UV or surface cracking, so an oil-based timber sealer is still worth applying, particularly to the visible deck boards.

For timber decking screwed down conventionally, the South African rule of thumb is 2 screws per board at every joist it crosses. For composite decking with a hidden clip fastening system, it's 1 clip per board at every joist it crosses instead, screwed down into the joist so no fastener shows on the surface. Where the boards you buy are shorter than the deck run, each row also needs a butt joint over a joist, and that joint carries two board ends, each fixed with its own screws or clip, so the calculator adds 2 screws (or 1 clip) per joint. As a worked example, the 4m by 3m deck above has 10 joist lines and 29 board rows, giving 290 board-joist crossings plus 29 butt joints from its 3.6m boards, so 638 screws, rounded up to whole boxes. Composite clip boxes are typically rated by coverage, for example Eva-Last's own 150-clip box covers roughly 6 to 9m², which is consistent with the same crossing-based count the calculator above uses.

Yes, every post needs its own footing so it doesn't sink or shift. As a general guide, a 114x114mm post is commonly set in a footing roughly 300mm in diameter, and a heavier 152x152mm post in a footing roughly 400mm in diameter, both commonly around 0.6m deep, with H4 treated timber below ground level. The calculator above works out how many posts you need and pre-fills our Concrete & Mortar Calculator with that exact post count and footing size, so you can get straight to the bags of cement, sand and stone the footings themselves need.

It depends on your budget and how much maintenance you want to take on. Treated pine is the cheapest option, the boards themselves cost roughly R52 to R55 a linear metre (around R510/m² of decking surface for the boards alone), but needs regular sealing or oiling to keep weather and UV damage at bay. Balau and other hardwoods cost more, the boards running roughly R95 a linear metre (around R1 056/m²), and are naturally more durable with less maintenance, but still benefit from periodic oiling to keep their colour. Composite decking, a wood-plastic blend, costs the most for the boards themselves, roughly R165 a linear metre (around R1 179/m²), but needs no sealing, sanding or annual maintenance, and resists rot, insects and splintering. A complete deck's total materials cost, once the joists, bearers, posts and fasteners the calculator above works out are added on top of the boards, typically comes out a good deal higher than the bare board price alone, see the cost example below. Composite also needs tighter joist spacing, roughly 350mm against 450 to 600mm for timber, which the calculator above accounts for automatically when you switch material. There's no single right answer, it's a genuine trade-off between upfront cost and ongoing upkeep.

As a rule of thumb, allow about 10% extra over your bare linear metres needed, to cover offcuts, trimming board ends square, and the odd mistake. Deck boards can butt-join at any joist along a row, so an offcut from one row isn't wasted, it can start the next row, which is why a flat percentage over the total works well here. Joists, bearers and posts are different, each is a single continuous run that has to come from one piece (or be spliced over a support if it's longer than one stock length), so buying an extra length or two as a spare for a mismeasure or a damaged piece is the better approach there rather than a percentage buffer, which the calculator above reflects in how it rounds those materials.

Split it into two or more rectangles and run each one through the calculator separately, then add the resulting shopping lists together. The substructure at the shared inside corner will come out slightly conservative, since it gets counted once on each rectangle's own edge, which is the safe direction to be wrong in, you may end up with a couple more joists or a bearer than a perfectly optimised L-shaped frame strictly needs, not fewer.

Yes, for the common case of a straight-sided pool surround. Treat each side of the pool as its own rectangle, run each through the calculator, and add the shopping lists together, the same approach as an L-shaped deck. A few pool-specific points are worth budgeting for: choose a grooved or slip-resistant board profile and keep the standard expansion gaps so splash water drains through, use stainless steel fasteners since chlorinated or salt water corrodes plated screws quickly, and make sure any timber touching the ground or set in concrete is H4 treated. Composite is popular around pools because it doesn't splinter under bare feet, though a dark composite board gets noticeably hotter in full sun than timber. Professional pool deck installs typically add roughly R200 to R300/m² over standard decking rates for drainage detailing and slip-resistant finishes. A curved surround is beyond a rectangular grid, ask a specialist installer to quote that.

A freestanding deck stands entirely on its own posts and footings on every side, independent of any building. An attached deck is bolted to an existing structure, usually a house, along one edge using a ledger board, which replaces that edge's bearer line, its posts and footings entirely. An attached deck is usually cheaper in materials, since one whole bearer line is removed, but the ledger connection itself is structural and carries real load into your house's frame, so it needs to be properly flashed against water ingress and fixed according to an engineer's or your local building inspector's requirements, this is not a connection to improvise. The calculator above models both, switching automatically between the two substructure layouts.

As a July 2026 reference point using this tool's own defaults, that 4m by 3m (12m²) freestanding treated pine deck above comes to roughly R14 592 in materials alone, boards, joists, bearers, posts and screws, not including the concrete for the footings themselves or any professional labour. That works out to a materials cost per square metre of roughly R1 216/m² for this size and material combination, smaller or larger decks won't scale exactly proportionally since the substructure has some fixed overhead regardless of size. The calculator above gives you this total from your own dimensions, material and prices.

Labour only, fitting materials you've already bought, typically runs roughly R300 to R400/m² for a professional crew, based on 2026 South African market rates. A full supply-and-fit quote varies far more by material, roughly R1 400 to R1 800/m² for treated pine, R2 000 to R2 800/m² for balau or another hardwood, and R2 200 to R3 850/m² for composite, since the material itself is a bigger share of a composite job's total cost. Always get a few written quotes rather than relying on one average figure, an uneven site needing extra excavation or a tricky attached-ledger connection can push a quote toward the top of these ranges.

Yes. It's brand-neutral, you enter your own board size, price, joist and bearer stock lengths and rates, so it works whether you're buying Cashbuild or Builders treated pine, a specific hardwood supplier's balau, or a composite brand such as Eva-Last, The Composite Company, or another range entirely. Board sizes and prices vary quite a bit by brand, so always check your own supplier's exact dimensions and current quote rather than assuming the calculator's shipped defaults match precisely.