The joists (the cross-beams your boards screw down to) decide how solid the deck feels. Here's how to work out the count.
At 60 cm joist centres you'll need about 1.7 linear metres of joist per m² of deck. For a stiffer feel, go to 40 cm centres – that's around 2.5 linear metres per m².
Joists run perpendicular to your deck length. Joist count = (deck length ÷ spacing) + 1, because you always need one at each end.
Total joist length = joist count × deck width.
60 cm centres is normal for 21–28 mm boards. Always check what your board manufacturer says, especially for thin or hollow composite boards.
The deck is 5 m long with joists every 60 cm: (500 ÷ 60) + 1 = 9 joists. Each is 4 m wide, so total = 36 linear metres. If you buy 4.5 m joists, that's 8 pieces (you'll trim two of them down to 4 m).
The calculator works out joist count, board count and screws from your dimensions and joist spacing.
60 cm centres for 25–28 mm boards. For 21 mm boards or hollow composites you usually need to go down to 40–50 cm – check the manufacturer.
Yes – they sit in damp shade. Use a high-grade pressure treatment and keep them clear of soil and standing water with proper post bases.
Yes – where two boards meet end-to-end, each board needs its own joist face to screw into.
At least 5 cm of airflow underneath. Aim for 10–15 cm for proper ventilation and longer joist life.
Keep going with one of these – they pair well with what you just read.
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