It is almost never the material that fails — it is the installation. A board that warps, a deck that moves underfoot, gaps that widen or a warranty that becomes invalid: behind each of these problems is usually an installation rule that was not followed. Here are the five most common mistakes and the correct way to avoid them. If you are installing the decking yourself, keep this checklist close at hand.

Spacing the joists too far apart

Space the joists too far apart and the deck can sag, lose its level and eventually warp. The correct spacing depends on how the deck will be used:

  • Residential (garden, balcony, terrace, private poolside area): 400 mm maximum;
  • Commercial (restaurant, walkway, hotel pool): 350 mm maximum.

Joist spacing is measured centre to centre, from the centre of one joist to the centre of the next. If you need additional strength, reduce the spacing — never exceed these maximum values.

Diagram of joist spacing for Quick-Fix joists and CDECK boards, in residential and commercial use

Supporting end-to-end board joints on a single joist

Where two boards meet end to end in a staggered layout, using a single joist beneath the joint weakens the installation. Each board needs its own support at the joint.

  • Use two Quick-Fix® joists at the joint, one for each board.
  • Never use fewer than three joists per board. This is what ensures a firm, level base.

Diagram of end-to-end board joints: poorly supported on a single joist versus correctly supported on two joists

Leaving joists loose instead of fixing them down

Everything starts with the base. The joists support the deck, provide ventilation and drainage, and keep the boards away from direct contact with the ground. They should not simply be laid in place — they must be securely fixed to create a stable structure. With CDECK, the joists are PVC or aluminium, and screw spacing matters:

  • Leave 40 to 80 mm between the screw and the end of the joist;
  • Use one screw every 500 mm (PVC) or 600 mm (aluminium).

Diagram of fixing PVC and aluminium joists, showing screw spacing

Ignoring expansion gaps

Composite decking moves: it expands and contracts as temperature and humidity change. Without enough room for this movement, boards can warp, lift or break, and may even put pressure on surrounding elements such as walls. Two measurements are essential:

  • Against a fixed element (wall, low wall): minimum 15 mm;
  • Between board ends: minimum 6.5 mm.

Diagram of expansion gaps: against a wall and between board ends

Fixing the boards incorrectly

This is the mistake with the most serious consequences, and it usually comes from the same misunderstanding of the Quick-Fix® system: either nothing is fixed, or every clip is screwed down. Neither is correct. The right method is:

  • Use just one screw per board, on the clip closest to the centre. This prevents the board from sliding lengthways;
  • Do not add any more screws, and never screw the clips to the joists.

When fixed correctly, the boards stay in position, remain aligned and maintain consistent gaps.

Fixing a Quick-Fix clip at the centre of the board with a screwdriver

Checklist before you start

Joist spacing: ≤ 400 mm (residential) / ≤ 350 mm (commercial), centre to centre.
End-to-end joints: two joists, with a minimum of three joists per board.
Fixed joists: screws 40–80 mm from the ends, then every 500 mm (PVC) / 600 mm (aluminium).
Expansion gaps: 15 mm from walls, 6.5 mm between board ends.
Boards: just one screw per board, on the central clip.

Install with confidence

Avoid these five mistakes and your deck is set up to last. For the full step-by-step process, follow our installation guide and installation videos; the exact dimensions and requirements are set out in the Technical Datasheet. The entire system is built around the patented Quick-Fix® system — one screw per board, and no more. You can also explore the CDECK Original and CDECK WUUDE ranges.