Avoid common mistakes when installing a composite deck.

Mistake #1: Neglecting the drainage slope

❌ The trap: "A deck should be flat!" This common misconception causes water infiltration and pooling. Standing water weakens the structure and can lead to mold (especially with pine joists treated to class 4).

✅ Preventive solution:

• Minimum required slope: 1% (1 cm per meter) away from the house
• Check technique: Use a garden hose filled with water to observe flow
• Key point: The slope must be created during ground preparation, not corrected afterward
• Pro tip: Water must drain around the entire perimeter, not just at one point

Mistake #2: Unstable or insufficient support

❌ The trap: Installing directly on compacted soil or poorly positioned pedestals. Result: sagging, squeaking, and warping within the first few years.

✅ Foolproof method:

• On soil: Excavate 20 cm + geotextile fabric + compacted gravel + concrete pedestals
• Pallet spacing: Follow DTU standards and manufacturer’s instructions
• Crucial check: Each pedestal must be perfectly stable (no rocking)
• Simple test: Jump on each pedestal — if it moves, redo it

Mistake #3: Insufficient ventilation height

❌ The trap: Leaving less than 5 cm under the boards to "gain height." Result: trapped humidity, mold, premature warping.

✅ Optimal ventilation:

• Absolute minimum: 5 cm of air beneath the boards
• Recommended: 8–10 cm for ideal airflow
• Mandatory check: Free air circulation around the entire perimeter
• Tip: Install ventilation grilles

Mistake #4: Incorrect joist spacing

❌ The trap: "The wider the spacing, the cheaper!" Exceeding recommended spacing leads to excessive board flexing and guaranteed cracks within a few years.

✅ Spacing based on installation angle:

• Perpendicular installation (90°): Max 40 cm between centers
• Diagonal installation (45°): Max 20 cm
• At 30°: Max 10 cm
• Measurement method: Always measure between joist centers
• Golden rule: When in doubt, space closer rather than further apart

Mistake #5: Poor choice of joists

❌ The trap: Using composite joists on pedestals (they require continuous support) or untreated class 4 wood outdoors.

✅ Correct choice depending on situation:

• On continuous concrete slab: Composite or aluminum joists (or class 4 wood)
• On isolated pedestals: Aluminum or class 4 wood joists only
• Very humid environment: Prefer aluminum
• Minimum dimensions: 50x50 mm for composite joists, 45x70 mm for wood

Mistake #6: Neglecting gaps

❌ The trap: "5 mm gap? No big deal..." Composite expands with temperature! Without proper gaps = bulging and destructive internal stresses.

✅ Mandatory gaps:

• Between boards (width): 5 mm minimum – never less!
• End to end (length): 5 mm minimum
• Against walls/obstacles: 15 mm minimum
• Essential tool: 5 mm spacers for consistent gaps
• Check: Always verify with a tape measure

Mistake #7: Inappropriate or poorly done fastenings

❌ The trap: Tightening "like traditional wood" or using unsuitable screws. Composite behaves differently and can irreversibly crack.

✅ Perfect fastening:

• Recommended solution: Stainless steel clips – invisible, self-spacing fastening
• If visible screws are necessary: Always pre-drill
• Critical distance: Minimum 25 mm from board edges
• Tightening: Moderate torque – composite should never be forced
• Material: Stainless steel mandatory in humid environments

Mistake #8: Rough cuts

❌ The trap: Cutting "by guesswork" without a rigid guide. Result: sloppy finishes, material waste, and amateurish appearance.

✅ Professional cutting:

• Essential tool: Circular saw with rigid guide
• Specialized blade: Fine tooth (at least 60 teeth) for composite
• Failproof method: Measure three times, mark once, cut once
• Safety: Protective glasses + dust extraction
• Finish: Light sanding of cut edges

Mistake #9: Installing in poor weather conditions

❌ The trap: Installing in extreme heat (+30°C) or cold (-5°C). Materials expand or contract abnormally, skewing all adjustments.

✅ Optimal conditions:

• Ideal temperature: 15–25°C for smooth installation
• Absolutely avoid: Rain, strong wind, full summer sun
• Necessary preparation: Store boards in shade for 48h before installation
• Optimal time: Morning or late afternoon (stable temperature)
• 7-day weather check: Verify forecasts before starting

Mistake #10: Neglecting material acclimation

❌ The trap: Installing immediately after delivery. Boards stacked for weeks carry internal stresses that release after installation.

✅ Proper acclimation:

• Minimum duration: 48h before first use
• Optimal storage: Flat
• Essential protection: In the shade, dry, ventilated
• Simple test: Boards should be at room temperature (15–25°C)