When a deck fails, the boards are usually fine. The failure points are curled edges, wide gaps, and rust stains. The fatal flaw of WPC decking isn’t the material; it’s the joints. The fate of those joints rests on three WPC decking accessories that cost less than 5% of the material: starter clips, spacers, and screws. They determine whether the deck stays flat for 15 years.     WPC expands 0.03–0.05 mm per meter per 1°C. A 4-meter board moves 4–6 mm in a 30°C swing. Insufficient gaps lead to snapped clips or buckled edges.Starter clips set the baseline. Hidden fasteners grip two adjacent grooves, but the first and last rows only have a single board. Starter clips slot into the side groove and lock directly to the joist, securing the edge while allowing movement. Omit them, and the first row creeps like a staircase. Use face screws, and the board splits at the head. Cheap clips are flimsy, burred, and deform easily; only heavy-duty stainless steel stamped clips survive. Rule: one starter clip for every perimeter edge and every joist.     Expansion spacers dictate life-or-death gaps. Board-to-board gaps (4–6 mm) are set by clip tabs. But perimeter gaps (10–15 mm) against walls and columns must be created with spacers. No gap = guaranteed buckling in the first heatwave. Thin plastic shatters under the sun; use high-toughness PP/PE spacers. Two strict rules: Remove spacers after installation to avoid locking the deck; leave 5–8 mm at butt joints and land them on double joists to prevent sagging.     Screws cost the least but fail the most expensively. Carbon steel rusts and stains permanently. Use a magnet: if it sticks, it’s iron—reject it. Stainless steel screws (A2/304 minimum, A4/316 for coastal) are non-magnetic. Four installation rules: Penetrate the joist by at least 25–30 mm; control clutch torque to avoid cracking the clip; pre-drill within 50 mm of board ends; absolutely zero exposed screw heads on the walking surface. For high-traffic commercial areas, joist spacing must be 300 mm.     Accessories cost 3%–6% of materials, but repairing a buckled edge costs 10 to 20 times that in labor and replacement. Don't just write "accessories included." Specify the details: UV-resistant clips (20–25 per sqm), matching starter clips, color-coded spacers, end caps for cut edges. Don't let a few cents of cheap parts ruin a multi-thousand-dollar project.