1. Scope and limits of this guide

WPC decking behaves differently from wood: it is heavier, it expands and contracts mainly with temperature, and it is not a structural material. The rules below reflect mainstream US practice under the IRC / IBC and the published instructions of major composite decking manufacturers.

2. Storage, handling and acclimation

• Store boards flat, fully supported, on a firm level surface and off the ground — never lean them against a wall, which will bow them permanently in hot weather.
• Keep the stacks covered and ventilated until installation. Composite is moisture-resistant, but trapped heat under a dark tarp will still distort a stack.
• Let boards sit on site at least 24 hours before fixing so they take ambient temperature. Install within roughly 40–90 °F (4–32 °C); below that the material stiffens and is more prone to splitting at fastener points — pre-drill and use a slower driver.
• Carry boards on edge, not flat — a long composite board carried flat will sag and can crack.

3. Substructure: footings, beams and joists

Composite decking is a wearing surface, never a structural element. It must never be used as a joist, beam, post or stair stringer, and it does not add bracing to the frame.

• Footings must reach below the local frost line and bear on undisturbed soil; size them for the tributary load.
• Use joists that are straight, dry and pressure-treated for ground contact. Crown all joists the same way and check tops are level before laying boards — composite telegraphs any dip in the frame.
• Joists should be at least 1-1/2 in. (38 mm) wide at the bearing face so clips and screws have proper purchase.
• Over a concrete slab, build a sleeper / batten system shimmed level and free-draining. Never glue composite boards directly to concrete.

4. Joist spacing and span

Spacing is driven by the laying angle and by how much traffic the deck takes. Use the tighter value whenever two rules overlap.

Condition	Joist spacing (on centre)	Note
Boards perpendicular to joists (90°), residential	16 in. / 400 mm	The standard detail for private decks and terraces.
Boards at 45° (diagonal)	12 in. / 300 mm	Diagonal laying shortens the effective span, so centres must be tightened.
Commercial / high-traffic	12 in. / 300 mm	Pool decks, hotel terraces, restaurant platforms and public boardwalks.
Stair treads	As spec sheet — typically 12 in. / 300 mm or tighter	Treads take concentrated point loads; never exceed the profile's rated span.
Maximum board overhang past the last joist	1 in. / 25 mm (up to 2 in. / 50 mm on some profiles)	Confirm against the spec sheet before trimming the last board.

5. Slope, drainage and ventilation

Composite will not rot, but the frame underneath it will. Water and air management decides how long a deck lasts.

Item	Requirement	Why
Clearance from grade to underside of joists	Minimum 12 in. / 300 mm	Room for air to move and for inspection; keeps splash-back off the frame.
Ventilation of enclosed or skirted decks	Openings on at least two opposing sides	A sealed box under the deck traps moisture and overheats the boards.
Surface slope away from the building	1/8–1/4 in. per ft (1–2 %)	Built into the joists, not into the boards — never shim the decking to create fall.
Ground below the deck	Geotextile + 2–3 in. / 50–75 mm gravel, graded away	Suppresses weeds, stops splash-back and keeps the underside dry.
Gutters, planters and downpipes	Discharge beyond the deck footprint	Constantly wet boards stain, grow mould and get slippery.

6. Expansion gaps and butt joints

Composite boards move lengthwise with temperature. The gap you leave on installation day absorbs that movement — the single most common cause of buckled decks.

Location	Gap	Detail
Board end to board end (butt joint)	1/8 in. / 3 mm	Both ends must land on a joist — provide a doubled joist or solid blocking under every butt joint.
Board side to board side	Set by the clip — about 3/16 in. / 5 mm	Hidden clips hold the spacing themselves. If face-fixing, space the boards by hand and do not butt them tight.
Against a wall, post, step or any fixed obstruction	Minimum 1/4 in. / 6 mm — use 3/8–1/2 in. / 10–12 mm in hot climates	Never trap a board between two rigid faces.

• Rule of thumb: the colder the boards are when you fix them, the wider the end gap should be.
• Stagger butt joints between adjacent courses by at least one joist bay.
• Do not fix two boards to each other and do not bridge a butt joint with a single clip. Each board end gets its own fixing.

7. Fastening: hidden clips and screws

• Use the clip system made for the profile. Clips are not universal.
• Start every run with a starter clip, then work across the deck clip by clip. Do not slide boards into clips already tightened.
• Fasteners: stainless steel (304 near the coast, 316 in marine exposure) or a quality polymer-coated composite screw. Do not use plain steel or aluminium — they stain the board black and corrode.
• Screw length: at least 1 in. / 25 mm of thread into the joist below the board.
• Never nail composite decking. Nails do not hold, they pop, and split ends are the usual result.
• Pre-drill within about 2–3 in. / 50–75 mm of any board end, and whenever working below 50 °F / 10 °C. Drive screws straight and flush — do not over-drive.
• Two fixings per board at every joist crossing where the profile has a wide face; one fixing at each end as a minimum.

8. Cutting, drilling and finishing

• Cut with a carbide-tipped blade — a fine-tooth finishing blade (60 teeth or more) gives the cleanest edge.
• Cut outdoors where possible and collect the dust. In enclosed spaces wear an approved dust mask and eye protection.
• Support the board fully on both sides of the cut so the offcut does not tear away.
• Do not sand, plane, paint, stain or varnish the boards. The co-extruded cap is the finish.
• Never burn offcuts — composite gives off dense, acrid smoke. Dispose of them as construction waste.

9. Picture framing, fascia and stairs

• Picture framing looks sharp but concentrates movement at the mitres. Mitre accurately, keep the 1/8 in. / 3 mm gap at each end, and support every mitre.
• Use the matching fascia / trim board for the exposed edge. Do not rip a deck board down to make a fascia — the cut face loses its cap.
• Fascia spans are shorter than deck-board spans. Fix with the specified trim screws, pre-drilled, leaving the same expansion gap at each joint.
• Stair treads: use a full tread board rather than two narrow strips, tighten spacing per section 4, and add a nosing / bullnose profile so the front edge does not chip.
• Where the deck meets a door threshold, keep the deck surface at least 1 in. / 25 mm below the finished floor level.

10. Cleaning and maintenance

• Routine cleaning: sweep, then wash with warm water and mild household detergent using a soft-bristle brush. Rinse clean.
• Pressure washing is permitted on most capped profiles at 1,500 psi / 100 bar or less, fan tip, at least 8 in. / 200 mm from the surface. Test a hidden area first.
• Mould and mildew: use a composite-approved cleaner or diluted household bleach, then rinse thoroughly. Avoid acetone, paint thinner and aggressive solvents.
• Snow and ice: calcium chloride or rock-salt de-icers are generally accepted; shovel with a plastic blade, never metal, and rinse afterwards.
• Use pads under heavy planters and furniture; never place a grill or fire pit directly on the surface.

11. What voids the warranty