Technical guide · Pandaboard

WPC and Co-Extruded Decking: Joists, Clips and Drainage

WPC and co-extruded deck boards are typically 140–200 mm wide and 25–30 mm thick, fixed to WPC, aluminum or hot-dip galvanized steel joists at 300–400 mm centers with stainless steel clips that sit in the side groove, so no fastener shows on the face. Leave 5–8 mm between boards and 3–6 mm at butt joints for thermal movement. The sub-base falls at 1% or more to a drain, with at least 50 mm of ventilated void beneath the joists and no point where water can stand. Use 304 stainless fittings, or 316 on the coast.

Exterior WPC decking (including co-extruded types) is used for gardens, terraces, roof gardens and boardwalks. The key differences from interior flooring: there must be a ventilated cavity beneath, the ground must have a drainage fall, and the board ends must have expansion gaps. Without these three, the deck distorts, molds and holds water within a year or two.

· Exterior boards face UV, rain and temperature cycles, so all fixings must be 304 or 316 stainless steel;
· Laying direction, framing spacing and joint width interact and must suit the board thickness and span.

01Key specifications and suitability

Suitable

• House terraces and roof decks

• Pool surrounds and spa walkways

• Park boardwalks and viewing platforms

• Commercial outdoor seating and dining decks

• Covered ground floors and waterside walkways

Not suitable

• Areas permanently under water

• Laid directly on sealed concrete that cannot drain or vent

• Under open flame or where a barbecue sits directly on the deck

• Driveways and loading areas with vehicle traffic

• Barefoot play areas where surface temperature matters

02Preparation Before Installation

2.1 Substrate and Drainage

• Substrate: concrete (strength ≥C20) or compacted gravel (compaction ≥95 %);

• Drainage fall 1 %–2 % toward the outlets, never a reverse fall;

• Concrete substrates need a waterproof layer (mandatory on roofs and terraces) and a flood test;

• Substrate flatness ≤5 mm/2 m (the framing can be packed, so the tolerance is looser than indoors).

2.2 Materials

Table 2-1 Material list

Parts list and reference quantities：Per 100 m² of deck, joists at 350 mm; includes normal wastage.

2.3 Site Conditions

• Ambient temperature 5–35 °C; avoid laying at midday in high heat (the boards are already expanded and the gaps end up too small);

• Do not work in rain;

• Deliver boards 24 h early and store them flat, out of direct sun.

2.4 Delivery and site conditioning

• Move panels indoors 24 h ahead and store flat; never lean them against a wall, which sets a permanent bow

• Keep one batch and one shade per elevation or room

• Spot-check 5 %: intact tongue or click, unmarked face, straightness within 2 mm over 1 m

• Cut and drill in a separate area and clear swarf at once so the face is not scratched

03Installation Steps

Figure 1 Build-up section Substrate with drainage fall → adjustable shims → framing → hidden clips → deck board; cavity ≥40 for ventilation.

1. Setting out - Mark framing lines and the board direction from the drawings; framing runs perpendicular to the board length.

2. Install shims - Place shims at ≤600 centers, level them to the design height and form a continuous cavity.

3. Install framing - Fix WPC / aluminum framing to the shims, stagger framing joints ≥200, and support every joint with a shim.

4. Level the framing - Level with a string line; framing top level difference ≤2 mm/2 m; check the drainage fall at the same time (a spirit level can be used to read the slope).

5. Lay the first board - Start at the wall or edge trim, leave 10–15 at the board end and fix clips to the framing.

6. Lay board by board - Place clips between adjacent boards, drive screws diagonally into the framing (do not drive vertically through the board), board gap 5–8.

7. Board ends - Leave a 10–15 expansion gap between board and wall, post or edge trim; end laps along the length must land on framing.

8. Movement joints - A movement joint every 3–4 m (matching edge trim or double framing) to release accumulated expansion.

9. Trims and cleaning - Fit edge trim and external corners; clean the surface and check that every screw sits inside its clip.

Laying in high heat is a hidden killerAt midday in summer the board surface can exceed 60 °C, and the board is already at maximum expansion. Lay it tight at that moment and it will open gaps of 5 mm or more once it contracts in autumn; lay it cold with gaps too small and it will bulge in summer. So: follow the product data for gaps and avoid midday installation.

04Junctions and Details

• Board end to wall: 10–15, covered by edge trim, never tight;

• Board to board: 5–8, for drainage and ventilation;

• Corners: matching external corner, or two framing members meeting at 90° with a gap at each board end;

• Steps and terminations: fit an end cap to stop the board end absorbing water;

• Pool surrounds: choose a slip-resistant face and keep the board surface 5–10 above the pool edge to prevent backflow;

• Junctions with lawn or planting soil: add a retaining edge or gravel strip to keep soil out of the cavity.

05Quality Acceptance

Table 5-1 Acceptance requirements

06Common Problems

Table 6-1 Defects

07Product Storage and Handling

Figure 2 Stacking Store flat, raised and shaded; exterior boards bow under long exposure, so never leave them in direct sun.

Table 7-1 Storage conditions

Handling: two people lift by the edges, long boards supported at several points; never drag (the face scratches easily).

08Transport and Safety

• Load flat, dunnage supports ≤800, stack ≤1.0 m, strap down;

• Avoid direct sun (high temperature inside the vehicle bows the boards);

• Wear gloves when loading and unloading; fit corner protectors;

• Wear a mask and goggles when cutting and drilling;

• Wear a harness for work at height or near edges; scaffolding must be inspected and approved.

09Appendix: Checklist

9.1 Goods-in Inspection

• □Size, thickness, color code and batch number match

• □Boards flat, no bow, no scratches

• □Clip groove complete

• □Packaging intact, corner protectors fitted

• □UV resistance and slip-resistance test reports supplied with the goods

9.2 Before Installation

• □Substrate strength and drainage fall within limits

• □Waterproofing complete and flood test passed

• □Framing, shims and stainless fixings on site

• □Setting out and board layout complete

• □Installation scheduled outside the hottest hours

10Frequently asked questions

Can I lay decking straight onto concrete?

No. Direct contact keeps the underside damp and leaves no room for movement. Raise it at least 50 mm on adjustable pedestals or joists so air can move under the boards.

Do I really need gaps between boards?

Yes. The 5–8 mm gap handles drainage and thermal movement. A dark board reaches 60–70 °C in summer, and a closed deck pushes against itself at midday and buckles. Allow 1.5 mm per meter at butt joints.

How far apart should the joists be?

Typically 300–400 mm. Use 300 for 25 mm board, high traffic or long spans, and land board ends on a joist with no more than 50 mm cantilever, or the ends deflect and rattle.

Composite or treated timber — which is better?

WPC needs no coating and does not rot, but it is less stiff than timber and hotter underfoot. Co-extruded boards carry a capped surface that resists staining and fading. Use light colors where people walk barefoot.

Can it be used around a pool?

Yes, with 316 stainless fittings, a ventilated void, fixed edges and a slip-resistant surface. Drain the first 1.5 m separately so water never sits under the deck.

Why does the deck buckle or work loose?

Usually tight butt joints, clips overdriven into the groove, joists spaced too far, or water standing under the deck. Lift that area, reset the joints, replace damaged clips and fix the drainage.

About Pandaboard

Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA

We manufacture and export gypsum board, mineral fiber and fiberglass ceiling tiles, calcium silicate and magnesium oxide boards, fiber cement board, aluminum ceilings, WPC and SPC wall panels, exterior decking, fencing and handrails, together with the matching grid and framing systems. The methods on this page are compiled from product behaviour and standard trade practice; for any project, follow the approved drawings, local codes and the specification confirmed on your order.

Website: www.pandamanufacturer.com Contact sales: contactus

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Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA
This guide is technical information for specifiers and installers; it does not replace project drawings or local codes.