Composite decking is not difficult to install, but it is not timber either. The boards are denser, the fixings are different, and — the part most guides skip — the material moves with temperature in a way you have to design for. Get the three numbers in section two right and the rest of the job takes a day or two. Get them wrong and the deck will tell you about it in the first summer.
Every figure on this page comes from our own test reports, not from a rule of thumb. Report numbers are given so you can check them.
What you are installing
| Profile | 140 × 25 mm hollow board |
| Standard length | 2.9 m, or cut to order |
| Fastening | Hidden clip system (no face screws) |
| Surface | Capped, co-extruded |
| Sample reference | BY-N113Y16 |
If you are comparing suppliers on specification rather than price, the full board data sheet is here
The three numbers that decide the job
Number one — joist spacing
For a premium 25 mm hollow board, set joists at 300-500 mm centre to centre. Do not exceed it.
This is the figure that matters most, and it is where cheap decks fail. The board itself is strong — after three freeze–thaw cycles down to −29 °C its bending strength fell by only 4.9%, to 54.4 MPa (SGS report GZIN2604000591CM01-1_EN). The deflection you see on a bad installation almost always comes from joists set too far apart, not from the board.
Three rules:
- Joists run perpendicular to the boards.
- You can always go tighter than 300-500 mm; never looser.
- Where two board ends meet over a joist, double the joist under the joint.
Number two — expansion gaps
Composite expands and contracts with temperature. Ours was measured by SGS at 3.195 × 10⁻⁵ per °C (report GZIN2604000591CM02_EN). On a 2.9 m board that works out to:
| Temperature swing the deck will see | Movement over a 2.9 m board |
|---|---|
| 20 °C (36 °F) | 1.9 mm |
| 30 °C (54 °F) | 2.8 mm |
| 40 °C (72 °F) | 3.7 mm |
| 50 °C (90 °F) | 4.6 mm |
| 60 °C (108 °F) | 5.6 mm |
Work out the widest temperature range your deck will actually see over a year — not the range on the day you install — and allow that much. For most of North America and northern Europe this lands on 4–6 mm, which is why 5 mm is the common rule.
Where a board runs between two fixed abutments, split the figure between the two ends. A board butted tight against a wall at both ends has nowhere to go, and the force goes into the clips and the trim instead.
Between boards, the clip sets the side gap at 4–6 mm. Never butt boards edge to edge.
Number three — clearance and ventilation
The frame has to be able to dry. A closed void under a deck traps moisture, and moisture is the one thing that does slowly change a composite board: measured absorption is 1.12% by weight, with thickness swelling of 0.70% — about 0.18 mm on a 25 mm board (SGS GZIN2604000591CM01-1_EN). That is small enough to be absorbed by the gaps above, provided the void below is not sealed.
Keep the underside open, keep the joists off the ground, and never lay boards directly onto concrete or soil without a drained, ventilated frame under them.
Tools and materials
| Item | Notes |
|---|---|
| Composite boards | 140 × 25 mm, 2.9 m standard |
| Joists | Composite or treated timber, at 300 mm centres |
| Starter clips | One run along the first edge |
| Hidden clips + screws | One per board per joist crossing |
| Edge trim / L-angle | Fixed to the joist, not through the board |
| Expansion screws / anchors | For fixing joists to concrete or masonry |
| Rubber mallet | For closing gaps — never a metal hammer |
| Fine-tooth carbide blade | Standard wood blades chip the capped edge |
| Spacers, string line, level | For set-out |
| Safety glasses, gloves | Composite chips are sharp |
Order for 5–10% waste, more if the deck has angles or many short runs. Our cost guide breaks down what boards, clips and trim actually add up to per square metre.
The installation, step by step
Step 1 — Set out and fix the joists
Set the joists at 300 mm centres, perpendicular to the direction the boards will run. Fix each joist with expansion screws at every crossing and along its run, then check the whole frame is in one plane — a 3 mm dip across two metres will telegraph straight through the boards and is the single most common complaint after installation.
Check levels before you lay a single board.
Step 2 — Fix the first board at the edge
Fix a starter clip along the first joist. Set the first board against it, and check the run is square to the building before going further — everything after this follows that line.
Leave the gap against the wall that you calculated in section two. It will look too wide when the deck is empty. It is not.
Step 3 — Run the field boards
Insert a hidden clip at every joist crossing and drive one screw per clip. Do not over-drive: the clip holds by clamping the board groove, and a screw driven too deep can let water in behind the cap. Knock the board home with a rubber mallet on the edge only, never with a metal hammer.
As you go, check the side gap stays even. Each fixing is rated to 2,380 N withdrawal resistance — about 243 kg per fixing (SGS report GZIN2604000591CM02_EN). The fixing is not the weak point in the system; the joist it goes into is. That is why the frame in Step 1 gets more attention than the clips.
Stagger the board ends so no two joints sit on the same joist in a straight line.
Step 4 — Cut and close the last run
Measure the last board at both ends — rarely the same, and the deck is the one place you cannot hide a 10 mm error. Allow for the expansion figure from section two when you cut.
Rip the board with a fine-tooth carbide blade and turn the cut face toward the trim, where it is covered.
Step 5 — Edge trim and fascia
Fix the edge trim to the joist, not through the board. The board has to be free to move underneath it — a trim screwed through the board pins it in place and you have built the expansion problem back in.
L-angle trim along the exposed joist face finishes the edge and hides the clip line at the same time.
Step 6 — Steps, corners and obstacles
Keep stringers on steps at the same 300 mm centres — steps are where impact loads land hardest, and the board is rated for 67 J impact (SGS GZIN2604000591CM01-1_EN), but the frame under it takes the repeated load.
Mitre outside corners with the two cut faces on the trim, not on the walkway. Around pipes and posts, cut a clearance hole with the same gap allowance and seal the cut edge.
On stepped and ramped surfaces, remember the surface figures: static friction measured 0.74–0.89, dynamic 0.72–0.90 (SGS GZIN2604000591CM01-1_EN). Wet and dry, that is a textured board rather than a polished one — but a board laid with a channel of standing water in it will still be slippery, which is a drainage and set-out problem, not a product one.
Seven mistakes that cause callbacks
- Boards butted tight against a wall. Expansion has nowhere to go; the deck buckles or pushes the trim off. Use the table in section two.
- Joists exceed 500 mm under a hollow board. The board deflects between joists and the clips pop. 300-500mm.
- Over-driven screws. The clip loses its clamp and the fixing strength is thrown away.
- A sealed void below. Trapped moisture is what the 1.12% absorption figure is measured against — do not give it a place to sit.
- A standard wood blade. Chipped cap at the cut edge is where water gets in first.
- Laying before the frame is level. A hollow in the frame becomes a visible wave in the deck.
- Laying boards cold and tight in winter. The board will be at its shortest when you install it and at its longest months later. Cut for the range, not for the day.
After the first season
Clean with a stiff brush and water; no sealing, no painting, because the cap is the finish.
What to expect from the surface, honestly: at 3,000 hours of accelerated weathering the colour shift measured ΔE 0.96 — a change most people would not pick out by eye. Past that point the rate increases: ΔE 1.87 at 3,500 hours and ΔE 4.58 at 4,000 hours (SGS GZIN2604000591CM01-1_EN). A deck in full southern sun will get there sooner than one under shade. That is why we publish the curve rather than only the first number.
Structurally, the board recovers 90% of a sustained load’s deflection once the load is removed (SGS GZIN2604000591CM02_EN), and the surface resists abrasion at 110.6 mg loss after 1,000 cycles of CS-17 under 1,000 g (SGS GZIN2604000591CM01-1_EN) — foot traffic, furniture legs and dog claws all fall well inside that.
Check the clips after the first full summer and again after the first winter. That is when movement either works or shows up.
FAQ
Do you need a gap between composite decking boards?
Yes — 4–6 mm side to side, set by the hidden clip. Boards butted edge to edge will rub, trap debris and squeak.
What joist spacing does composite decking need?
300-500 mm centre to centre for a 25 mm hollow board, perpendicular to the boards, doubled under any butt joint. Never wider.
Can you install composite decking in cold weather?
You can, but the board is at its shortest then. Work out the gap from the annual temperature range, not from the temperature on the day — a deck installed at −5 °C and designed for a 50 °C swing needs the full 4.6 mm per 2.9 m board.
Can composite decking be laid over an existing concrete slab?
Only on a drained, ventilated frame — adjustable pedestals or a joist grid. Laid directly on concrete, water has nowhere to go and the deck sits in it.
Does composite decking need to be sealed or painted?
No. The cap is the finish; sealing it adds nothing and painting over a capped board will not hold.
Ordering, quantities and lead times
We manufacture the boards to this specification in Foshan. Standard length is 2.9 m, cut to order; first orders can run below a full container with mixed colours. If you are pricing a job or checking a specification against a quote, send us the profile and quantities and we will come back with the loading and the price rather than a brochure.