Types of Composite Decking: Which Profile and Build-Up Each Setting Needs
Composite boards look interchangeable in a catalogue and are not interchangeable on site. What separates them is rarely the colour: it is the section, whether it is capped, what it is allowed to span, how it is fixed, and which test evidence exists for the way you intend to use it.
This page is the index to the rest of our technical pages. It sets out the five decisions in the order they have to be made, routes each setting to the page that answers it, and ends with a plain list of the evidence we hold and the evidence we do not.
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Two generations
Co-extruded and classic, 15 or 10 years
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350 / 300 mm
Flat decking and stair treads differ
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22.4 kg/m²
Boards only, 140 × 23 mm
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CE / CPR
M.2024.206.C97619 · UDEM
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Evidence listed
Including what we cannot evidence
Five Decisions, in the Order They Have to Be Made
Choosing a board first and checking the constraints afterwards is how projects end up re-ordering. These come in this sequence because each one narrows the next.
What is underneath
Soil, an existing slab, a waterproofed roof, a balcony, or a stair carriage. This decides the build-up before it decides the board, and on an elevated structure it decides whether anything may be fixed through the surface at all.
What the structure is allowed to carry
On the ground this rarely binds. On a roof, a balcony or a podium it is the governing figure and it belongs to the building, not to the deck. Our boards weigh 22.4 kg/m²; everything else in the calculation comes from your engineer.
How much height you have
A threshold, a door sill or an existing level sets a ceiling on board plus joist plus support. Where the height is too tight for a framed build-up, direct-lay tiles are usually the only route that keeps the door working.
How it will be fixed
Hidden clips into a groove, or face-fixed. Whether the perimeter can be lifted later for inspection. On stairs the fixing has to sit below the walking surface, which rules out face fixing entirely.
What evidence the project will be judged against
Slip classification, fire classification, wind, structural. Find out which method the authority accepts before shortlisting products — the two tables at the bottom of this page set out what we can and cannot evidence today.
Where You Are Laying It, and Which Page Answers It
Each setting has one binding constraint that decides everything else. Find yours in the first column.
| Setting | The constraint that binds | Where the detail is |
|---|---|---|
| Terrace against a house | Door threshold height and the damp-proof course | Garden terrace detailing page |
| Backyard platform on grass or soil | Ground preparation, drainage and uneven settlement | Ground-level backyard page |
| Roof terrace, balcony or podium | Permitted load, and nothing drilled through the waterproofing | Lightweight decking and pedestals page |
| Outdoor stairs | Regulated rise and going, and a tighter support spacing | Composite stair treads page |
| Anywhere slip is specified | Which of three unrelated test systems applies | Slip resistance page |
| Pool surround, barefoot | Barefoot wet classification, which we do not hold | Slip resistance page — read it before specifying |
Start Here, Then Follow the Thread
Composite decking for garden terraces — the junction where a deck meets a house: threshold height, damp-proof course clearance, which way the fall runs, and the ventilation gap at the wall.
Ground-level decking for backyards — building on grass or soil: what to do about the formation, drainage and support points so the platform does not settle unevenly.
Lightweight decking on adjustable pedestals — roofs, balconies and podium decks: weight per square metre, pedestal load and height, and fixing without penetrating the waterproofing.
Outdoor composite stair treads — the dedicated 138 × 22.5 mm moulded profile, why two boards make one compliant tread, and why the support spacing is 300 mm rather than 350 mm.
Slip resistance — the three test systems, why a result from one cannot be converted into another, and exactly which classification our report covers.
Six Questions Every Outdoor Deck Has to Answer
Whatever the setting, these six come up. The pages above differ mainly in which one dominates.
Common to every application
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What carries the load
Prepared ground, a slab, pedestals or a stair carriage. Everything above it is sized from this.
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Where the water goes
Boards laid with open gaps drain through the gaps, so what needs a fall is the surface underneath — not the deck.
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Whether the cavity breathes
A sealed perimeter traps moisture in the place that dries slowest. Ventilated details and a liftable edge are not decoration.
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What it is allowed to span
350 mm maximum for the 140 × 23 mm flat board. Stair treads are 300 mm. These are not interchangeable figures.
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How much it moves
Composite moves with temperature. Side, end and perimeter allowances have to be left, and long runs move most at their ends.
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What evidence is required
Slip, fire, wind and structural. Establish the accepted method early; retro-fitting evidence after installation is the expensive path.
The Range, and What Separates the Sections
Two generations and several profiles. The practical differences are the cap, the section and the warranty term.
Co-extrusion Decking Board
23 × 138 mm · second generation
Four-sided cap including back and edges. 15-year stepped warranty. The profile our R10 slip result was tested on.
View product →
Co-extrusion Wide Board
23 × 148 mm · second generation
Wider face, fewer joint lines across an open area. Same build-up height and the same 15-year cover.
View product →
Classic Board
140 × 25 mm · first generation
Uncapped classic section, 10-year stepped warranty. Lower cost per metre where the area is sheltered and budget-led.
View product →
Composite Stair Tread
138 × 22.5 mm · second generation
A dedicated moulded profile with a side clip groove and a start-clip groove, not a deck board cut down. Solid section.
View product →
Co-extrusion Deck Tile
Direct-lay · second generation
For thresholds too tight for a framed build-up. Lays onto the existing surface with no bearers beneath.
View product →
Adjustable Decking Support
24–175 mm · 8 kN each
Five overlapping height steps. Levels a deck over an existing fall without fixing through a waterproof membrane.
View product →
Estimate Quantity and Compare Offcut
Whatever the setting, the quantity arithmetic is the same. It is calculated from the 145 mm effective coverage width rather than the nominal board width, which is the usual reason a container order arrives short.
Evidence We Hold
Reference numbers and scope, so any of this can be checked rather than taken on trust. Ask the same of every supplier you shortlist.
| Subject | Standard or method | Reference | Scope it covers |
|---|---|---|---|
| Quality system | ISO 9001:2015 | IAS25924Q1473R0M, valid to 29 Apr 2027 | R&D, production and sale of WPC building products |
| Environmental system | ISO 14001:2015 | IAS25924E0394R0M, valid to 29 Apr 2027 | As above |
| CE marking | CPR, EU 305/2011 Annex V | M.2024.206.C97619, UDEM International Certification, a notified body, valid to 11 Mar 2029 | Product certification |
| Slip, shod and oil-wet | AS 4586-2013 Appendix D | Intertek 230625005SHF-001, 15° measured, classified R10 | Decking profile ZSGJ138H23A only — not the wide board, tread, tiles or fencing |
| Substance compliance | REACH, EC 1907/2006 | Manufacturer's declaration of conformity, 28 Jan 2026 | Fencing profiles T180H24 and T90H24. A declaration, not a third-party certification |
| Wind, horizontal load | In-house method — not a recognised standard | Intertek 250710027GZU-001, 372.1 N/m² sustained, pass | One 1.8 × 1.8 m fencing configuration. Not a decking or uplift figure |
| Bending load | GB/T 24508-2009 | In-house QC, not third party | Fencing profiles. Quoted as in-house testing only |
| Warranty | Manufacturer's limited warranty | Generation 2: 15 years. Generation 1: 10 years | Both stepped by year, from arrival and signature at destination port |
Evidence We Do Not Hold
Published deliberately. If your project requires any of these, we cannot meet it today — and it is cheaper for both of us if you know that now.
| Subject | Standard | Where it usually comes up |
|---|---|---|
| Slip, barefoot and wet | DIN 51097, Class A / B / C | Pools, spas, changing areas |
| Slip, pendulum | BS 7976, PTV | UK specifications and HSE practice |
| Reaction to fire, EU | EN 13501-1 | Commercial and public projects in the EU |
| External fire, roofs | BROOF(t4), EN 13501-5 | Roof terraces and podium decks in the EU |
| WPC product standard, US | ASTM D7032 / D7031 | US distribution and specification |
| Surface burning, US | ASTM E84 | US commercial projects |
| Evaluation report, US | ICC-ES | US code compliance |
| Wildland-urban interface | CAL FIRE BML listing, SFM 12-7A-1 | California WUI zones |
| Substance declaration, US | California Proposition 65 | US consumer-facing supply |
How to Read Those Two Tables
Scope is the part most suppliers leave out
A classification belongs to the profile that was tested, not to a catalogue. Our R10 covers one decking section. It does not cover the wide board, the stair tread, the tiles, the classic range or any fencing product, and we will not present it as though it did. When another supplier quotes a rating, ask which exact profile was tested, by which laboratory, under which standard, and on what date. Five questions. A figure that cannot survive them is not evidence.
A declaration is not a certification
Our REACH document is a manufacturer's declaration of conformity, and it covers fencing profiles. That is a useful document and it is not a third-party certificate. Anyone writing “REACH certified” about it — including us, if we ever slip — is overstating it.
A report to an in-house method is not a standard
The wind report is real, was carried out by Intertek and passed. It was also run to an in-house method agreed with a customer rather than to a recognised standard, on one fencing configuration. It is evidence of that test, not a compliance credential, and it says nothing about uplift on a roof deck.
If you need something in the second table
Tell us at design stage. Testing can be arranged on the specific profile you intend to use: the cost is paid up front and credited in full against your order. We will give you the cost and the lead time before you commit.
Choosing Composite Decking: Common Questions
Types and differences
What are the main types of composite decking you supply?
Two generations. Co-extruded boards are capped on four sides, including the back and edges, and carry the 15-year stepped warranty. Classic boards are uncapped and carry the 10-year stepped warranty. Within those there are flat board sections, a dedicated stair tread profile and direct-lay tiles. The practical difference is the cap, the section and the warranty term, not the colour.
What is the difference between capped and uncapped?
A capped board has a co-extruded protective layer fused over the core. Ours is capped on four sides rather than three, so the back and edges are covered as well as the face. Uncapped classic boards cost less per metre and are the budget-led option where the area is sheltered.
How do I choose between the sections?
Work through the five decisions at the top of this page in order: what is underneath, what the structure may carry, how much height you have, how it will be fixed, and what evidence the project will be judged against. The board is normally the last thing the sequence decides, not the first.
Which board is used for stairs?
A dedicated 138 × 22.5 mm moulded tread with its own clip grooves, not a deck board cut to width. It also has a tighter maximum support spacing — 300 mm rather than the 350 mm used for flat decking.
Specification and evidence
What can composite decking span?
350 mm maximum joist spacing for the 140 × 23 mm flat board. Stair treads are 300 mm because the load arrives at the front edge rather than spread across the span. Diagonal layouts, heavy use and point loads all call for tighter centres.
What does it weigh?
22.4 kg/m² for the 140 × 23 mm board, about 0.22 kN/m², derived from 3.25 kg per linear metre at a 145 mm effective coverage width. That figure is boards only — joists, supports and anything the deck carries are additional.
Do you have a fire classification?
Not one that most European projects will accept. We do not hold EN 13501-1, and we do not hold BROOF(t4) for roof build-ups. We hold an Australian fire test to AS ISO 9239.1, which is a different framework and should not be presented as a substitute. If a project requires an EU classification, we cannot meet it today.
Are you compliant for the US market?
No, and this is worth being direct about. We hold no ASTM D7032, no ASTM E84, no ICC-ES evaluation report and no Proposition 65 declaration. Our documentation is European, Australian and Chinese. A US project that requires those cannot be served from our current evidence.
Supply
What lengths are the boards supplied in?
2.9, 3.6, 4.0 and 5.8 m. Check how your longest run divides into those before estimating quantity — the calculator above compares offcut across all four.
Can we see the boards first?
Samples are free — you cover the courier, and we credit it against your first order. Sending a sample to your own laboratory is a perfectly reasonable step and several buyers do it.
What is the production lead time?
20 days for standard catalogue sections.
Tell us where it is going and what it has to satisfy
The setting, what is underneath, the height you have, the standard your project is written around and the destination port. We come back with the profile, the build-up and a quantity take-off — and we will say plainly where our evidence stops.
Start from the constraint, not from the catalogue
Tell us what is underneath and what the project has to satisfy. We will point you at the right section and the right build-up, and tell you which evidence we can and cannot provide.