Coastal Composite Decking: Choosing the Substructure and Fixings That Survive Salt Air
Salt does not rot wood-plastic composite. It has nothing to feed on — there is no lignin to break down and no open grain to wick into. That is the easy half of the answer, and it is the half most suppliers stop at.
The hard half is everything else in the build-up. The joists, the clips, the screws, the brackets holding the frame to the structure — those are metal, and salt-laden air is what metal fails in. On a coastal deck the board is rarely what goes first. This page is about specifying the parts that do.
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4-sided cap
Back and edges included, co-extruded
🛡️
15-year warranty
Stepped, co-extruded boards
⚠️
No salt-spray test
Stated plainly, not implied
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350 mm max
Joist centres for 140 × 23 mm
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22.4 kg/m²
Boards only
Salt does not rot the board.
It eats everything you fix it with.
Which is why this page spends more words on joists, clips and fixings than it does on boards.
Is Composite Decking Suitable for Coastal Areas? The Answer Has Two Halves
Worth separating the halves, because the usual sales answer gives you only the first one and stops there.
The board is the part that is fine
Wood-plastic composite has no lignin to break down and no open grain to wick salt water into. It does not rot, and it does not corrode, because it is not metal. A supplier telling you composite is "ideal for coastal projects" is telling you something true about one component and nothing about the other five.
Everything holding it down is metal
Joists, clips, screws, angle brackets, the fixings tying the frame back to the structure, and on a raised deck the balustrade and its anchors. Salt-laden air is exactly the environment those fail in, and they fail from the inside of the build-up where nobody looks until a board lifts. On a coastal deck the substructure is the specification. The board is a finish.
So the question to ask us is not "is it suitable"
It is: what does the substructure need to be made of, what fixing grade goes with it, what does the warranty cover in that environment, and what evidence do you hold. Those four are answerable. The first three are answered below; the fourth is answered honestly, including where the answer is none.
The Numbers That Do Not Change at the Coast
Salt does not alter the span, the weight or the slip result. These carry over from any other deck — which is precisely why they are not where the coastal risk sits.
350 mm
Max joist centres
140 × 23 mm board
22.4
kg/m², boards only
At 145 mm coverage width
R10
Shod wet ramp
ZSGJ138H23A only
20 days
Production
Standard catalogue sections
Component by Component: What the Coast Does to Each Part
The same deck, broken into what it is actually made of. The middle column is general material behaviour, not our test data — we hold no corrosion testing of any kind. Read the third column as what to settle with your engineer before ordering.
| Component | What salt-laden air does to it | What to settle before ordering |
|---|---|---|
| Composite board | Nothing structural. Surface salt deposits are a cleaning task, not a degradation mechanism | Capped or uncapped — a UV and staining decision, covered further down |
| Steel joist / keel | Zinc coating is sacrificial and is consumed faster in salt air than inland. Once it is gone, the steel goes | Coating class and expected service life in your exposure, or substitute a different material |
| Aluminium joist / keel | Forms a passive oxide and does not rust, but is vulnerable to galvanic attack when coupled to dissimilar metals, and to salt standing in crevices | Alloy and temper, isolation from other metals, and whether crevices can drain |
| Clips and screws | The smallest sections in the build-up, so they lose proportionally the most. Usually the first visible failure | Fixing grade, and that it is compatible with the joist material rather than merely available |
| Structural fixings and brackets | Hidden, load-bearing, and the most expensive to replace | Specified by the project engineer to the local exposure category, not chosen from a decking catalogue |
| Balustrade and hardware | Exposed on all faces, handled constantly, and safety-critical | Outside what we supply for decking — specify separately |
The Decision This Page Exists For: Steel or Aluminium
We supply both and we hold no corrosion testing on either. What follows is material behaviour and the questions it forces — not a recommendation we are qualified to make for your site.
| Zinc Plated Steel KeelSubstructure | Aluminium KeelSubstructure | |
|---|---|---|
| Rust mechanism | Zinc is a sacrificial coating and is consumed over time — faster in salt-laden air than inland | None. Aluminium forms a passive oxide rather than rusting |
| Main vulnerability | Once the coating is gone, the steel beneath it goes | Galvanic attack when coupled to dissimilar metals, and salt standing in crevices |
| Stiffness for the same depth | Higher | Lower — check the section against your span |
| Handling on an exposed site | Heavier | Lighter |
| What to settle before ordering | Coating class, and expected service life in your exposure category | Alloy and temper, isolation from other metals, and whether crevices can drain |
| What we can evidence | Nothing on corrosion. We hold no salt-spray testing | Nothing on corrosion. We hold no salt-spray testing |
| View product | View product |
Specifying a Seafront Deck, in Order
The board is chosen last. Everything above it in this list constrains what the board can be.
Step 01
Establish the exposure, in writing
Distance from breaking surf, prevailing wind direction, and whether the deck is sheltered by the building or faces the water. A terrace on the landward side of a house 800 m back is a different specification from a balcony directly above a beach, on the same plot.
Step 02
Have the substructure decided by someone who owns the consequence
Steel with a coating specification, or aluminium with an alloy specification. We supply both and we will give you what we hold on each, but the corrosion design for your exposure is an engineering decision, not a catalogue choice.
Step 03
Match the fixings to the joist, not to the box they came in
Fixing grade and joist material have to be compatible as a pair. Dissimilar metals in permanent contact in salt air is a known failure mode, and it is designed out at this stage rather than diagnosed later.
Step 04
Set joists at 350 mm centres or tighter
That is the maximum for the 140 × 23 mm board. Tighten it where the deck carries concentrated loads or where the span direction changes.
Step 05
Allow for movement at the ends of long runs
Leave the side, end and perimeter allowances for the profile and for the local temperature range. A long straight run moves most at its ends, which is where the allowance has to be.
Step 06
Write the rinse regime into the handover
Fresh water removes salt from the surface and, more importantly, from the fixings and the gaps. It is the one maintenance task on a coastal deck that changes the outcome, and it costs nothing to specify.
Already know your distance from the water and your substructure material?
Send the deck area, the substructure you are considering and the destination port. We will come back with the board quantity, the clip and keel list, the build-up weight, and a plain statement of what we hold and do not hold for that environment.
⚠️Chemical corrosion is an explicit warranty exclusion
We are telling you before you buy rather than after you claim. The board warranty covers three inherent defects arising naturally — peeling, cracking and overall uneven fading — stepped by year, running from the date the goods are signed for at the destination port. 15 years on co-extruded boards, 10 on uncapped classic.
The exclusions are improper installation, external or deliberate damage, natural disaster, chemical corrosion, improper use, and after-sales service.
That does not make a coastal project uncovered — peeling, cracking and uneven fading on the board remain covered on the same terms as anywhere else. It means damage attributable to chemical attack sits outside the clause, and since we hold no salt-spray testing, neither we nor you can point to evidence about where that line falls in a seafront environment. If your contract needs that settled, settle it in the contract rather than assuming it from a brochure.
What We Can and Cannot Evidence for a Coastal Climate
Stated now rather than at approval; we would rather lose the argument early than be found out late. Where the answer is no, testing can be arranged on the specific profile your project intends to use, with the cost paid up front and credited in full against the order — worth raising at design stage, while the programme still has room for it.
What we hold
- R10 shod-wet slip, profile ZSGJ138H23A — Intertek 230625005SHF-001, AS 4586-2013 Appendix D, measured at 15°
- ISO 9001 and ISO 14001, valid to April 2027
- A stepped warranty schedule in writing, 15 years capped / 10 years classic
- A REACH declaration of conformity — but read the caveat opposite
What we do not hold
- Any salt-spray or accelerated corrosion testing, on boards or on substructure sections. Not a weaker version, not an equivalent from another standard: none
- Immersion or leaching data for a board in sea water
- Accelerated weathering or colour-retention data, on either grade
- Wind uplift data for decking. Our only wind report used an in-house method on a 1.8 × 1.8 m fencing system, and we will not quote it at you as if it answered this
- DIN 51097 barefoot classification, for a pool surround or any wet barefoot area
About That REACH Declaration
Because it is the document most often mistaken for something it is not.
What it is
A manufacturer's declaration of conformity
EC 1907/2006, Article 33 SVHC below 0.1%, Article 67 Annex XVII.
What it is not
A third-party certification
We declare it. No external body assessed it. Buyers who ask for laboratory names and report numbers are right to, and we would rather they asked us than assumed.
Scope
Issued for the fencing profiles T180H24 and T90H24
Not for decking. If your consenting body or specifier needs a decking-scoped document, it has to be issued, not reinterpreted.
Sun and Salt Together: Capped or Uncapped
Coastal sites are usually high-UV as well as high-salt. Here is what we can and cannot tell you about colour.
The physical difference is real and simple
Classic boards leave the wood-polymer composite exposed on all four faces, so UV reaches the core material directly. Co-extrusion bonds a polymer cap onto the core as the core is formed — and ours caps all four faces including the back and the edges, not a three-sided budget version that leaves a face exposed where water and salt sit. The cap does not make the board stronger; it puts a layer between the sun and the core.
What we cannot tell you: a fade rate
We hold no accelerated weathering test and no colour-retention data, on either grade. So we cannot give you a percentage, a timeline, or a point at which the colour settles. If a supplier gives you one of those, ask which standard it was measured to. We would rather leave the box empty than fill it with a number we cannot source.
What we can tell you: how the warranty treats it
Overall uneven fading is one of the three inherent defects the warranty covers, on both grades — alongside peeling and cracking. The difference is the term: 15 years stepped on co-extruded boards, 10 on uncapped classic. For a seafront elevation we would specify co-extrusion, on the cap and on the term together. If budget forces classic, tell the client in advance that an uncapped surface weathers in full view on an exposed elevation, rather than fielding it as a complaint later.
The Two Board Grades
Colour, variants and the full specification are on each product page.
Co-extrusion Decking Board
23 × 138 mm · 15-year cover
Capped on four sides including the back and the edges, so no uncapped face is left exposed to weather. The profile our R10 shod-wet result was tested on.
Co-extrusion Wide Board
23 × 148 mm · 15-year cover
Wider face, fewer joint lines across an open terrace. Same build-up height as the 138 mm board.
Coastal Composite Decking: Common Questions
Suitability and evidence
Is composite decking suitable for coastal areas?
Half of it is. The board does not rot and does not corrode — salt has nothing in wood-plastic composite to attack. The substructure and the fixings are where a coastal deck actually fails, and those are a material and grade decision for your engineer. Treat any answer that stops at “yes, ideal for coastal” as having answered one component out of six.
Does “coastal” here mean a colour or an environment?
An environment. Several manufacturers sell board colours with coastal in the name, so the word gets used both ways. This page is about building on a site exposed to salt air. Our own colours are listed on each product page.
Do you have salt-spray or corrosion test data?
No. We hold no salt-spray testing and no accelerated corrosion testing, on boards or on substructure sections. We would rather say that plainly than offer you a report from a different standard and let you assume it covers this.
Can the boards be used in direct contact with sea water?
We have no immersion or leaching data, so we cannot give you a supported answer for a board that is periodically or permanently in sea water. Tell us what the structure will actually experience; if the project needs evidence for immersion, it has to be tested.
What about wind uplift on an exposed seafront deck?
That is a structural design question for the project engineer. Our only wind report used an in-house method on a fencing system with aluminium posts, not on decking, and we do not offer it as an answer to this.
Substructure and installation
Steel keel or aluminium keel for a coastal site?
We supply both and we will give you the specification we hold on each. Zinc on steel is a sacrificial coating and is consumed faster in salt air; aluminium does not rust but is vulnerable to galvanic attack when coupled to dissimilar metals and to salt standing in crevices. Which is right depends on your exposure, your fixings and your required service life — that is a corrosion design decision, and it belongs with your engineer rather than with a board supplier.
What joist spacing should a coastal deck use?
350 mm maximum centres for the 140 × 23 mm board, the same as inland — salt does not change the span. Tighten the centres where the deck carries concentrated loads or where the span direction changes.
What do the boards 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 is boards only. On a raised or cantilevered coastal terrace the engineer will want the whole build-up, including joists and fixings.
What maintenance does a coastal deck need?
Rinse it with fresh water. That removes salt from the surface and, more importantly, from the gaps and the fixings. There is no sanding, staining or sealing cycle on composite — but rinsing is the one task at the coast that actually changes the outcome, so write the frequency into the handover.
Warranty and supply
Does the warranty still apply at the coast?
The board warranty covers peeling, cracking and overall uneven fading, stepped by year, running from the date the goods are signed for at the destination port — 15 years on co-extruded boards, 10 years on uncapped classic. Chemical corrosion is an explicit exclusion in the clause. That does not make a coastal project uncovered, but it does mean damage attributable to chemical attack sits outside it, and we hold no salt-spray evidence to draw the line for you. If your contract needs that settled, settle it in the contract.
Which boards do you recommend for a seafront elevation?
Co-extrusion, capped on four sides including the back and the edges. Coastal sites are usually high-UV as well as high-salt, and the cap puts a polymer layer between the sun and the core where an uncapped board leaves the composite exposed. We hold no accelerated weathering or colour-retention data, so we will not quote you a fade rate or a timeline — what we can point to is that overall uneven fading is a covered inherent defect on both grades, and the co-extruded term is 15 years stepped rather than 10.
Are the boards slip resistant when wet?
Our co-extruded decking board, profile ZSGJ138H23A, is classified R10 under AS 4586-2013 Appendix D — the shod, wet ramp test, measured at 15°, mid-band in the 10–19° R10 range. That result belongs to that one profile and does not extend to the other boards. It is a shod rating: for a barefoot wet area such as a pool surround the relevant method is DIN 51097, and we do not hold it.
What is the lead time, and can we see the boards?
20 days production for standard catalogue sections. Samples are free — you cover the courier, and we credit it against your first order. On a coastal project, sending a sample and a length of keel for independent assessment is a reasonable step.
Send the project details
Deck area, distance from the water, whether the deck is sheltered or faces the sea, the substructure material you are considering, and the destination port. We come back with a board quantity, the clip and keel list, the build-up weight, and a plain statement of which evidence we can and cannot supply.
Start from the exposure, not from the board
Tell us how close the deck sits to the water and what the substructure is. We will confirm the board, the substructure list and the quantity — and tell you plainly where our evidence stops.