The most common composite decking installation mistakes include leaving incorrect gaps between boards, spacing joists too far apart, using an unsuitable subframe and choosing the wrong fixings. Poor drainage, inadequate board-end support and incorrect storage can also cause problems that are expensive to put right.
Most of these mistakes are avoidable. Knowing what to look out for can help prevent boards from moving, lifting or becoming damaged after installation. If you’re preparing to build a deck, our article on how to install composite decking explains the complete fitting process. Here, we’re focusing on the mistakes, why they matter and how to prevent them.
Quick checker: What could go wrong with your composite decking?
| Problem or warning sign | What might be causing it? |
| Boards lifting, buckling or pressing together | Incorrect expansion gaps |
| Decking feels springy or bouncy | Excessive joist spacing or inadequate support |
| Uneven or moving board ends | Poorly supported board-end joints |
| Boards becoming loose | Incorrect clips, screws or fixings |
| Water collecting beneath the deck | Poor drainage or restricted ventilation |
| Entire deck feels unstable | Weak subframe or inadequate foundations |
| Boards damaged before fitting | Poor storage or handling |
| Boards becoming distorted in strong reflected sunlight | Unusually high, concentrated heat exposure |
These are possible causes rather than definitive diagnoses. Several problems can produce similar symptoms, and structural movement should always be investigated before the deck continues to be used.
1. Leaving incorrect gaps between composite decking boards
Composite decking naturally expands and contracts as temperatures change. If boards are installed without enough room for that movement, they can press against one another or nearby structures.
This is especially important where two boards meet end-to-end or where decking finishes against a wall.
What can go wrong?
- Boards can lift or develop raised joints.
- Pressure between boards can cause distortion.
- Fixings and board edges may become damaged.
- Inconsistent gaps can spoil the appearance of the finished deck.
Warning signs: Board ends pushing together, raised joints or uneven spacing between boards.
How to avoid it
Use the expansion gaps specified for your decking range, board length and installation temperature.
On compatible Ecoscape grooved decking, the approved hidden clips establish the normal side-to-side spacing. Board-end gaps and clearance around fixed structures must follow the relevant expansion-gap table.
Don’t rely on a single gap measurement found online. What is suitable for one decking system or temperature may not be appropriate for another.
2. Positioning decking joists too far apart
If the joists underneath composite decking are spaced too far apart, the boards may flex excessively underfoot.
This can be particularly problematic when replacing timber boards with composite decking on an existing subframe.
What can go wrong?
- Decking feels springy when walked on.
- Boards can move or deflect between supports.
- Unsupported areas may suffer unnecessary stress.
- The deck may require costly corrective work.
Warning signs: Noticeable movement between joists, excessive flexing or uneven board support.
How to avoid it
Check the maximum joist spacing for the specific decking range.
Ecoscape’s February 2024 installation specifications state:
| Decking range | Maximum joist centres |
| Forma, Clarity and Grande | 400mm |
| Linea | 350mm |
These are maximum centres for the ranges covered by those instructions. Particular layouts or conditions may require closer support.
The important thing is not to assume that an existing timber subframe automatically meets composite decking requirements.
3. Using an unsuitable or weakened subframe
A deck is only as reliable as the structure supporting it.
Even correctly fitted composite boards can develop problems if the frame beneath them is deteriorating, inadequately secured or unsuitable for the loads involved. This is one reason reusing an old timber decking frame without inspecting it can be a mistake.
What can go wrong?
- Joists or connections can deteriorate.
- The deck may become uneven or unstable.
- Fixings may loosen as the frame moves.
- The supporting structure may not safely carry the intended loads.
Warning signs: Movement across larger areas, visibly deteriorating timber, loose structural connections or an uneven deck surface.
How to avoid it
Check that the subframe is structurally sound, securely supported and compatible with the chosen decking system.
Pressure-treated timber should be in good condition, with exposed cut ends appropriately protected. Aluminium and compatible plastic substructures may also be suitable, but each system has its own requirements.
Don’t assume that an existing frame is suitable simply because it previously held wooden decking.
For more information, see our guide to laying composite decking over an existing wooden deck. Ecoscape composite boards must not be installed directly over existing decking boards.
4. Failing to support decking board-end joints
Where two boards meet end-to-end, both ends need adequate support.
A mistake is to leave a board end unsupported or use a joint arrangement that doesn’t provide the support required by the manufacturer.
What can go wrong?
- Board ends may move when walked on.
- Joints can become uneven.
- Fixings may be placed under unnecessary stress.
- The gap between board ends may become irregular.
Warning signs: Visible deflection or movement at the joint, particularly when pressure is applied near the board ends.
How to avoid it
Make sure both board ends are independently supported in accordance with the decking manufacturer’s jointing requirements.
Ecoscape’s installation guidance illustrates a double-joist arrangement, providing support to each board end while maintaining the necessary expansion gap.
The joint support needs to be right from the outset. Correcting it after installation may mean removing decking boards.
5. Using the wrong clips, screws or fasteners
Not all composite decking fixing systems work in the same way.
Using leftover clips from another project, ordinary screws or fixings intended for different boards can interfere with how the decking is secured and how it moves with temperature.
What can go wrong?
- Boards may become loose.
- Fixings may damage board edges.
- Boards may not expand and contract as intended.
- Incorrectly secured boards can become distorted.
Warning signs: Loose or damaged clips, unusual movement around fixings or boards appearing to be held under pressure.
How to avoid it
Use the approved fixing system for your particular decking range.
Ecoscape’s compatible grooved decking uses hidden T-clips and locking clips, with specified screws to secure the components.
Locking clips are important because they control the board’s movement as temperatures change. Incorrect positioning or unsuitable substitutions can affect how the system performs.
Certain fascia boards, edge trims and finishing components use approved screw-fixing methods, so the problem isn’t using screws in every circumstance. It’s using an inappropriate fixing method for the component involved.
6. Ignoring drainage and ventilation beneath the deck
Water needs somewhere to go, both across the surface of the decking and underneath the subframe. Poor drainage or restricted airflow can create persistently damp conditions, particularly around timber supports.
What can go wrong?
- Water can collect beneath the decking.
- Timber components may remain damp for prolonged periods.
- Debris can obstruct drainage routes.
- Moisture-related problems can develop around the supporting structure.
Warning signs: Persistent standing water, visibly damp subframe components or blocked drainage openings.
How to avoid it
Make sure the deck has adequate drainage and ventilation. Ecoscape’s installation guidance specifies:
- A minimum of 25mm clearance beneath the decking for drainage and airflow.
- A recommended fall of approximately 1% to help water run off.
- A supporting structure that allows water to escape rather than collect underneath.
Keep drainage routes clear and avoid enclosing the underside so tightly that ventilation is obstructed. If animal-proof mesh is fitted around the base, it should not block the required airflow.
7. Building on inadequate or unstable foundations
Problems with a decking structure sometimes begin below the subframe. If the ground or supporting foundations are unsuitable, movement can eventually affect the decking above.
What can go wrong?
- The frame may settle unevenly.
- Board joints can become misaligned.
- The deck may move or feel unstable.
- Serious structural problems can develop where supports are inadequate.
Warning signs: Uneven settlement, movement across the deck or noticeable changes in level.
How to avoid it
Ensure the foundations are suitable for the ground conditions, deck height, intended loads and supporting structure. Foundation requirements are not identical for every installation.
Raised decks, complex ground conditions and structures carrying substantial loads may require professional structural advice. Don’t treat a foundation method suitable for a small, low-level garden deck as automatically suitable for a larger or raised structure.
8. Storing or handling composite decking incorrectly
Some problems begin before installation. Decking boards can become damaged if they’re stored unevenly, left unsupported or dragged across rough surfaces.
What can go wrong?
- Boards may become distorted before fitting.
- Surfaces can be scratched or marked.
- Damaged edges may affect the finished appearance.
- Poorly stored materials can become dirty or unnecessarily weather-exposed.
Warning signs: Visible distortion, surface damage, damaged edges or boards that won’t sit correctly.
How to avoid it
Ecoscape recommends storing composite materials under cover, in shade and protected from the weather until they’re required.
Boards should be supported above the ground on a clean, flat surface at intervals of no more than 500mm, with supporting battens aligned through the stack.
Lift boards rather than dragging them over one another, and avoid sliding tools or equipment across finished surfaces. The board temperature must also be considered when determining expansion gaps. Don’t assume that a universal acclimatisation period applies to every decking system.
9. Ignoring the fitting instructions for your decking range
One of the easiest mistakes is assuming all composite decking systems can be fitted in exactly the same way.
Board profiles, joist spacing, clips, expansion requirements and finishing details can vary between manufacturers and even between ranges from the same supplier.
What can go wrong?
- Incompatible clips or fixings may be used.
- Board movement may be restricted.
- Joist spacing may not provide enough support.
- The finished deck may not meet the manufacturer’s installation requirements.
Warning signs: Components that don’t fit correctly, conflicting spacing advice or problems appearing shortly after installation.
How to avoid it
Check the technical instructions for the exact decking range and fixing system being used. Ecoscape’s Composite Decking Installation Guide (PDF, February 2024) explains the requirements for the products covered by that edition.
If you’re working with a different or newer system, make sure you have the appropriate approved instructions.
One additional risk: Excessive reflected heat
Another issue worth being aware of is concentrated sunlight reflected from certain types of glazing, particularly low-emissivity (Low-E) glass.
In unusual circumstances, concentrated reflected heat can cause decking surfaces to reach significantly higher temperatures than normal.
Ecoscape notes that excessive heat exposure may contribute to unusual expansion, distortion or surface damage, although it considers extreme conditions unlikely in the UK.
What to watch for: Areas exposed to concentrated reflections from nearby glazing, particularly where repeated intense heating occurs.
If you suspect a reflected-heat problem, seek advice rather than assuming it is simply an expansion-gap issue.
Final composite decking mistake checklist
Before considering the installation complete, use this quick checklist to identify anything that might have been overlooked:
- The subframe and foundations are sound and stable.
- Joist spacing meets the requirements for the decking range.
- Board-end joints have adequate support.
- Expansion gaps meet the manufacturer’s specifications.
- The correct clips, screws and fixings have been used.
- Drainage routes and ventilation are unobstructed.
- Boards have been properly stored and checked for damage.
- The manufacturer’s relevant technical instructions have been followed.
If something appears wrong, investigate it before the deck is put into regular use. Structural movement or uncertainty about the supporting frame should be assessed by a suitably qualified professional.
Avoiding problems with composite decking
Most installation mistakes come down to inadequate support, incorrect spacing, unsuitable fixings or poor attention to drainage. They are generally easier to prevent than to correct afterwards.
By understanding the common warning signs and following the specifications for your decking system, you can avoid many of the problems that lead to damaged boards, uneven surfaces and unnecessary repair work.
If you’re unsure whether your proposed subframe or existing structure is suitable, contact the Ecoscape team for advice before proceeding.
