Decking is traditionally supported by timber substructures, but as alternative decking solutions evolve so do the types of deck substructures. What solutions are available and what do you need to consider when choosing the right solution.

Is it wise to install a composite board on a timber substructure? Does this defeat the point?

The answer to both of these questions are Yes and No. When designing a deck various factors play part in deceasing the best materials fro your deck substructure. Below we will discuss the benefits of each solution and the issues each may face.

composite decking over timber structure

Composite Decking on Timber


  • Timber can rot, warp, split and can be damaged by insect infestations. The lifespan of any deck can be reduced if timber is used in the incorrect way.
  • Timber will swell when wet. This usually happens when it is colder. When timber is expanding, composite decking is likely to be contracting, so both systems are working against each other. Composite boards screwed directly into the timber substructure is likely to fail, screws coming loose and being visible above the boards. 
  • The same happens the other way when hot. Timber will dry out and will shrink & split / crack. At the same time composite boards will expand, warping against the substructure moving the other way.
  • Clips that secure decking boards can work loose overtime as timber swells and dries out.
  • Joist centres on timber substructures are less than those made of alternative materials. This leads to needing more timber as compared to alternatives such as a Aluminium Substructure such as Qwickbuild.
  • Can have structural limitations for decks at height that need to span larger distances or need to cantilever.
  • Can have limitations for deck areas that need to be very low and close to the ground or roof level.
  • Reduce the amount of airflow under a deck so will slow down or prevent water under the deck drying, leading to acceleration of water adsorption and rotting. 
  • Labour intensive install, takes time to build.
  • Ideally can only be built by a competent deck builder or DIY’er


  • Cheaper upfront material costs
  • Widely available
  • Can be pressure treated to stabilise the timber and extend its lifespan
  • If well designed it can have a reasonable airflow under the deck keeping the under deck dryer
  • Great at managing more difficult designs such as step areas
  • Can be easily cut and customised onsite during the build.
  • Can be an environmental sustainable product.
  • Is recyclable.

Composite Decking on Composite Joists


  • Hollow joists not as structurally strong and timber. Larger or more solid profiles needed to compensate these issues.
  • Can be more expensive than timber.
  • Can warp or bend with larger loads or heavier pedestrian traffic.
  • Poor quality joists (softwoods and low grade plastics) have same issues as low quality composite decking and can have a shorter working life.
  • Labour intensive install, takes time to build. 
  • Ideally can only be built by a competent deck builder or DIY’er


  • Will not rot or get damaged by insect infestations.
  • Good quality solid  composite joists will have reasonable strength.
  • Will expand and contract in line with the composite decking so will not be working against them.
  • Made from recycled materials and is recyclable itself.

Composite Decking on Aluminium Joists (Qwickbuild)


  • More expensive cost of materials.
  • Technical product that does not have full industry understanding.
  • Not as widely available.


  • Will not rot, warp, or split.
  • Not effected by insect infestations.
  • Can be used in wet area and marine environments. Ideal around swimming pools and hot tubs.
  • Can be a structural solution, larger spans and cantilever up to 50% of its own supported length.
  • Can be used on very low level deck areas, such as on membrane roofs, over exciting patios or hardstanding areas
  • Can be pre-cut, brackets and pre-assembled before delivery.
  • Quick and easy to install cutting days of professional style installs.
  • Can be installed by most DIY’ers without special tools or skills.
  • Will expand and contract with the composite boards, prevent board failure.
  • Secure decking with screw free twist clips. Again speeds up installations and prevents boards splitting at screw points
  • No screws visible and will not work loose.
  • Can easily control expansion and contraction of boards
  • Will last a lifetime. Using high strength marine grade structural aluminium profiles.
  • Can also support artificial grass and structural tiles on the same system.
  • Can be designed into complex designs and can incorporate planters & seating
  • Joist centres larger than that of timber and composite joists, reducing the amount of material used on a build.
  • Encourages greater airflow under the deck, drying out the ground or a platform as compared to timber of composite joists.

Do your homework and consider the cost benefits of the various solutions against use and lifespan of the deck. If a deck is installed well, and the application is not in a wet area and you have good airflow under the deck, a timber substructure can last for years. If looking for a structural solution or using on a roof top terrace with a need for an ultra-low level deck, or in close proximity to water such as a pool, a Qwickbuild deck would be a great solution that allows for an easily and speedy install. If you’re looking for a non-structural deck subframe that could be in close proximity to water, or over wet ground, a composite substructure could be a budget option to create a wet friendly deck.

At Elite Balustrade Systems we offer composite decking for use with various sub-structure and we specialise expensively in one of the most advanced structural deck solutions found in the UK. Speak to our team of decking advisors who will discuss your project requirements and advise you on the best decking build, to support your desired range of composite decking. We also offer solutions for a Qwickbuild deck to accommodate Artificial Turf and a Qwickbuild sub-frame for Structural Tiles. Or why not consider a mix of deck, tile and turf. Ask our team for more information.


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