Sub frames


Sub structure

Gripsure manufactures our products in accordance with the requirements of the ‘Deckmark Plus’ quality assurance and performance scheme operated by the Timber Decking & Cladding Association (TDCA). The TDCA is an independent technical and advisory organisation formed to establish and promote the materials, design and installation practices required to create high performance decks and associated outdoor structures in timber.

The TDCA has a professional’s manual for installing deck boards. We recommend following the installation guidelines within the manual which can be purchased on their website -

There are a number of ways that the sub structure can be constructed. The choice of sub-structure will be dependant on the application of the deck. We would always recommend using a competent and experienced deck installer as the sub structure gives the whole deck it’s strength and is extremely important in giving a long lasting and high performance deck.



In all cases, deck boards are fixed into joists below. Depending on the species of the joists, the dimensions can vary. The majority of joists are constructed from softwood and pressure treated to usage class 4 (External use and in ground contact or exposed to constant wetting). A minimum of C16 graded softwood joists should be used and we would recommend all joists to be graded to C24. These are generally sourced from a timber or builder’s merchant with common dimensions and lengths kept in stock ready for quick delivery.

If using hardwood, bamboo or composite deck boards, we would recommend using a species for the joists which will match the service life of the deck board. Hardwood and bamboo deck boards will last for over 30 years so the sub structure needs to last the same period of time or the sub structure could fail before the deck boards. Gripsure offer a range of joists, so please enquire with the sales office for prices. As the sub structure won’t be seen, a different species can be used to the deck boards, but still offer the same service life at a cost saving.




The most popular way is face fixing deck screws into the deck boards and into the joists below. If a secret fix is required, deck clips can be used on low movement timbers (hardwoods & bamboo) by using deck clips. The clips are screwed into the joists below and the clips sit in side grooves within the deck boards to keep them in place. Our Gripsure Ecodek range is available in six colours and we provide screws with colour match deck screw heads to blend in with the boards and give a secret fix finish.

If budget allows, we would recommend stainless steel fixings for all components, especially for the deck boards themselves. However, for the sub structure, hot-dipped galvanised steel fixings can be used at a more economical price.





The joists can then be either supported by adjustable pedestals or fixed against timber posts which can either be supported by pre cast concrete pads, steel supports or concrete foundations.

The adjustable pedestals are available in a range of sizes to best suit the application. Pressure pads can also be provided so movement of the pedestals is reduced. The joists simply sit on top of the pedestals and fixed by a screw and if the surface is uneven, such as pre cast concrete for inset balconies or on top of old paving slabs, the pedestals can be adjusted to give the deck an even surface (a slight fall away from buildings should be incorporated).



Bearers (beams)

In some cases, joists are fixed to bearers (also known as beams) and then the bearers are fixed onto the posts. Two sets of bearers can be fixed to either side of the timber posts offering even greater strength. This will give a more solid sub structure but may not be suitable for all applications as this will add height to the deck. If there is limited space to meet a door line for example, this needs to be considered at the design stage.


Joist centres

Typically, the joist centres are spaced out between 400 and 600mm depending on the species and dimensions of the deck boards. We have the recommended joist spans for each of our products in the technical specification tab on each of our product pages which we have based on the TDCA’s recommended guidelines which were worked out by a structural engineer from TRADA for both domestic and commercial settings.

It is also worth noting that hollow composite boards require much shorter joist centres at around 300mm. Therefore, it is worth considering the cost of the whole deck system rather than just the deck boards as these boards can be a false economy.