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What should I know about deck joists?

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Whilst you might have given a great deal of thought to your choice of deck boards and other elements such as balustrades, fixings and lighting, you might not have thought about deck joists.

Whilst these bits of timber cannot be seen once your deck is complete, they are perhaps more important than your deck boards! Deck joists give you the framework to attach your timber decking boards to and play a very important part in ascertaining the strength and durability of your decking. If you skimp on this part of the project you stand to compromise its service life, regardless of the quality of the deck boards. You can also create a safety issue and so it is important to get it right.


As from January 2020, the TDCA recommend that deck joists or any timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in ground contact or not. This is to ensure the enhanced durability and safety of the deck substructure and is in line with BS 8417 (Preservation of Wood: Code of Practice) which is to adopt this change in due course this year. See our blog article here. 


Having ensured the preservative treatment of your joists is Use Class 4, consider the size of joists that you need. Take into account the weight bearing that is required and whether the deck joists will sit directly on the ground or be raised. If you are using professional help, your installer will be able to specify the size of deck joists that you require. Alternatively, many of our TDCA approved suppliers will be able to offer you guidance on sight of your decking plan, helping you to specify the optimum timber for your project.

Service Life options

British / European Standards allow you to specify pressure treated softwoods with a 15, 30 or 60 years desired service life – achieved through material selection and installation – the industry default specification is 15 years.  If you want longer than this you will often need to specifically request it, the TDCA have produced a 60 year Code of Practice for balconies on new homes - see here.

If you have any specific queries about deck joists, please do get in touch with the TDCA and we will be happy to help you.