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Joists for Decking. The right timber and Size for your deck joists.



Selecting the right timber joists for your decking is absolutely top priority! Whether your decking project is to be a raised deck or a flat-on-the-ground- deck, the right joists will determine the success or otherwise of your decking.

Decking Joists Size - for decking frame construction

The standard size joists for decking is 150x50mm (6x2in) with lengths up to 6.2metres (20ft). As well as the dimensions of the decking joist, the quality also has to be taken into account.

The normal 'standard' for such construction quality joists, are in grades C16 and C20. C20 being the highest grade, though C16 is quite adequate for domestic decking - raised or flat. 

If your project is to be built using composite deck boards, you should access the manufacturer's design specification sheet to ensure that your deck conforms to the different joist sizes and spans used in construction of composite decks.

A change in direction with the decking joists to allow the chevron effect of the deck going around the corner on the large low level deckGround level deck using 6x2 decking joists

All softwood decking timbers should be Tanalith treated (Tanalised) The decking joists are no exception. The Tanalised timbers - pressure treated - give a life span well in excess of 15 years Under no circumstances should non-treated timber be used. Get quality construction timbers from a reputable builders/timber merchants.

The should be no compromise on decking joist size just because it is a ground level deck. Together with this you will still need decking joist supports. You should not simply lay the deck frame on the ground without suitable supports. Decking frame supports will play an important part in the stability of the deck - taking into account lifting is possible in strong wind and also because of ground shrinkage and heaving - particularly on clay soils - but also because of natural settlement over time.

Do NOT use ready notched DIY joists for raised decks. Decking Frame construction is one of the most important aspects of building a satisfactory deck. Get the decking frame construction part right - the rest falls into place.

Joists may have a few slight splits (Shakes) in them. This is not normally a problem - virtually all timber will split when used outdoors. (There are treatments available to minimise this). 

Sturdy 6x2 joists are the minimum for a raised deck. Even this slightly raised deck needs the larger joists sizePart  Raised deck using 6x2 decking joists.

Try to ensure that the joists for decking are as straight as possible. Slight curves are acceptable and unavoidable. Do not expect to be able to sort through a stack of construction timber at your timber Merchants!  Slight bends and distortions in joists for decking, are run of the mill for professional carpenters - you just have to make adjustments to your construction techniques.

Joist for decking packs often include C16 and C20 qualities in the same pack. (This does not refer to the decking joist size but to the quality. Unless you specify - and pay for -  C20 timbers, you will receive the decking joist timbers as they come out of the pack. C16 joists are quite adequate for decking purposes. The construction of the deck sub base is the important factor. Decking joist supports are clearly shown in the images on the page.

In some cases on ground-level decks, it is virtually impossible to use 6x2 joists and you may have to opt for 4x2 - or worse - 3x2! There are ways of constructing low level decks that have to use such joists to ensure a satisfactory outcome. These are discussed in our sub frame construction section.

There are 'Decking Joist Span tables' that tell you how often any particular joist has to be supported in various ways. Do not exceed the recommended span width.





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