Joists for decking. Select the right timber for your decking 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.

The standard size used by Top Deck 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.

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 deck
Ground level deck using 6x2 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.

Do NOT use ready notched DIY joists for raised decks.

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 size
Part Raised deck using 6x2 joists.

Try to ensure that the joists 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 are run of the mill for professional carpenters - you just have to make adjustments to your construction techniques.

Joist pack often include C16 and C20 qualities in the same pack. Unless you specify - and pay for ) C20 timbers, you will receive the timbers as they come out of the pack. C16 are quite adequate for decking purposes. The construction of the deck sub base is the important factor.

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.