What Size Are Decking Joists?

When planning a decking area, there’s a lot of thought about those top boards, the ones you see.  But the most essential part of the construction is timber decking joists.  These things are under the boards, out of sight and critical for the strength and longevity of your decking area.

The confusing part is that there are various sizes and types available.  So, what size are decking joists, and how do you know which ones you need?

We supply a range of timber joist sizes, including 4x2, 6x2, and 8x2 Inch. 

The most suitable wood for the job

Before you start looking at the size of the wood, it is essential to check you have the right product for the job.  You need treated timber joists for decking because these are treated to make sure they are up to the job.

For most decking areas, the C16 standard will be sufficient for the job.  Softwoods with a strength class rating of C16 are the minimum for decks above 600mm in height and comply with the building regulations.

If you have a higher strength requirement due to smaller component sections, longer spans or a commercial decking standard, then quality Treated C24 Joists will be needed.

Timber moisture content

The other part of picking the timber decking joists is to look at the timber moisture content.  Because wood is a natural material, so there’s always some water.  It is natural and a good thing.

For example, wood outdoors in the UK typically has a water content of around 19% in winter and as low as 12% in summer.  But this can cause cracking and warping.  So, with pressure-treated wood, it is best to get a moisture content of around 16%, give or take 3%.

Also, if you have a decking layout that means the wood is close to the ground, usually around 150mm or so, then you will need to ensure plenty of drainage under the decking so the water doesn’t build up and cause problems.

What size are decking joists?

Once you have wood to the right standard and moisture content, you can find what size you need and purchase your decking joists.

The key to working out the needed size joist is to understand the deck board span.  This is the gap between joists and is usually decided by the length of the decking board.

For example, for a maximum span of 2 metres, 400mm joist centres would need to have a joist section that’s 45mm wide by 97mm deep.  Similarly, a 4-metre joist span with 400mm joist centres would need a joist section that was 45mm wide by 220mm deep.

If you plan to use double-member beam spans, then a 1.99-metre clear beam span would need beams that are 45mm x 122mm (2) with a beam centre of 1.2mm.

Getting the right joists

Choosing the right decking joists is one of the most complex parts of the decking equation and also one of the most important.  That’s why getting a little help can be a good idea we have a composite decking calculator that many people find useful

Armed with your measurements and an idea of how you want the decking to look, you can buy your timber decking joists and your timber decking or Trex decking at Armstrong Cheshire.