Joists for composite decking

Independent comparison. Teosad may earn a commission from some retailer links; this never changes the order or the numbers. Disclosure

What are decking joists?

Joists are the beams across the frame that the boards fix to. They run at right angles to the boards, rest on bearers or supports, and set how firm the deck feels. Composite boards are less stiff than many people expect, so makers require closer joists than timber boards.

How far apart should composite decking joists be?

EnviroBuild gives 300 mm as the standard maximum for composite. Ecoscape says to reduce centres for hot tubs and heavy furniture. A UK builder building for composite uses 400 mm centres, and another installer says his joists should be no more than 350 mm. A US installer who likes a firmer deck uses 305 mm and goes closer for diagonal boards. These are installers’ choices, not requirements. Only the maker’s document counts for the warranty.

What joists can you use?

Treated timber is common. UK builders on video use graded structural timber (C16 and C24 are mentioned), noggins, and preservative on every cut end. Joist tape protects the tops.

Prices are VAT-inclusive where the seller shows it, dated 30 October 2026. Some warranties tie to the subframe: Cladco’s Signature board warranty requires its recycled-plastic subframe. Plastic and aluminium joists don’t rot, so joist tape is for timber.

How long can a joist span?

That is a separate question from spacing. Spacing is set by the board. Span, the distance between supports, depends on the joist’s size, material and load. Millboard’s DuoSpan page gives no span figures, and aluminium and plastic joists come with their own tables. For timber, a builder should size joists to the load and support spacing, and our frame guide explains the layout.

Where do the joists sit?

On bearers, posts or pedestals, with a gap to the ground. See the frames guide for clearances and the supports guide for pedestals and posts.