Plastic Joists

A decking joist is a horizontal structural member that supports the decking boards from beneath. Joists span between bearers or sit directly on a prepared base, and are fixed at regular intervals to provide a consistent, level surface for the decking boards to attach to.

The correct joist size depends on the span between bearers and the weight of the decking boards. BM Steel offers plastic decking joists in 125 x 50 x 3000mm and 50 x 50 x 2400mm. As a guide, joists should be spaced at 250–350mm centres depending on your decking manufacturer's recommendations.

Yes. Leave a gap of at least 8mm between joist ends at butt joints, and at least 15mm between joist ends and fixed structures such as walls. Recycled plastic will expand and contract slightly with temperature changes, so allowing for movement is essential.

Yes, but the ground should be levelled first or adjustable pedestal supports used to create a consistent base. Uneven ground can lead to deck instability or water pooling, so ensuring a level sub-base before joist installation is important.

Plastic joists perform well for ground-level and low-height decking. For elevated or load-bearing structures, consider combining them with galvanised steel or treated timber cross-supports for added structural strength.

Start by setting out the deck area and marking post positions. Space plastic posts at 1,000mm intervals and fix them to a third of their length into the ground. Attach bearers along the post network with a 10mm expansion gap. Lay joists onto the bearer network at 250–350mm spacings, fixing the outer joists first, then working inward, screwing diagonally through each joist into the bearer. Use a double joist wherever two decking board ends are expected to meet.