Plastic Joists

A decking joist is a horizontal structural member that forms the direct fixing surface for decking boards. Joists are laid across bearers at regular intervals, creating a level, load-bearing grid that ties the subframe together and provides consistent support across the full deck surface. 

The correct joist size depends on the span between bearers and the weight of the decking boards. IMS offers plastic decking joists in 125 x 50 x 3000mm and 50 x 50 x 2400mm. For residential builds, the 50 x 50mm profile is typically sufficient for standard spans, while the 125 x 50mm section is better suited to wider spans or commercial installations with higher load requirements. Always refer to your decking manufacturer's recommended joist centres before specifying. 

Yes. IMS recommends leaving a minimum of 8mm between joist ends at butt joints, and at least 15mm between joist ends and any fixed structure such as a wall or fascia board. Recycled polyethylene expands and contracts with ambient temperature, so adequate movement allowance is essential to prevent buckling or joint stress over time. 

Yes, though the supporting base should be levelled before installation. Adjustable pedestal supports are a practical solution where ground conditions are uneven, as they allow precise height adjustment without groundworks. A consistent sub-base prevents water pooling beneath the deck and ensures the joist network sits true. 

IMS plastic joists are well suited to ground-level and low-rise decking applications. For elevated or heavily loaded structures, we recommend consulting our technical sales team to discuss whether additional structural support, such as galvanised steel cross-members, is required alongside the plastic subframe system. 

Mark out the deck area and establish a post grid at 1,000mm intervals. Once posts are set and bearers are fixed with a 10mm expansion gap, lay joists across the bearer network at 250 to 350mm centres. Fix outer joists first, then work inward, driving fixings diagonally through each joist into the bearer below. Where two decking board ends will meet, install a double joist to provide a full bearing surface for both boards.