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2.9m Composite Joist 40x30mm
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Composite Decking Calculator
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Product Description
Our composite joist is a 2900 × 40 × 30 mm black, two-chamber composite profile. It is the joist for composite decking laid on a concrete base, and the batten behind our composite wall and slatted cladding. Sold per length and stocked at our yard in Didcot, Oxfordshire.
- £10.00 per length inc. VAT (October 2026)
- 2900 × 40 × 30 mm, two-chamber composite, black
- Decking: 400 mm centres (300 mm for commercial or diagonal laying)
- Cladding: battens at 400 mm centres maximum
For decking: on a concrete base only
Composite joists are laid on a flat, stable concrete base with a waterproofing layer, levelled with packers and fixed down, so they are supported along their whole length. They must not span between pedestals and are not laid on soil — for a deck on adjustable pedestals or on concrete footings, use aluminium joists. Board ends meet over a double joist.
On a new frame at 400 mm centres, allow about 2.5 linear metres of joist per m² of deck (about 3.3 m at 300 mm centres). A 20 m² deck at 400 mm centres needs about 50 linear metres — 18 lengths.
For cladding: the batten frame
Fix battens to the wall across the board direction at 400 mm centres maximum, plus one each side of every opening and corner, keeping at least 25 mm of ventilated air gap behind the boards. For vertical cladding, fix horizontal battens over vertical counter-battens so air and water can drain. Board ends meet over a double batten.
Technical specification
| Product code | OCW-DKJST-290-STD |
|---|---|
| Size | 2900 × 40 × 30 mm |
| Profile | Two-chamber composite, black |
| Decking centres | 400 mm residential · 300 mm commercial or diagonal |
| Decking support | Full length, on a concrete base |
| Cladding centres | 400 mm maximum |
Questions
Can I put composite joists on pedestals or on the ground?
No. Composite joists need full-length support on a concrete base. For pedestals or concrete footings, use aluminium joists.
How many joists do I need for my deck?
At 400 mm centres allow about 2.5 linear metres per m² of deck, plus double joists where board ends meet. Send us your deck size on WhatsApp (07709 913846) for a full kit count.
Can I use the same profile behind cladding?
Yes — it is the batten specified for our Oxford Heritage 3D™ Wall Cladding and Slatted Cladding, at 400 mm centres maximum.
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