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Decking Joists
Decking joists are the beams your deck boards are fixed to, and they decide how firm and long lasting the deck will be. We stock PuraFix composite joists in recycled plastic from 40×30 mm to 100×100 mm, and timber joists in Douglas fir, oak, planed hardwood, MicroShades and Ecowood in longer lengths and deeper sections. Choose composite for no maintenance or timber for length, stocked in our warehouse and delivered across Ireland.
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Decking joists for composite and timber decks
Decking joists are the beams the boards are fixed to, so they set how flat, how firm and how long lasting a deck will be. We stock two kinds of decking joist: PuraFix composite joists made from recycled plastic, and timber joists in Douglas fir, oak, planed hardwood, MicroShades and Ecowood. Choose the material first, then the section and length your frame needs.
PuraFix composite joists
PuraFix composite joists are made from 100% recycled plastic and are fully recyclable. They will never rot or need treating, so the frame under your deck needs no maintenance for as long as the boards are down. They are the natural partner for composite decking. The 100×50 mm section is the everyday composite decking joist, the composite version of a 4×2. For deeper frames there are the heaviest sections, 150×50 mm and 100×100 mm, and the range also includes a compact 50×50 mm square section and a lighter hollow 40×30 mm joist.
Because of the recycling process, small metal fragments can end up inside the material, so cut plastic joists with a tungsten carbide tipped saw blade rather than a standard one.
Timber decking joists
Timber joists give you longer lengths and deeper sections, and they suit timber decks and frames built the traditional way. Each species has its own page with every size, and all of them are on our timber joists page:
- Douglas fir joists: sawn sections from 70×50 mm to 200×70 mm in one of the strongest softwoods, for everyday and heavy frames alike.
- Oak joists: sawn 150×70 mm and 200×70 mm heavy joists, Durability Class 2, for deep, strong frames.
- Hardwood joists: planed 190×45 mm, 90×45 mm and 65×42 mm sections in a dense hardwood, for long lasting frames.
- MicroShades joists: treated softwood at 100×44 mm and 150×44 mm, supplied with the protective finish already applied.
- Ecowood joists: kiln dried softwood at 95×45 mm, with consistent quality from piece to piece.
Choose the length on each product page; longer joists mean fewer joins in the frame on long runs.
Composite or timber decking joists?
Composite joists are the low maintenance choice: nothing to treat, nothing to rot, and the right match for a composite deck. Timber joists win on length and depth. Species below Durability Class 1 last longest when you seal cut ends and notches as you build, and on MicroShades a touch up sealer on cut ends keeps the factory treatment continuous. Most customers keep a tin from our oils and stains range on hand while cutting.
How many decking joists do I need?
Our board calculator allows about 3 m of joist for every square metre of composite decking, which puts joists at roughly 330 mm centres, inside the 350 mm or closer that our Pura composite boards need. For timber decking it allows about 2.4 m per square metre, which is joists at around 400 mm centres, the maximum for residential timber decks; use 300 mm centres or less for heavier loads. Divide the total by the joist length you choose on the product page and round up. The calculator on each board page works out the boards, and the cart suggests the matching joists and fixings.
Decking joists in Dublin and across Ireland
Every joist is stocked in our Dublin warehouse, delivered across Ireland with your boards. On a slab or patio, lift and level the frame on decking pedestals. For pedestals, joists and everything else under the boards, see all our decking substructures.
Frequently Asked Questions About Decking Joists
What centres should decking joists be at?
For timber decking, our joist guidance is a maximum of 400 mm centres on residential decks and 300 mm or less for heavier loads. Our Pura composite boards need joists at 350 mm centres or closer.
How many joists do I need?
Our board calculator allows about 3 metres of joist for every square metre of composite decking, which puts joists at roughly 330 mm centres. For timber decking it allows about 2.4 metres per square metre, which is joists at around 400 mm centres. Divide the total by the joist length you choose on the product page and round up.
What sizes do PuraFix composite joists come in?
PuraFix composite joists come in 50×50 mm, 100×50 mm, 150×50 mm and 100×100 mm, plus a lighter hollow 40×30 mm joist. The 100×50 mm section is the everyday choice under a composite deck.
How do I cut recycled plastic joists?
Use a tungsten carbide tipped saw blade. Because of the recycling process, small metal fragments can end up inside the material, and a standard blade will dull quickly.
Which timber joists do you stock?
Sawn Douglas Fir, sawn Oak heavy joists, planed hardwood joists, Ecowood 95×45 mm and factory-finished MicroShades joists in 100×44 mm and 150×44 mm. Each product page shows the section and length.
Do I need to treat cut timber joists?
Yes, for species below Durability Class 1, seal cuts and notches with an end-grain preservative. For MicroShades joists, reseal cut ends with a touch-up sealer to keep the factory treatment continuous.