What size lumber do I need for deck joists spanning 12 feet in NB?

For a 12-foot joist span on a residential deck in New Brunswick, you need 2x10 pressure-treated lumber at 16 inches on centre, which safely handles spans up to 13 feet. A 2x8 joist will not work at this distance because its maximum recommended span tops out at 10 feet for standard residential deck loading, leaving you well short of the 12-foot requirement with no safety margin.

The span ratings for deck joists are based on the National Building Code of Canada's prescribed residential deck loads: 40 pounds per square foot of live load (people, furniture, snow) and 10 pounds per square foot of dead load (the weight of the structure itself), totalling 50 PSF combined. In New Brunswick, where winter snow loads on a deck surface can be substantial, particularly in northern communities like Bathurst and Edmundston where seasonal snowfall totals exceed 300 centimetres, that 40 PSF live load figure covers the worst-case scenario of heavy wet snow accumulation before you clear the deck.

The standard joist span table for SPF (spruce-pine-fir) lumber, which is what pressure-treated lumber in New Brunswick is made from, gives these maximum spans at 16 inches on centre: 2x6 joists span up to 8 feet, 2x8 joists span up to 10 feet, 2x10 joists span up to 13 feet, and 2x12 joists span up to 16 feet. Your 12-foot span falls squarely within the 2x10 range with a foot of margin. That margin matters because real-world conditions in New Brunswick introduce variables that the span table cannot fully account for, including the extra weight of wet snow, the concentrated loads from large planters or hot tubs, and the gradual weakening of wood fibres exposed to decades of Maritime humidity cycling.

You could technically tighten joist spacing to 12 inches on centre and use 2x8 joists for spans approaching 11 to 12 feet, but this is not recommended. You end up using more total lumber than the 2x10 at 16-inch option because the tighter spacing means more joists across the same deck width. On a 16-foot-wide deck, 16-inch spacing requires 13 joists while 12-inch spacing requires 17. Additionally, if you are installing composite decking from Trex, TimberTech, or Fiberon, the manufacturer warranty typically requires 12-inch on-centre joist spacing regardless, so starting with 2x10s gives you flexibility to meet those requirements.

For your 12-foot span, specify No. 2 grade or better SPF lumber with MCA pressure treatment rated UC3B for above-ground use. Each joist should sit in a properly rated joist hanger at the ledger board and beam connections. The standard for New Brunswick deck construction is the Simpson Strong-Tie LUS series at 18-gauge minimum steel thickness. These hangers are designed to carry the full load capacity of the joist they support and provide the mechanical connection that prevents joists from rotating or pulling free under load.

The connection at each end of your 12-foot joists is where the structure either succeeds or fails over time. At the house end, the joists typically attach to a ledger board that is itself lag-bolted to the house rim joist. At the outer end, they rest on a beam supported by posts on footings. In New Brunswick, those footings must reach the local frost depth: 1.2 metres in Moncton, Saint John, and Fredericton, extending to 1.5 metres in Bathurst and Edmundston. A beam supporting 2x10 joists spanning 12 feet on a standard-width deck should be built from doubled 2x10s or a single engineered beam rated for the load, supported by posts no more than 8 feet apart.

All fasteners connecting your joist system must be stainless steel or exterior-rated coated fasteners approved for use with MCA-treated lumber, as the copper in MCA treatment reacts with standard galvanized hardware over time.

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