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<\/p>\nJoists are one of the key components of a deck’s foundation and, together with the footings, posts and bearers, determine the safety, strength and longevity of a deck. I’ve seen the tragic results of deck failure, and too many times it is due to an incorrectly installed substructure. It’s critical to give careful attention to joist installation.<\/p>\n
Joists are the repeated structural horizontal framing components that run parallel to each other, sitting between the primary bearers and ledger boards. When combined with the blocking or bridging (short pieces between joists), they provide lateral stability to the framing.<\/p>\n
Because building a safe structure is your top priority, it is critical to heed joist installation requirements.<\/p>\n
Joists support the load above from the deck boards, railing, hardware, other accessories (pergolas, outdoor kitchens), and foot traffic. Correct joist spacing will prevent sagging and a springy feeling underfoot as you walk across the deck. If joists are spaced too far apart, boards will sag over time, adversely affecting the structural integrity as well as your safety and enjoyment of the deck.<\/p>\n
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Local building codes dictate joist requirements. Codes will have strict minimum standards to account for the weight the deck must hold.<\/p>\n
Joists are most often made from pressure-treated timber, the most affordable and readily available material. Alternatives include composite and aluminium. For joists made from timber, joist tape, such as Trex Protect<\/a>, can protect the tops of joists from water damage, further extending the life of your framing and entire deck.<\/p>\n A number of factors influence the correct spacing of joists. They are:<\/p>\n Joist spacing matters because it directly affects the structural integrity of your deck and safety. Incorrect joist spacing will lead to the sagging or cupping of deck boards above. Closer joist spacing reduces or eliminates the bouncy feeling underfoot.<\/p>\n Joist sizes are typically 2″x8″ ,2″x10″ or 2″x12″. The size chosen depends on the distance the joist needs to span to carry the load. The larger the span and the more weight it has to support, the larger the joists will be.<\/p>\n Joist spacing is the distance from the centre of one joist to the centre of the next. You will often see it written as 400 mm o.c. or 16 inches on centre. As previously pointed out, the correct spacing depends on a variety of factors.<\/p>\n For softwoods, spacing is generally 400-600 mm on centre, depending upon the size of the deck board. For heavier composites, spacing is usually 300-400 mm on centre.<\/p>\n Installing joists 300 mm or less may also be required for joists that carry heavier loads, have longer spans, or hold intricate deck patterns.<\/p>\n Stair stringers are typically installed 300 mm or less because of the added force from stepping on the treads.<\/p>\n If you are replacing timber decking with composite decking and find that the existing joists are spaced 400 mm on centre, adding one new joist between each of the existing joists is a simple fix.<\/p>\n This recommended deck joist spacing chart, based on TDCA Decking Handbook<\/a>, covers the most common decking scenarios in the UK and serves as a quick reference:<\/p>\nFactors That Determine Joist Spacing<\/h2>\n
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Why Joist Spacing Matters<\/h2>\n
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Other considerations include:<\/p>\n\n
Recommended Deck Joist Spacing<\/h2>\n
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