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Step 1
Check with your city's building codes for decks before attempting to calculate the joist dimensions. It will likely have minimum joist dimensions for your new deck to meet building code requirements.
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Step 2
Evaluate the kind of wood you will be using. Each species or class of wood behaves differently and can bear different levels of loads under specific circumstances and environments. Softer woods like cedar may splinter easily, so you'll want to select cedar joists with slightly larger dimensions than a hardwood like ironwood.
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Step 3
Determine how far your joists will need to span in your deck design. If you have chosen to design your deck with wider gaps between beams then take this into consideration when you calculate the dimensions of your deck joists. Narrower pieces of lumber like a 1-by-6-foot board will not be able to span the same distance as a wider piece of lumber like a 2-by-6-foot board.
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Step 4
Consider the dimensions of your ledger board. Most professional deck consultants will match the dimensions of their ledger boards to their joists for the best structural integrity.
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Step 5
Refer to a deck-building calculation chart to determine your joist dimensions quickly and easily. James Dulley Deck Building Basics page has a few easy-to-read charts that even the most amateur of deck builders can understand (see Resources below).
