The spacing between deck joists is commonly referred to as joist span in professional lingo. It doesn't matter what you call it -- it's still a critical calculation when it comes to building your new deck. Though it may seem complicated to the first-time deck builder, the way to calculate the spacing on deck joists is based on common sense and a knowledge of your materials.
Determine what kind of wood you will be using for your joists. Different woods expand and contract at different rates and will vary in softness, which will affect how much weight the wood can bear before beginning to sag. Softer woods like cedar will give more than Brazilian Ipê or ironwood, which are harder and less flexible.
Determine the size of the timber you will be using for the joists. Longer, thinner joists will need to be set closer together for optimal structural integrity, while shorter, wider joists can be set further apart.
Measure the length of the floorboards you will be using. Most decks are built with long, consistent pieces of lumber for the deck flooring. However, some patterned floors require short, angled pieces of wood to create intriguing designs. The designs that use shorter floorboards will require the joists to be set closer together to accommodate these patterns.
Take into consideration the climate where you will be building your deck. Not only will wood warp or crack under less than optimal conditions for their species, but if you live in an area that gets heavy snowfall each winter, then you must also take into account the overall dead weight that the deck must hold, sometimes for months at a time. In cooler climates, keep the joist distances smaller than you would in warmer climates.
Consult a deck joist span table to determine the best spacing. You can find tables and calculators online at sites like Ace Hardware and Best Deck Site (see Resources). You can also check with the local planning department since it will be able to help make sure that the joist spans you calculate fall within its regulations.
Always calculate your timber lengths based on your actual measurements, plus at least 10 percent extra for waste.