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Deck Joist Span Calculator

When it comes to the anatomy of a deck, joists are part of a deck’s framing system. Each joist is one of a series of long, horizontal structures that support the decking itself. While beams are vertical, joists lay horizontally on top of each beam to provide a sturdy, level surface to lay your decking materials across. Joists can be made from wood or steel.

As a critical part to a deck that will last you for years to come, it’s important to find out just how many joists you’ll need to safely support your decking -- and that depends on the size of your deck and approved building permit plans. As a rule, the larger the deck, the larger the joists. For example, joists spaced 16 inches from the center of the joist next to it can span 1.5 times in feet the depth of their inches. To learn more about deck joist sizing, spacing, and allowable span, reference this chart.

Ready to start calculating how much lumber you’ll need for your joists? Use the joist span and spacing calculator below to plug in your desired type of wood, the size of your joists, and the required spacing between each joist.

Choosing Wood for Your Joists

The area where you live plays an important role in choosing wood for your joists. Not every species of wood is available in every region. For instance, California Redwood, Hemlock, or Douglas Fir are not available in the Southeast. Similarly, Southern Yellow Pine is not available on the West coast.

There are also strength differences between Redwood, Fir, Pine, and other types of wood. Check your local building codes for find out which species of wood are available in your market and -- most importantly -- check into the strength and spanning capabilities of a species.

You can order a specific type of wood from the supplier, but shipping costs can increase the amount you will spend on lumber for joists and framing.

Even if you like the look of Redwood, it may not be the strongest material available to you. Treated Fir may be a better solution. You can always stain a particular type of wood to match your aesthetic. It’s the strength and span that counts!

Steel Alternatives to Wood Joists

If you’re building a deck, you’re not solely limited to wood to create joists and framing. Steel joists offer a stronger, straighter, and potentially safer alternative to wood. Even when using the strongest wood to build joists for a deck, that wood is still subject to the elements and can bend, bow, or warp with time.

Steel joists, such as Trex® Elevations®, are machine-made, making each joist an identical twin. Similarly, steel will not warp over time, allowing your decking materials to lay flat over your joists.

Another advantage to steel joists is that they’re non-combustible. If you live an an area prone to wildfires or want to be extra cautious when BBQ-ing outside, steel joists and framing leave the structure of your deck intact in the event of a fire.

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