How to Determine Joist Size for Decks

The frame is the most important part of the deck. It bears all the weight of whatever is placed on top and must be built with care to hold up to predicted loads. The sizing and spacing of joists depends on the dimensions of the deck and the predicted tolerance necessary. For example, if a hot tub will be put on the deck, the joists will need to be more substantial.

A deck adds value to a home.

Step 1

Measure the available space using the tape measure.  Decide on the the height of the deck, the length and width of the deck.

Consider what type of wood you'd like to use; different woods have different hardnesses and strengths. 

Step 2

Decide on the tolerance needs you have for your deck.  Will you often be throwing large parties that will take place on the deck? Will you have a hot tub or a grill on the deck? Larger joists will be needed if the deck is to withstand these higher pressures.

Step 3

Determine joist sizes based on the decisions made in steps one and two.  The dimensions of the joist typically are: 2 by 6, 2 by 8, 2 by 10 or 2 by 12.

2 by 10 is the most common, general-purpose size.  For a smaller simpler deck, like a ground-level 8-foot-by-10-foot deck, 2-by-6 joists every 16 inches is an acceptable design.

For a tall, long and wide deck that will be supporting a lot of weight and traffic, 2-by-10 or even 2-by-12 joists mounted at 12 inches on center is a preferable option. 

Things You Will Need

  • Tape measure


  • If you're unsure about the dimensions of your deck, contact a professional carpenter who is familiar with building codes.

About the Author

Andrew Ford is a journalist based in Florida. He has contributed to newspapers such as the "Tampa Tribune," "St. Petersburg Times" and "North Florida Herald." Ford is completing his bachelor's degree in journalism from the University of Florida.

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