How to Determine Joist Size for Decks

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.

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.

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.