Decks.com. Deck Joist Sizing and SpacingBefore building a composite deck always read the installation instructions from the manufacturer paying special attention to the required joist spacing. Most composite decking materials require 16" on center spacing for straight decking and 12" on center joist spacing for 45 degree angle diagonal decking. Also many composite decking materials require 12" or even 10" on center stair stringers spacing to support composite stair treads. If you are reinstalling composite decking over an ...