• A deck needs strong support underneath to prevent it from bouncing or being damaged.deck image by Albert Lozano from Fotolia.com
  
  The type of joists that are best for your deck depends on your priorities. Each material has its own strengths and weaknesses regarding sustainability, durability, cost, and appearance. The size of the joists can be reduced, if desired, by placing the posts that support them closer together. Conversely, you can build a deck with fewer posts by using larger and stronger joists.
  
  

Cedar

• Cedar is a beautiful wood that is more resistant to rot than most wood. Since the joists of a deck are really not visible, cedar's resistance to rot is more of a factor than its appearance. Cedar is substantially more expensive than other wood; it's really only preferable for someone looking for a top quality deck and has plenty of money to spend on it. Cedar makes more sense in areas where it is highly available, such as the west coast of Canada, Washington, and Oregon.
  
  

Pressure Treated Lumber

• Pressure treated lumber is wood that has been impregnated with chemicals to make it rot resistant. Some people believe the chemicals in this lumber are unhealthy, and avoid it. The treatment gives the wood a slightly greenish tinge, which some people find unattractive. For very wet environments, pressure treated lumber is preferred by many people, because many other types of wood tend to rot quite quickly. Some decks are made using pressure treated lumber for the posts and joists, and cedar or other more attractive wood for the decking and visible parts.
  
  

Logs

• If you are interested in minimizing your costs, and you have access to a woodlot, raw logs straight from the forest can be used for joists. It takes a certain skill to make them even enough that you have a functional deck, but the savings are substantial. Logs should be cut in the summer when the sap is running, as the bark is far easier to strip off than if they are cut in winter. Removing the bark decreases the speed of rot and deprives wood boring insects of habitat. Logs that are straight enough to be used as joists are most commonly found in young, crowded, even-aged stands of softwood. When the trees are crowded closely together, they all reach for the light above and thus tend to grow very straight and tall.