How to Determine Joist Size

• To determine Joist size and spacing we need to know
  • The load - If this is for residential then a table in the building code can apply to the joist size design.
  • type of wood - In Ottawa the most common is #1 Spruce
  • the span

• the type of bridging and strapping

• Refer to the joist span tables in the building code to determine the size of joist required.
  • Floor Joist Size Table from NRC

   

• Optimizing The Design
  • Larger sizes of wood and larger lengths of wood are much more expensive because large trees are more rare. In the examples below we can see that although a 2x12 contains about 21% mor wood than a 2x10 it is in fact about 59% more expensive. For this reason many designs are optimized to use the smallest joist size allowable to meet the building code. It may be in your interest to calculate the actual span exactly especially if a few inches could make the difference between a joist size. Sometimes the tables give us different options and the most economical can be chosen. (Personally I am not in favor of 24" oc option since a thicker flooring thickness is often required which can increase cost. I have not seen any construction which chooses this option.)