I am planning to insulate my unfinished garage, which leads to adding a ceiling. There are 4 2x6 joists spacing around 50”. How do I add more joists to support drywall and fiberglass rolls? Location north California. 20 feet span seems too long even for none storage ceiling. Do I need to add a beam from back wall to garage door? joists hang on beam. The other ways is to hang to roof joists? The problem is the roof is not flat, contains many planes. I updated pictures. Hopefully it could show the structure of the roof. Additional question, how do I ceiling the flue pipe area? The huge duct from coil also block one joist.