you don't need temporary support, you probably need permanent support. These ceiling joists are probably more accurately called collar ties. They may well be helping to hold the sides of the house together. Most houses have the rafters coming down to the sidewalls. If the collar ties weren't there then the roof would want to fall down and the bottom of the rafters would push the top of the walls away. There are ways around needing collar ties. You can have a structural ridge beam that is posted up on either end, with footings too, that holds up the ridge so it won't fall. Or, usually you can get away with a collar tie spaced only every 4 feet or so, instead of the current 16" on center - Only your engineer knows for sure... I don't think your insurance company would be happey to pay for your house falling down because you didn't get the plans and permit done right (worst case scenario).