joining

Hi all, so im doing my flat garage roof and need to join the joists over the steel beam and just trying to find out how much overlap you need, is a total 600mm join ok ? so 300mm eitherway from where the arrow is on the picture Also can they be screwed together or do they need to be bolted with...

I'm going to be doing a few 14 m2 high artex ceilings next week and i'm looking for some clear detailed guides on how to do them in two halves, there is suprisingly little on the net about this, no youtube videos, no articles, just a few lacking mentions in forums. I've heard of the scrim...