The OP might want to fix a plate against a wall to support the high end of a lean-to roof.
Indeed he might or even at the low end for the ceiling joists.
When fixing any timber that is in excess of 50mm thick i would always recommend 4" 12's. Again, it makes sense to use screws initially so that they don't interferes with the joist or rafter settings, however in order to satisfy B.C. you must then pop a few sleeve anchor bolts between the rafters, just for show of course!