Our bathroom vent pipe which extends through the ceiling, through the unheated attic space and through the roof has been leaking after a shower. The is a bend in the pvc pipe and it is leaking at the joint to the 90 degree bend. Since it is difficult to access due to the slope of the roof, we are wondering what se can seal it with on the outside of the pipe to avoid the leak onto the drywall ceiling.

Any suggestions?

Yes you can. Dry the joint off with a dry cloth and a hair dryer. Paint primer all around the joint and while the primer's still wet slop on PVC Cement around the joint getting it into all the cracks and openings. Good luck, Tom