Fixing (at least temporarily) roof damaged wooden beam (rafter?) in an outbuilding
judging by the shape and cut of the wood, the method of assembly and the size of the members, i would assume you have a tile roof?  either way, it looks like it was built for a very heavy roof load.  

the rot you are seeing is just part of the problem, it most likely extends far into the timber and may even be present in the adjacent rafters.

bracing the beam (thats the transverse thing under the rafter) may not help much as it looks rotten as well. 

the best thing to do is put in a post.  this is a vertical column from ground to the bottom of the rafter.  put down a wide block of wood or a concrete slab first to spread the load over the paving stones below.  use a metal bracket at both ends and concrete anchors, lag bolts or nails to secure the post to the base and to the rafter.  if you use a good solid post and are careful in your work, it should last until you can replace the roof.