best method to shield exposed horizontal roof beams
The top of the beam is just part of the problem. The beam soaks up way more moisture from end grain exposure. Also, whatever you use to cover (protect) the beams, you need it to allow to dry out each time after a storm too. 

Where I live, our community college had a similar problem and a local architectural firm designed a stainless steel cap to protect the tops and ends of the beams. However, after about 3 years the beams had even more dryrot and the problem accelerated, because moisture blew in and became trapped between the beam and flashing. 

Even pressure treated material can rot. Good design does not included leaving wood structural members exposed in wet climates.