Source: Wikipedia


Rolls are notated as tremolos, with slashes through the note stem. Each slash indicates dividing the note in two.

And for longer notes


More info available in the notation guide. Also, there is a difference in how the rolls are played depending on the tie between the notes:

head other  
is played as
is played as

in terms of the roll length (not the number of notes comprising the roll itself).

Videos: buzz roll (link).