Dotted or dashed, whichever. Indeed, you’d play it the same as a regular tie, but supposedly it makes more sense visually since a roll is made up of many notes. On the other hand it’s more work for a copyist. If you haven’t even seen them, that’s info for me.
More discussion of notation for those interested …
I mentioned in another topic a couple months ago (regarding strings) that I had worked with a composer who insisted on dashed ties. She held up Bartók’s scores as the model, and regarded it as standard modern notational practice. Having much more experience reading older scores, I doubted its usefulness, especially for multiple bars.
Percussion rolls traditionally were notated like trills, at first with a new tr on each bar, then later with a wavy line continuing over barlines as appropriate, which clarifies the continuous nature. Where a rearticulation was desired, a new tr~ made it clear. Some time in the 19th century composers started using tremolo slashes (presumably borrowed from string writing) instead.
IMO ties are usually not needed for string section players, as the idea of making a slight break in a tremolo all together is impracticable. (But see the first page of Bartók Concerto for Orchestra for a famous example of how ties on tremolos can clarify the rhythm.)