Not sure if 'roof' is the proper term when talking about boats/ships.
When you google "longships", 9/10 images show longships without roofs. Were the Vikings really so though that they skipped creating any form of protection from big waves, wind, and rain on their ships?
Longship is one of those terms that has become genericized out of all meaning. That said, the surviving examples of longships (warships, esneccas) that we have found were not decked, but there were other Viking craft that had a small half-deck (the knarr, for example). The lack of decking, of course, doesn't preclude tents, awnings or other temporary cover.