So, i should preface this by saying I've never watched Vikings, mostly because I like my television and I'm afraid I'd throw things through it if I did. :-) But I can speak generally as to the type of ships the Norse built (that we know of) and why they didn't have decks, which I think is what you mean when you say "roofs."

First off, the traditional ship we think of when we think of "Vikings" is most likely a longship, the most famous example of which is preserved at the Viking Ship Museum in Norway. The Oesberg ship has is a 15 "room" ship, meaning it has 15 pairs of oars, or "rooms" for 15 pairs of rowers (30 men in all). The Norse and early English ships seem to be divided that way into "rooms," from what we know from record-keeping; Alfred the Great seems to have ordered several ships of 20 rooms and there were some that were larger. In any case, the Oesberg ship is fairly well preserved and has served as a model for perceptions of Norse ships since then.

So, why didn't longships have "roofs" or shelters? The main reason is that they weren't really built for very long voyages, but rather for raiding or short sailing voyages that might last a few days or a few weeks. It's quite possible to be totally fine in an open boat for a limited amount of time at sea, and the "ships" really were more like boats, placing a premium on shallow draft so they could penetrate up rivers and creeks. They were also surprisingly small compared to modern ships, and were built using clinker construction, meaning that they did not have internal ribs or frames on which to hang decks (those were added later).

There were other Norse ship types that had decks, or at least a small half-deck that could shelter a bit of the crew or cargo. The knarr is the most famous type of this ship, and was used during the "Viking expansion" era to carry cargo or larger groups of people from place to place than the longship could. A knarr would have been the type of ship to take supplies to and from Norse colonies in Greenland, for example. The only surviving example of a knarr that we have is "Skuldelev 1," so called from being part of a group of ships that were found in Skuldelev, in modern-day Denmark. It's not a particularly large ship but seems to have had a half-deck at the front of the ship.