Now the death rolla is and always has been an AP- weapon. I ask you: What is the point of a cover save against a death roller? This question must have been asked by a Nid player who wants his gaunts to get get a better save.
I would have to say that the cover save would have to be dependent on the cover the target unit is in. If a unit of gaunts is in the open (most likely scenario) then there is no cover to get a save from and they would have to rely on their normal armor save. If the gaunts are in a ruin, then they could scatter and hide, so the cover save from hiding in ruins would be better than their normal save.
As for vehicles getting a cover save, if your vehicle can somehow put cover between it and the rolla, then maybe it could be argued that the tank took less damage from the rolla plus having a wall falling on it.
My personal opinion should be that additional hits should be incurred as there is now a building falling on your tank as well as a death roller ramming it. So it should be D6+1 per floor/story of the ruins/building that the target tank was trying to hide in/behind. Add in an AP rating as well, AP3 for a one story building, AP2 for a two story building, and AP1 for three or more stories.
Of course, you could get a cover save as there might be a lucky I-beam that lands next to your tank and absorbs all the impact before it can crush your tank into a frisbee.