Roll top bags seem similar to dry bags I get for kayaking. However, I noticed the last dry bag I bought says it's not meant to be waterproof... which makes me wonder! Anyway, as long as it's not submerged, it would be fine.
I have a full set of Carradice panniers (Super-C rears, smaller fronts) and they are made of duck canvas. Not completely waterproof but waterproof enough. I put anything important in a ziplock bag and that works fine.
Be careful with the Madden bags from ACA, I dislike how they attach via one hook in the center. It's doesn't seem as stable for off road tours as my Carradices are and they are incompatible with certain racks, like my OMM front rack. I had to ship the panniers back to ACA when I bought the rack and bags from them. The last I heard, Madden was going to redesign them but I have no idea if that ever happened or even if ACA actually put a note saying that compatibility was an issue.