The simple design of a roll top bag is one of the most reliable ways of keeping your kit dry, which is why they're a common site on the decks of dive boats the world over. Apidura have used this roll closure design for their fork bags, saddle pack and their bar bag too.
With just one hole at the top of the bag that is sealed by rolling the top over on itself before clipping it together with a quick release buckle, there is no way for water from above or below to get in.
