A roll-top dry bag has no zipper and no valve, which is exactly why it works. The seal is made by the material folding against itself under tension. Get the fold wrong and the best bag in the world leaks.
The three-fold rule
Squeeze the opening flat, align the two sides, then roll down at least three full turns before clipping the buckle. One or two rolls will not hold a seal under pressure. Three or more is what turns a tube of fabric into a closed vessel.
Leave some air in
Counter-intuitively, do not squeeze all the air out. A cushion of trapped air makes the bag float if it goes overboard and helps the roll hold its shape. A fully evacuated dry bag sinks, which rather defeats the purpose.
Submersion is not the design case
Roll-tops are splash-proof and rain-proof, and they will survive a brief dunk. They are not dive bags. If your phone absolutely must survive being underwater, it needs a hard case, not a roll-top.
Care
- Rinse salt off the closure strip — grit there is what breaks seals
- Store it unrolled and dry so the fold line does not set
- Check the buckle before the trip, not at the ramp
Ours is a 30L roll-top, which is the size that fits a day's gear for two people without becoming awkward to carry.
