The roll-seal drybag has been around for a long, long time; at least since the second world war when it was used by soldiers to protect kit from the elements. But something so familiar is quite often used in a way which means it won’t be sealed properly and you end up with damp gear.
The secret is to fold, not roll the seal!
The waterproofing in a drybag comes from creating a series of ever-tighter folds in the material of a bag, wrapping them over each other so that by the time you’ve folded 4 or 5 times, the first fold is under great pressure.
If you just roll the bag down casually, you won’t get those folds, and you’ll allow water to seep up through the bag and in to your gear.
When you have a lightweight bag, folding material is easy, but always look for a lightweight bag that has a stiffened opening so you get a nice crisp fold. You can see the stiffener used on a very lightweight bag in the video at the bottom of this page.
But when you have a heavyweight bag, the problem is slightly different – you want a material that’s not too thick to fold easily. A good heavyweight drybag will also have a stiffener at the opening to help that first all-important crease. British brand Aquapac have an example here.