If you want your bag to be properly waterproof then a roll-top is the way to go, and the Altura Vortex 25 is a solid example of the genre. It has a decent harness and it's pretty comfortable on the bike too.
Made from heavy duty 420 denier waterproof fabric, with fully taped seams, the Vortex features a roll-top closure that buckles at either side to keep things snug and dry.
Like most roll-tops it can be folded over a couple of extra times to pack the bag down a bit if there's not much in it; to guarantee that water won't get in you need to make sure you're folding it over at least twice. Once it's shut you won't be getting any water inside.
I've done hours in the pouring rain followed by a hose-down to get the mud off the bag, with no ingress at all. You could probably take it canoeing and do eskimo rolls and stuff, if that's your thing. So, waterproofing: tick.
Inside there's basically one big 25-litre compartment that's big enough to swallow a change of clothes, your lunch, some tools and more with room to spare. At the back there's a pocket that's referred to as a laptop sleeve, and if you have a small laptop or a tablet then it'll fit fine. I can just squeeze my 15in Macbook into it, although it pokes out of the top a fair way. The sleeve is held in place by Velcro strips so you can take it out if you don't need it.
The main issue with any roll-top bag tends to be finding stuff – because you can't unzip it you always end up rooting around in the bottom, or tipping everything out. One solution to that is to put things in a plastic bag you can pull out if you need to. Generally I've only got a change of clothes, a tool pack and maybe my laptop in my backpack, so finding the right thing isn't too much of a chore; if you're the kind of person who can live out of their rucksack for weeks at a time then you'll probably want something with a bit more internal organisation.
There's a waterproof pocket on the outside that you can put a phone in, and a mesh pocket to hold a lock or your gloves, or other small stuff. And you get helmet hooks for attaching your lid to the back when you're not wearing it. If you're tucking down a bumpy hill then it's possible to lose stuff out of the mesh pocket, so don't put anything important in there. Like, for example, your keys. Just saying.
The harness is well-padded and comfy, with a sternum strap to keep it secure on the bike and a removable waist strap too. The back pads have air channels to try to keep you cool, and they do a decent job; the back's not the airiest I've used but it's not bad, and it's not uncomfortable against your back. The top of the bag sits low enough that it doesn't tend to interfere with your helmet, unless you've filled it really full.
Overall the Vortex 25 is a likeable bag that's very well sealed against the weather. If that's your main concern then it's certainly one to consider.
Very waterproof backpack that's big enough for a fully packed commute
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road.cc test report
Make and model: Altura Vortex 25 Waterproof Backpack
Size tested: 520 x 190 x 290mm, Grey
Tell us what the product is for, and who it's aimed at. What do the manufacturers say about it? How does that compare to your own feelings about it?
Altura says: "A fully seam welded backpack pack to keep your gear dry and protected for the allweather commuter."
Tell us some more about the technical aspects of the product?
* Welded seam construction
* Large main compartment
* Protective laptop sleeve (removable) fits 13' laptop
* External U-Lock pocket
* Waterproof front pocket
* Helmet attachment points
* Chest strap
* Removable waist belt
* Strategically located retroreflective trims for increased visibility
Fabric: 420D TPU coated Nylon
Dimensions: 520 x 190 x 290mm
Tell us how the product performed overall when used for its designed purpose
It's a solidly built backpack that's completely waterproof.
Tell us what you particularly liked about the product
Great weatherproofing, good external fittings.
Tell us what you particularly disliked about the product
Laptop sleeve too small, no internal organisation.
Did you enjoy using the product? Yes
Would you consider buying the product? Maybe
Would you recommend the product to a friend? Maybe
Use this box to explain your score
Depends on your use case really. If you're looking for proper waterproofing it's a very good option; for more general use it's decent too, but a zipped bag would probably be better. Not cheap, but nicely built.
About the tester
I usually ride: whatever I'm testing... My best bike is: Kinesis Tripster ATR, Kinesis Aithein
I've been riding for: Over 20 years I ride: Every day I would class myself as: Experienced
I regularly do the following types of riding: road racing, time trialling, cyclo-cross, commuting, club rides, sportives, general fitness riding, fixed/singlespeed, mountain biking, Mountain Bike Bog Snorkelling, track
Dave is a founding father of road.cc and responsible for kicking the server when it breaks. In a previous life he was a graphic designer but he's also a three-time Mountain Bike Bog Snorkelling world champion, and remains unbeaten through the bog. Dave rides all sorts of bikes but tends to prefer metal ones. He's getting old is why.