I have a supplier's spec sheet on my desk, printed out because I wanted to mark it up by hand. There is exactly one waterproof line on it: waterproof ripstop fabric, hydrostatic head tested. I read it three times looking for the rest. Nothing about how the seams are finished. Nothing about how the roll-top was tested or in what state. Just a fabric claim sitting on a page, presented as if it answered the question I actually have, which is whether this bag keeps a sleeping bag dry in the bottom of a canoe through a long river float.
The fabric figure is the weakest of the three claims in a dry bag, and it is the one suppliers volunteer first, because it comes from a mill and not from their own sewing floor. Seam construction and the closure set the practical limit. What follows is the checklist I run on paper before I approve anything, and the questions I send back when the paper does not answer.
What I Check First on a Dry Bag Spec Sheet
I read spec sheets for what is missing before I read them for what is there. On this one the fabric column is full and confident, and the seam column does not exist. I checked the closure description next and found a sentence about a roll-top with a buckle, phrased as a feature rather than a claim, with no test condition anywhere near it. That pattern is common enough that I treat it as a signal: the further a document stays from the two hard parts, the more the fabric number is doing the persuading.
The tell is where the word waterproof attaches. In most sheets I have marked up, it modifies the fabric, as in waterproof coating or waterproof laminate, and never the assembled bag. A buyer skimming a forwarded PDF reads the headline and assumes the bag was rated. The supplier has not said that. Ask for the page that describes the finished bag as a unit, and if that page does not exist, you have learned something before you have spent money on a sample.
Fabric, Seam and Closure Are Three Separate Claims
Three different questions get folded into one word. The fabric question is what the material resists before anything is sewn into it. The seam question is what happens at the stitched line, which is where a waterproof fabric stops being waterproof unless that line is finished. The closure question is what the roll-top will hold once the bag is rolled, buckled and put under load.
Seam construction is the cleanest separator a buyer can read off paper. Gear built to be waterproof has seams that are taped or welded, and products without that kind of seam construction are usually water-resistant rather than waterproof. Sea to Summit states that as a general rule of thumb on its own product-care page, and it is useful precisely because it stays binary at the document level. Either the spec names tape or welding, or it does not. There is no middle phrasing to argue with.
The closure is the layer buyers underestimate. A roll-top closure only supports a relatively modest amount of water pressure. That is geometry rather than a defect: two layers of coated fabric folded over each other and clamped by a buckle resist water through contact pressure, and the harder the water pushes, the more it works at the fold edge. The fabric behind that fold may take a tall column of water in a lab clamp while the assembled bag still leaks at the roll.
Why a Hydrostatic Head Number Proves Little
Sea to Summit's product-care page makes the point directly: the hydrostatic head of a dry bag fabric is only one factor to consider when thinking about keeping things dry. Read that as a statement from a brand that sells these bags and has the mill data in hand, not as a rule I can hand you for every factory.
The same brand puts advisories on its dry bags and dry sacks explaining that they are not suitable for submersion use, and the same page treats double-bagging of sensitive or electronic devices as essential. A company holding the fabric figure still declines to promise the bag survives being underwater. That should recalibrate how you read a sheet offering a fabric number and nothing else.
So the hydrostatic head answers a question about material, and buyers keep spending it as an answer about the bag. When a supplier's only waterproof evidence is a fabric number, the honest reading is that the seam and closure are unverified. That is not a prediction of failure. It is a statement that nobody has told you what happens at the two places water actually gets in.
Running a Three-Layer Claim Audit
Here is the audit I run, entirely on paper, before I will approve a sample. I draw four columns, one each for fabric, seam and closure, plus a fourth for whatever the document leaves out, and I fix the passing criteria before I read a single line. A column passes only if the document names both the condition and how the thing was established. An adjective on its own scores as blank. Waterproof, taped, welded, tested, all of them: with no condition attached, the cell stays empty.
Fixing the criteria first matters more than it sounds. Read the sheet first and you will grade it generously, because the language slides and every line is written to be read that way. It is easy to accept the word reinforced as if it were a seam specification; it is not one. If the sheet does not say taped or welded, the seam column is empty no matter how many other words share the sentence.
My preference is not subtle. I would rather get back a single page that treats fabric, seam and closure separately and states the condition behind each one than a catalog with waterproof printed across the cover and no conditions inside it.
What the Claim Audit Turned Up
I ran those columns against the one public document I could read line by line, the Sea to Summit piece on how waterproof waterproof is. It scores as a model of a complete answer, which is why I use it as a reference rather than as proof of any product. The fabric column comes back negative in a useful way: that page treats hydrostatic head as one factor among several, so it refuses to let the fabric stand alone. The seam column is settled outright, with the tape-or-weld marker appearing as a plain statement rather than an implication. The closure column is addressed as well: the roll-top is named and its limit is described as modest rather than generous.
Then the fourth column fills in. That page carries no figure for the assembled bag, no description of how the closure was tested, and no statement of what water exposure the bag survived. It explains why the three layers differ; it is not a test report on a product. That distinction is the one I want every buyer on my team to hold, because a supplier can be completely honest, answer all three layers, and still leave you without the bag-level figure that would settle the question. A good answer moves you from unknown to bounded, never from unknown to guaranteed.
The Questions I Send Suppliers Before Sampling
These are the questions I send in writing before a sample is approved. Which layer was tested, the fabric on its own or the assembled bag? If the answer is the fabric, everything after this is about the seam and the closure. With the roll-top in what state was the bag tested: rolled how many times, buckled or unbuckled, loaded or empty? What water exposure was applied, whether spray, rain, a brief dunk or sustained contact, and for how long? And what will the supplier explicitly not claim?
That last question does the most work and is the one suppliers least expect. Ask whether the finished bag carries a submersion advisory of its own, in the hangtag, the insert or the product page. A supplier who has written one has thought about the closure limit. One who has not may still build a good bag, but you now know the limit was never written down, and that is your call to make rather than theirs.
Put the questions in an email rather than on a call. The answer becomes part of the approval record, and when a shipment arrives with a different seam tape than the approved sample, you have paper to point at.
What Would Change This Verdict
My conclusion flips under one condition. Suppose a supplier comes back with a whole-bag result and the conditions it states match how my customer will actually use the bag: the closure in the same rolled and buckled state, a comparable water exposure, a stated duration. Then all three layers are answered in one document and the fabric figure stops being a proxy, and I stop asking at that point. I keep asking now not because suppliers misrepresent fabric, but because a tested-bag answer with conditions attached is rare to get in writing and easy to substitute with a material data sheet.
One trade-off belongs on the table, because this checklist gets misread as a demand for maximum protection. Heavier seam construction and a more secure closure cost weight and money. Tape adds a process step and a material; a beefier closure adds hardware. On a bag that takes paddle splash and rides in the bottom of a boat, that spend does real work. On a bag carrying a hoodie to a festival it does not, and the difference lands in your unit cost with nothing the end user will notice.
Which is where the line sits. If you are ordering a branded give-away that has to survive a walk from the parking lot to the gate, this checklist is more scrutiny than the product deserves, and an honest water-resistant label serves you and your customer better than a waterproof claim you cannot document. If you are specifying OEM bags for paddling, packrafting or electronics, send the three-layer question set before you approve a sample, and treat any reply that offers a fabric figure alone as unproven on seams and closure until the other two answers arrive.