How the AATCC 35 Rain Test Works and What It Proves

Purpose and History

AATCC 35 is the textile industry rain test. A specimen is mounted flat, backed by a weighed piece of blotter paper, and exposed to a shower of water falling from a fixed head for a fixed period. The blotter is weighed again and the increase in mass is reported as the amount of water that penetrated the cloth. The method was written by a textile association for cloth, which explains both its strength and its limits when you apply it to bags.

The test answers a comparative question, not an absolute one. It tells you whether candidate A lets more water through than candidate B under the same shower. It does not tell you whether a finished bag keeps a camera or a laptop dry in a storm, because a finished bag fails at seams, closures and stress points long before the face fabric reaches its own penetration limit. Buyers who treat an AATCC 35 pass as a product-level warranty end up with retail claims they cannot substantiate when a customer complains.

Use AATCC 35 where it belongs: as a screening tool during material selection. Once you have shortlisted fabrics and finishes, move the decision to a hydrostatic head determination under ISO 811 or AATCC 127, then verify the assembled product in its service configuration. Our screening sequence on the production floor runs face-fabric head tests first, seam integrity checks second, and whole-product ingress testing last, because that order finds the cheap failures before the expensive ones.

For context on where the method sits in the wider landscape, see our notes on seam sealing methods and how they are verified and the release discipline described in our AQL 2.5 inspection plan.

Nozzle, Head and Distance

The rain tester is a gravity device. A reservoir sits above a spray head, the water level in that reservoir is held constant during the run, and the column of water standing above the nozzles sets the impact energy at the specimen face. Because the driving force comes from a standing column rather than a pump, the machine is mechanically simple and highly repeatable, but it is also sensitive to anything that changes the height of that column during a run.

Nozzle geometry matters as much as the head. A spray head with partially blocked holes produces an uneven fan, and an uneven fan produces a result that reflects the blockage rather than the fabric. Operators should inspect and clean the spray head on a fixed schedule and log the last maintenance date next to the result, so a drift in the data can be matched against a drift in the machine.

Distance from nozzle to specimen fixes droplet size and velocity at impact. Move the specimen closer and you raise impact energy; move it further away and the droplets break up and lose momentum before they arrive. Both directions move the number without changing the fabric at all, which is why the method fixes the distance, and why you should never compare results from two machines until both have been confirmed at the same geometry and the same head height.

• Confirm the water level in the reservoir is at the marked height before every run, not once per shift.
• Check the spray head for blocked or damaged holes at the start of each batch and record the check.
• Verify specimen-to-nozzle distance with a gauge rather than by eye, and re-check it after any maintenance.
• Record water temperature on the report, because cold water penetrates more slowly and shifts the reading.
• Run a reference fabric of known behavior at least weekly to catch slow drift in the apparatus.

Blotter Paper and Absorbency End Point

The blotter is the sensor. A piece of absorbent paper of known mass is placed directly behind the specimen, and any water that passes through the cloth is taken up by that paper. Reweigh it after the run and the difference is the result. The method depends on the blotter being dry at the start, fully in contact with the back face of the specimen, and weighed on a balance with enough resolution to make small differences meaningful rather than noise.

Contact is where most bad data comes from. If the blotter bridges away from the fabric, or if the specimen is mounted so that it sags away from the paper, water can pass through the cloth and miss the blotter entirely, producing a false pass. Mount the specimen under even tension and confirm the blotter lies flat against the back face across the full exposed area before the run starts, then again when the run ends.

The end point is a mass, not a moment. Some buyers expect a visual pass or fail at a specific minute, but the method as written reports how much water got through across the full exposure. If your supplier quotes a 0 to 100 style rating, ask what conversion they applied to the mass reading, because that conversion is a commercial convention layered on top of the method and different suppliers apply different ones.

Rating and Reporting

A report that says only passed is worthless to a procurement team. Demand the raw number, the units, the exposure time, the water temperature, the machine identification, the number of specimens per sample, and the conditioning state of the specimens before testing. Without those fields you cannot reproduce the result, and without reproducibility you have nothing to enforce in a dispute twelve months later.

Sampling deserves the same discipline as the measurement itself. A single swatch tells you about that swatch. Pull specimens from at least three positions across the roll or across the production lot, because coating weight and finish application are rarely uniform from edge to edge. At lot release we apply AQL 2.5 lot acceptance to finished goods and treat fabric test results as a first-off and in-process control rather than the final gate.

Set the acceptance language before you place the order, not after the first disagreement. Write the threshold, the exposure, the sample size and the retest rule into the tech pack so both sides are reading the same sentence. Our welding and seam-sealing lines run against documented work instructions for exactly this reason, and every station record feeds the lot file.

• Test method name and the year of the version used.
• Specimen identification, including roll or lot number and position within the roll.
• Conditioning time, temperature and relative humidity before testing.
• Exposure duration, water temperature and reservoir head height.
• Raw blotter mass before and after, with the balance resolution stated.
• Number of specimens tested and the mean, maximum and minimum result.
• Machine identification and the date of the last spray head maintenance.

Variables That Skew Results

Water temperature changes viscosity and surface tension, and both change how easily water moves through a fabric structure. A run performed with cold mains water in winter will look better than the same fabric run in summer. Record the temperature on every report and refuse to compare results generated across a spread of more than a few degrees, because you would be comparing two different tests.

Specimen tension is the second large variable. A fabric stretched taut on the frame opens up the interstices between yarns and lets water through more readily than the same fabric mounted slack. Train operators to mount at consistent tension, and where the frame allows it, use a spring-loaded or torque-limited clamp instead of hand-tightened screws that different operators tighten differently.

The durable water repellent finish dominates early results. A fresh finish sheds water and the blotter stays dry; abrade the surface for a few hundred cycles and the same fabric behaves very differently. If your product will see abrasion in service, require the rain test to be run after a defined abrasion conditioning, not only on virgin fabric, and pair it with an abrasion resistance program so the two numbers are generated together.

Two smaller variables are worth naming because they are easy to miss. Residual moisture in the blotter from a humid room adds mass that has nothing to do with penetration, and water carried over on the specimen holder between runs does the same. Dry and condition the blotter in the same space where the test is run, and wipe the holder between every specimen.

Compared With ISO and JIS Methods

AATCC 22 is the spray test, and it grades how a surface sheds water rather than how much penetrates. It is fast, it produces a visual rating, and it is the right quick screen for a shower-resistant outer shell. AATCC 35 applies far more water for far longer and measures penetration, so it is the harsher of the two. A fabric can earn a good AATCC 22 rating and still fail AATCC 35, which is why a specification that names only one of them leaves a real gap.

Hydrostatic methods ask a different question again. ISO 811 and AATCC 127 raise a column of water against one face of the fabric until water breaks through, and report the height in millimeters of water column. That number is stable, comparable across laboratories, and easy to write into a contract. It is still a face-fabric number, and it says nothing at all about a seam, a weld boundary or a roll-top closure.

JIS L 1092 covers low-pressure and high-pressure water resistance approaches in the Japanese system, and buyers selling into Japan will meet it in local retail specifications. IEC 60529 defines the ingress protection code, but it was drafted around rigid enclosures for electrical equipment. Hold it in reserve for hard-shell electronics cases and do not build a soft-goods waterproof claim on it alone.

Sequencing With a Head Test

Order the tests so each one filters cheaply before the next one costs more. Start with the face fabric. A hydrostatic head determination is quick, returns a number in millimeters, and eliminates weak candidates before you spend sampling time and tooling money on them. Incoming coated fabric for our programs is screened against an agreed floor in the 10,000 to 20,000 mm band and rejected below it.

Run AATCC 35 next on the survivors. It is slower and messier than a head test, but it stresses the finish under continuous wetting and exposes fabrics that survive a short pressure ramp and then soak through. Treat it as a confirmation step rather than the primary gate, and always run it on the same fabric state you intend to ship.

Finish on the assembled product. Ingress into a real bag happens at the roll-top closure, the zipper tape, the weld boundary and the point where a strap is stitched through the shell. Test the finished article with the closure in its service configuration, then release the lot against AQL 2.5 under ISO 2859-1. Our finished waterproof bag programs are released on that full sequence rather than on any single fabric number.

1. Screen incoming fabric with a hydrostatic head determination against your agreed floor value.
2. Confirm the shortlisted fabric with AATCC 35 after abrasion conditioning.
3. Build samples and test the assembled product, closure included, in its service configuration.
4. Verify weld or seam integrity before whole-product ingress testing, because a failed seam invalidates the rest.
5. Release production lots against AQL 2.5 (ISO 2859-1) with documented retest rules.

Using Results in a Spec

Write the method, the threshold and the sample size into one sentence, and name the year of the method version you are citing. A line that reads resistant to rain is unenforceable. A line that reads AATCC 35, fixed exposure, maximum permitted blotter mass gain in grams, three specimens per lot with the mean reported, is enforceable, because both sides can repeat it and a third party can arbitrate it.

Tie the test to a commercial framework so the number has consequences. Development runs through a custom development programme with sampling in 6 to 10 working days, extending to 12 to 15 working days when the construction is complex. Bulk production runs 35 to 50 days after sample approval. Standard terms are FOB Xiamen with T/T 30/70, and repeat programs are released against the same AQL 2.5 plan used at first production.

MOQ is 500 pcs per style and colourway. That floor exists because a waterproof program needs its own weld tooling setup, its own first-off verification and its own test record. Splitting a smaller total across more colourways multiplies the verification cost per unit without improving the product, and it usually lengthens the sampling cycle rather than shortening it.

Close the loop by naming who pays for a retest and what happens to a lot that fails one. Write the disposition path into the contract: rework and retest once at the supplier's cost, reject and remake on a second failure, and require the full raw data set before either decision is taken. A supplier who will not release raw readings is telling you something about their process control.

• Name the method and its version year, the exposure, and the numeric threshold with units.
• State the number of specimens, the sampling positions and the statistic that governs acceptance.
• Require raw readings, not a summary pass or fail statement.
• Define rework, retest and lot disposition, including who carries the cost.
• Require retention samples from each lot with a stated retention period.

Frequently Asked Questions

Q1: What does the AATCC 35 rain test actually measure?

A: It measures the mass of water that penetrates a fabric specimen during a controlled shower and is absorbed by a weighed blotter paper mounted behind it. The result is reported as a gain in grams, so a lower number means better resistance. It is a face-fabric measurement and does not evaluate seams, closures or hardware.

Q2: Is AATCC 35 the same as AATCC 22?

A: No. AATCC 22 is a spray test that visually grades how well a surface sheds water after a light, short spray, while AATCC 35 subjects the fabric to a heavier and longer rain exposure and measures how much water actually penetrates. A fabric can score well under AATCC 22 and still fail AATCC 35, so specifying only one leaves a gap.

Q3: Can AATCC 35 replace a hydrostatic head test?

A: It should not replace it. Hydrostatic methods such as ISO 811 and AATCC 127 return a stable pressure value in millimeters of water column that is easy to write into a contract and compare across laboratories. Use the head test as the primary screen for face fabric, then use AATCC 35 to confirm behavior under continuous wetting.

Q4: How many specimens should a supplier test per lot?

A: Three specimens taken from three different positions across the roll or lot is a practical minimum, because coating weight and finish application are rarely uniform edge to edge. Ask for the mean, the maximum and the minimum rather than a single representative figure, and require the raw blotter masses behind each result.

Q5: Why do two laboratories return different AATCC 35 results for the same fabric?

A: Differences usually come from the apparatus and the operator rather than from the cloth. Reservoir head height, spray head condition, specimen-to-nozzle distance, water temperature, mounting tension and blotter contact all move the number. Without those parameters printed on the report there is no way to tell whether the difference is real.

Q6: Does AATCC 35 tell you anything about seams or zippers?

A: It does not. The method evaluates a flat piece of fabric, so a passing result says nothing about a welded seam, a stitched and taped seam, a zipper tape or a roll-top closure. Finished bags fail at those features far more often than at the face fabric, so always add an assembled-product ingress test.

Q7: What water temperature should be used for the rain test?

A: Record whatever temperature the laboratory actually used and hold it constant across every run you intend to compare. Water temperature changes viscosity and surface tension, and therefore changes how easily water moves through the fabric structure. Comparing a winter result generated with cold mains water against a summer result is not a valid comparison.

Q8: How does abrasion affect the AATCC 35 result?

A: Abrasion degrades the durable water repellent finish, and a degraded finish wets out and lets far more water through. Testing only virgin fabric flatters the material and misrepresents what happens after a season of use. Require the rain test to be repeated after a defined abrasion conditioning so the number reflects the product as it will actually be used.

Q9: Can a supplier report a 0 to 100 rating instead of grams?

A: They can, but you should ask what conversion was applied to the underlying mass reading. Any 0 to 100 style scale built on top of this method is a commercial convention rather than part of the test itself, and different suppliers apply different conversions. Insist on the raw gram values so results remain comparable between vendors.

Q10: How should AATCC 35 be written into an RFQ?

A: Name the method and its version year, the exposure period, the maximum permitted blotter mass gain with units, the number of specimens and sampling positions, and the statistic that governs acceptance. Also state the conditioning requirement and whether the test is run before or after abrasion. Vague wording such as rain resistant is unenforceable in a dispute.

Q11: What is a realistic commercial threshold for a bag fabric?

A: Set the threshold from the product promise rather than copying a number from a supplier sheet. A shower-resistant daypack needs a different floor than a bag sold for kayaking or professional camera use. Agree the floor with your supplier during sampling, verify it on production fabric, and re-verify it whenever the coating or laminate source changes.

Q12: How does AATCC 35 relate to IEC 60529 and IPX claims?

A: IEC 60529 defines the ingress protection code, but it was written around rigid enclosures for electrical equipment and it returns a single pass or fail for one artificial configuration. AATCC 35 measures fabric penetration under rain. They answer different questions, and neither one covers seams, closures or long-term wear on a soft bag.

People Also Ask