"Is this fabric waterproof?" is not a testable question. Three standard tests exist because water reaches fabric in three different ways, and a fabric can be excellent at resisting one and poor at resisting another.
What each test actually asks
| Test | Standard | The question it answers |
|---|---|---|
| Hydrostatic head | ISO 811 | How much water pressure before it leaks? |
| Spray rating | ISO 4920 / AATCC 22 | Does the surface shed water, or wet out? |
| Bundesmann | ISO 9865 | How does it behave under rain, while flexing? |
These are not three grades of the same property. They are three properties.
Hydrostatic head — pressure resistance
Water pressure is applied to one face of the specimen and raised at a controlled rate until droplets appear on the other side. The result is a pressure, usually in millimetres of water column or kPa.
This is a membrane and coating test. It tells you whether the barrier layer holds under standing or driven water — sitting on wet ground, kneeling, a pack strap pressing a wet shoulder.
What it does not tell you: whether the outer surface stays dry. A fabric can hold a very high hydrostatic head and still soak up water like a sponge on its face, because the barrier is inside.
Spray rating — surface repellency
A measured volume of water is sprayed onto a specimen mounted at 45°, and the wetted pattern is compared against reference photographs.
This is a finish test. It measures the durable water repellent treatment on the outer surface, not the barrier underneath. AATCC 22 reports 0–100; ISO 4920 reports a 1–5 grade. They are related but not identical scales, so do not mix them on one report.
Spray rating is the test that degrades first in service. DWR finishes wear off with abrasion and washing long before a membrane fails. If customers complain that a jacket "wets out" after a season, this is the test that shows it — not hydrostatic head.
Bundesmann — rain, with movement
Specimens are mounted over cups and exposed to artificial rain from a bank of nozzles, while being rubbed from underneath to simulate the flexing of a garment in wear.
Three things come out of it: the visual appearance after exposure, the water absorbed by the fabric, and the water that penetrated into the cup.
This is the closest to real service of the three, and the most demanding. It combines surface repellency, absorption and penetration under dynamic conditions. A fabric with good spray rating but poor absorption behaviour is exposed here.
How to choose
- Specifying a membrane laminate? Hydrostatic head is your primary number, spray rating your secondary.
- Specifying a treated woven with no membrane — a trench coat, a technical shell? Spray rating is primary; hydrostatic head may be almost meaningless.
- Writing a specification for outerwear that will be worn in rain? Include Bundesmann. It catches failures the other two miss.
- Testing after washing? Repeat the spray rating. Repellency is the property that changes; the membrane usually is not the problem.
A common specification error: requiring a high hydrostatic head for a garment whose real complaint is a wet outer face. The test passes, the customer stays unhappy, and nobody understands why.
Conditioning and water quality
All three tests are sensitive to things outside the fabric.
Use distilled or fully deionised water. Tap water carries dissolved solids and, depending on the region, surfactant residues, both of which change surface tension and therefore repellency results.
Condition specimens to ISO 139 before testing. A specimen that has picked up moisture from a humid store room does not behave like one at equilibrium.
And keep the water temperature consistent — surface tension varies with temperature, and repellency tests feel it.
Reporting
Name the standard and the scale:
ISO 4920, grade 4–5 (not "spray rating 4", which could be either scale) ISO 811, 1,850 mm H₂O, rate 60 cm/min ISO 9865, appearance grade and water penetration in ml
The number without the method is not a result. It is a rumour.