What is a water column?
What is a water column?
The millimetre figure behind every waterproof claim
A water column rating states the water pressure a fabric withstands before water passes through it. The technical name for it is hydrostatic head, it is given in millimetres and measured under ISO 811. This page explains how the figure is produced, why no test standard says at which point a fabric becomes waterproof, and what our own fabrics reach.
What the figure on a data sheet measures
The value comes from a laboratory test in which water is pressed against a fabric sample at a steadily rising pressure. Four points explain what comes out of it.
Hydrostatic head explained in one sentence
It is the water pressure a fabric holds back before water passes through. The name describes the principle: 1,000 mm means the fabric withstands the pressure exerted by a column of water one metre high.
Millimetres, centimetres, millibars
The standard itself allows centimetres or millibars and notes that the SI unit pascal is unusual for this test. The trade settled on millimetres. For conversion: one metre of water equals 98 millibars, so 1,000 mm is roughly 98 mbar.
A pressure value, not a duration
The standard relates its result to fabrics under water pressure for short or moderate periods. How long a cushion stays dry in continuous rain cannot be read from the figure.
The test standard is ISO 811
The current edition is ISO 811:2018, adopted in Europe as EN ISO 811. It replaced the older EN 20811, whose national versions were withdrawn at the end of November 2018. Many data sheets still carry the former designation, including our Agora documents, which cite UNE-EN ISO 20811. The method behind it is the same.
How the water column rating is produced
ISO 811 fixes every step. These requirements explain why two figures are only comparable when they were produced under the same conditions.
| Requirement | What the standard specifies | Why it matters |
|---|---|---|
| Test area | 100 square centimetres, clamped flat and free of bulges | a larger area contains more weak spots, so the area is fixed |
| Water | grade 3 water to ISO 3696, at 20 or 27 degrees Celsius | warmer water yields lower values, so the temperature belongs in the report |
| Rate of increase | 10 or 60 centimetres of water per minute | both rates are permitted, but they do not produce the same result |
| End of test | the pressure at which water breaks through in the third place | the first drop does not end the test, the third one does |
| Number of specimens | at least five, taken from different places in the fabric | a single reading would be chance, the mean is reported |
| Invalid run | water penetrating at the edge of the clamp | such runs are rejected and repeated |
At what point does a fabric count as waterproof?
ISO 811 describes a measuring method and nothing else. It contains no level at which a fabric may be called waterproof, and no classes either. Anyone quoting a fixed limit such as 1,500 mm is not quoting a textile test standard.
Thresholds exist only in product standards for particular goods. The best known is EN 343 for protective clothing against rain. There the material is pretreated before measurement, that is washed, abraded and flexed, and only the value measured afterwards decides the class. Those classes do not transfer to cushions, shade sails or covers, because neither the pretreatment nor the type of exposure is the same.
We call a fabric waterproof when it carries a laminate on the reverse and therefore reaches 1,000 mm. Without that laminate we call it water repellent. This is a convention of our own, based on the manufacturer data, not a requirement of any standard. Pure cover fabrics such as Cover PVC sit well above that at 4,000 mm. The difference between the two constructions is set out in our water repellent versus waterproof comparison.
What our fabrics reach
The figures come from the manufacturers' technical data sheets. Where a value is not stated there, this table says so.
| Fabric | Hydrostatic head | Test standard | Classification |
|---|---|---|---|
| Agora Liso, Lines, Rayure, Panama, Life Liso | 180 mm | UNE-EN ISO 20811 | water repellent, plus a spray test result of 5 out of 5 |
| The same fabrics with PU laminate | 1,000 mm | manufacturer data | waterproof, the laminate adds 44 grammes per square metre |
| Agora Liso Waterproof, sold as weatherproof | 1,000 mm | manufacturer data | laminate faces inwards, the fabric still passes 240 to 320 grammes of water vapour per square metre in 24 hours |
| Dralon Acrisol Liso with laminate on request | 1,000 mm | manufacturer data | the laminate is not a stock item and has to be ordered from the mill |
| Cover PVC | 4,000 mm | ISO 811 | the highest value in our range, intended for covers left out in all weather |
| Agora Life Panama, Agora Life Wabi | not stated by the manufacturer | not stated | no tested value is available to us for these two qualities |
What a water column rating does not tell you about a finished product
What is measured is a piece of new fabric, clamped flat, in a laboratory. A cushion on a terrace lives under different conditions. Four of them matter.
The seam is the real limit
Every needle puncture breaks the coating. A cover made from laminated fabric only becomes tight when the seams are taped as well. That sealing needs build-up height and can therefore only be done cleanly from 6 cm cushion thickness upwards. Which seam types we produce is shown in our overview of cushion seam types.
Body weight is not test pressure
Sitting down on a wet cushion produces, at that point, a multiple of the pressure used in the laboratory. Even a high figure is therefore no promise for every situation.
Time is not part of the number
The standard relates its result to short or moderate exposure. For lasting wet conditions, what counts besides the fabric is whether the product can vent and dry. That is why waterproof pads are given air holes for pressure equalisation.
Water repellency wears off and can be restored
The surface finish is worn down by sun, dirt and cleaning. A later impregnation restores most of the water repellent behaviour. The laminate on the reverse is not affected by this, but it must be kept out of direct sun and away from pressure washers.
Frequently Asked Questions
Sources
- ISO 811:2018, Textiles - Determination of resistance to water penetration - Hydrostatic pressure test. Freely available standard preview covering scope, principle, rates of pressure increase and the end-of-test criterion
- Catalogue entry for EN ISO 811:2018 at the European standards body CEN, noting the superseded EN 20811:1992
- All Germes fabrics with their technical values
- Weatherproof cushions with a laminated cover
- Buying guide
Last reviewed: 21.08.2026 · Reviewer: Germes-Team
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