What "10,000 mm Waterproof" Actually Means
Waterproof fabric does not equal a waterproof jacket
The most expensive — and most commonly failed — piece of mandatory gear is the waterproof jacket. Rules usually compress the requirement into one line: "hooded waterproof jacket, rated at least 10,000 mm". That line packs three concepts you may never have heard of.
The first number: hydrostatic head, in millimeters
The test is refreshingly simple: fabric is clamped over a tube and water pressure is raised from below until it seeps through the top. The height of the water column at that moment is the pressure the fabric withstands, measured in millimeters of water column, also called Schmerber (1 Schmerber = 1 mm). The test standard is ISO 811.
So 10,000 mm means the fabric can hold back a 10-meter column of water. That sounds extreme, but note — it is a static test. Kneel on wet grass, have your pack strap press on your shoulder, or sit on a wet rock, and the localised pressure can far exceed it. So do not buy right at the pass mark.
| Rated waterproofness | What it really handles | Typical use |
|---|---|---|
| 1,500 – 5,000 mm | Light rain, brief exposure | Commuter jackets — cannot serve as trail mandatory gear |
| 5,000 – 10,000 mm | Moderate to heavy rain, general outdoor use | Hiking, light trail use; the floor for some races |
| 10,000 – 20,000 mm | Sustained heavy rain | The common mandatory-gear band for trail races |
| Above 20,000 mm | Extreme sustained rain | Alpine and long distances — the safer tier |
The second key: waterproof fabric ≠ waterproof jacket
This is the point beginners most often miss. Sewing needle holes punch straight through the waterproof membrane, creating leak channels that the fabric rating does not protect at all. A 20,000 mm fabric with untreated seams will still wet out at the shoulder seams in moderate rain.
The fix is seam sealing: waterproof tape bonded over the seams on the inside of the garment. Two tiers — taping only the high-stress seams (shoulders, underarms) is "critically taped"; taping all of them is "fully seam sealed". Trail races normally require fully taped.
- Before buying, look for the words "fully seam sealed". A jacket that only says "waterproof" and nothing about seams deserves suspicion.
- It must have a hood, and the hood must cinch down. Rules usually state "hooded" explicitly.
- Windbreakers, softshells and water-repellent jackets never count. They have no membrane: they repel spray, they are not waterproof.
The third number: breathability — the direction is reversed
A waterproof jacket that cannot vent moisture traps your sweat inside; after a short while the inner layer is soaked — the sauna effect. You are waterproofed from outside while drenching yourself from inside, which is just as dangerous. Two metrics measure breathability, and they run in opposite directions — check which one the label uses.
| Metric | Unit | How to read it | Reference values |
|---|---|---|---|
| MVTR (moisture vapor transmission rate) | g/㎡/24h | Higher = more breathable | 15,000+ good; 30,000+ excellent |
| RET (resistance to evaporative heat transfer) | — | Lower = more breathable (ISO 11092) | Below 6 very breathable; 6 – 13 good; above 20 effectively not breathable |
"My jacket is leaking" — do not throw it out yet
This is the most useful thing to know: most "leaks" are not a failed membrane but a worn-out DWR — the durable water-repellent coating on the face fabric. Once the outer fabric saturates, the humidity gradient across the fabric collapses, breathability stops, and you feel wet and cold inside — but no water has actually passed the membrane. This is called wet-out, and the jacket can be revived.
- To restore it: wash off the grime, then tumble low per the care label — heat reactivates the DWR.
- A bottle of DWR spray-on treatment costs far less than a new jacket.
- DWR is a consumable: pack-strap abrasion and repeated washing both wear it down. Reapplying it periodically is normal maintenance, not a defect.
While we are at it: waterproof trousers, gloves, hat
Long-distance rules often demand more than a jacket: waterproof trousers, insulated waterproof gloves, and a hat or buff. These get forgotten constantly, and the on-site check will look for them.
- Waterproof trousers: usually held to the same tier as the jacket, and again "waterproof", not "water-repellent" — distinct from tights; rules often read "waterproof over-trousers worn over tights".
- Gloves: insulated + waterproof. Ordinary fleece gloves lose essentially all insulation when soaked.
- Hat / buff: usually either one suffices; the job is sun and wind protection.
This article covers general practice. For the race you actually enter, the organiser's rules and on-site check list prevail — even the same race in the same year can add gear at short notice.