How Waterproof is Waterproof?
July 13, 2025One of my biggest gripes with the outdoor industry is that companies are allowed to market water-resistant gear as "waterproof." This leads to frustration and can even be dengerous when customers take gear into the wild and find their clothes don't keep them dry.
How waterproofness is measured
You'd think that "waterproof" means "impermeable to H2O" but when it comes to apparel and gear like tents, it is actually a scale measured by how much pressure is needed to push water through the fabric. (Similar to watches and phones). The most common units used for measuring water resistance are mm H2O, which describes how high a water column (WC) stacked on top of the fabric would have to be before it gets through to the other side. Anything above about 1,000mm WC can be marketed as waterproof (there don't seem to be any actual regulations about this, but that seems to be the cutoff).
Practical applications
Whether particular water column rating is suitable for your activity depends on a few factors. If a fabric is free-standing and water can shed easily, like a tent wall, it doesn't need as much water resistance as something like a jacket or pant leg that will be pushed against other materials like backpack straps and bushes. Fabrics intended to keep out snow rather than liquid water don't usually need to be as water-resistant either. Materials that are going to be fully submerged, like the walls of drybags, need to be fully impermeable.
It's also worth noting that the seams, zippers, and openings are weak points in gear that may affect overall waterproofness.
| Water Column Rating | Good Uses | Misleading Uses |
|---|---|---|
| 1,000mm - 5,000mm | Tents, tarps, and rainflies. These applications don't experience much mechanical pressure so low ratings are OK. Lightweight shell jackets and windbreakers. | Don't get mislead thinking a "waterproof" trail running jacket will keep you dry all day. |
| 5,000mm - 15,000mm | Ski/snowboard jackets. Heavy-duty tents. | A lot of less-expensive hiking jackets are in this range and many people are frustrated to find they wet out quickly. |
| 20,000mm | Hiking shells. This is the minimum you need for extended time in the rain. Most Gore-Tex jackets have this rating. | |
| 25,000mm - 30,000mm | Mountaineering shells. Windy, wet conditions with a lot of abrasion require heavy-duty three-layer jackets which are typically rated around 28kmm WC. | |
| Impermeable | Fully waterproof materials are used for cold-weather fishing gear, classic "macintosh" rain jackets, drybags, inflatable boat materials, etc. | These materials don't breathe, so they are not well-suited for high exertion activities. Very thin plastic materials are also used for disposable ponchos and things like that, which are waterproof but puncture easily. |
How waterproofness is achieved
In chronological order...
- Since ancient times, oil and wax have been soaked into natural fibers like leather and cotton. This provides a durable but heavy waterproof solution that requires a lot of care to maintain.
- In the industrial era, rubber (and eventually flexible plastics like vinyl) became common for waterproofing. These can be extremely waterproof, but prone to UV damage, puncture, don't breathe, and are heavy.
- At some point in the late 20th century, "durable water resistant" coatings were invented, which are essentially wash-in or spray-on hydrophobic coatings that cause water to bead op and roll off materials. The downside is they eventually wear off and can never turn a loose-knit material fully waterproof on their own. DWR treatment are often applied to nylon or polyester fabrics, which are the main materials used for outdoor gear these days.
- Membrane fabrics like Gore-Tex (and competitors like Toray) become popular in the 1970s. These work by sandwiching a thin, flexible fabric like polyethylene between a more durable fabric, which often has a DWR treatment on the outside. These are the most common types of rain shell available these days, because they balance breathability, durability (through the choice of face fabric), and comfort (by being lightweight and flexible). However, they aren't fully waterproof like a vinyl rainsuit.
- Polyurethane (PU) coatings are common inside nylon and polyester fabrics in backpacks, tents, and rain gear. (This is that sticky coating on the inside). It degrades after about 10 years and is not easy to replace.
- Silicon-impregnated nylon (silnylon) is an increasingly-popular fabric for tents and tarps because it does not wear off like PU coatings
- Dyneema and similar ultra-high-molecular-weight-polyethylene (UHMWPE) fabrics laminate extremely-waterproof membranes between a variety of other materials like Mylar or polyester. This creates a light but not breathable fabric. Can be very durable, depending on the coating. Over time, Dyneema shrinks.