Hydrostatic Headache: Waterproofness Ratings Explained
How is waterproofness measured in tent fabrics, and what do the ratings actually mean?
Whether or not your tent beads water like this has less to do with the fabric's waterproofness rating than you might think.
What is “Waterproof”?
Many people have asked us about the minimum waterproofness rating they should look for in a tent. What’s the minimum waterproofness I need if I’m camping in the rain? Above treeline? The waterproofness rating, or “Hydrostatic Head” (HH), also colloquially called the “mm rating”, is one of the most misunderstood (and mis-marketed) specs in the tent world. But wait- either water can get through or it can’t… right? I hate to break it to you, but even waterproofness is wishy-washy (so to speak). Nuance rears its ugly head again! In this article, we’ll clear up some common misconceptions about waterproofness ratings and explain what they actually mean.
Given enough pressure and time, water can force its way through pretty much anything. Woven fabric is inherently porous because it has lots of little gaps between the fibers. Waterproof fabric coatings fill those gaps, but with enough pressure, water can push those little fibers out of the way and force its way through the fabric. Some fabrics are more waterproof than others, and no woven fabrics are 100% waterproof.
If waterproofness is a spectrum, we need to agree on what “waterproof” actually means. Qualitatively, as far as the outdoor industry is concerned, “waterproof” means it won’t leak during normal use of the product. Take your tent out in rain? You’ll be fine. Try to plug a fire hose by stretching your fly over the nozzle? Not so much. Quantitatively, a fabric is generally considered waterproof if its hydrostatic head is greater than 1000mm. So what is hydrostatic head? And why is it measured in millimeters? Good questions.
What Is Hydrostatic Head?
Hydrostatic head measures the pressure required to force water through a piece of fabric. Specifically, it’s how high a column of water has to be before the pressure exerted by the weight of the water is enough to penetrate the fabric. The higher the column, the more pressure the water exerts on the fabric. Imagine a PVC pipe with a piece of fabric stretched across one end with a rubber band. You keep adding water to the pipe until it starts coming through the fabric. Measure the height of the water in your pipe in millimeters (mm) right before the water comes through and that’s the hydrostatic head. If you want to be pedantic, the unit is technically “millimeters of water”, or mmH2O, but it’s almost always shortened to “mm” because it’s assumed you’re talking about water in the context of hydrostatic head. This is why hydrostatic head results are sometimes called the “mm rating”. It’s also why the hydrostatic head test is also known as a water column test, and the result is also called the “water column” of a fabric.
In reality, it’s pretty unwieldy to be adding water to a pipe that can be several meters tall, so HH tests are usually done with a pressurizing water pump that measures pressure in pounds per square inch (PSI) and the results are then converted to mmH2O. 1 psi is equivalent to about 700mmH2O of pressure.
The machine used to run hydrostatic head tests is called a Suter tester. This is the setup in SlingFin's fabric lab that we use for our testing.
What Is The Minimum Hydrostatic Head Rating I Should Get?
If the HH rating of a fabric describes how waterproof it is, more is better, right? That’s certainly what many marketing teams would have you believe. But remember, a 700mm water column is equivalent to 1 psi of pressure. That means you would need wind-driven rain that exerts 1psi on your tent to push water through the fly if it has a 700mm HH rating.
But what conditions are ACTUALLY required to achieve that kind of pressure? For wind to exert 1psi on a flat surface that’s perpendicular to the wind direction, you would need winds well in excess of 200mph! If you can design a tent under 10lbs that fits two full-sized humans and can handle 200mph winds then I have a job offer with your name on it. The lowest water column we use in our flysheets is 1200mm, which would require wind speeds of over 300mph (by the most conservative estimates) to exert enough pressure to push water through the fabric. For reference, the strongest wind gust ever recorded on earth was 253mph. I think you’ll be okay.
Water begins to penetrate a weathered fabric during a HH test. The pressure pictured here is about 1.75 PSI, which is equivalent to the pressure of wind-driven rain at over 300mph. This tent has spent a lot of time in the sun and was then run over by a car. The HH has decreased from 1200mm when it was new to about 1000mm now.
The math changes when it comes to floor fabric. Pressure exerted on your tent’s floor is from YOU, not the wind. The formula for pressure is Pressure=Force/Area (P=F/A), where force represents the weight of a person in the tent divided by the area (how large of an area the weight is spread out over). In simple terms, the more you spread out a force (by sitting/lying on a sleeping pad, for example), the lower the pressure. The more concentrated the force (e.g., kneeling directly on the floor), the higher the pressure. A 200lb person lying on a standard 20” x 72” sleeping pad exerts only about 100mmH2O of pressure on the tent floor, assuming even distribution of weight on the sleeping pad. Even if you triple it to account for uneven weight distribution, that’s still well under the 1800mm of even our lightest weight floor fabrics.
Point pressure, like kneeling, is a different story. Kneeling directly on your tent’s floor fabric can exceed 10,000 mmH2O, but since you would have to be kneeling long enough for water to soak all the way through, which takes time, chances are your knees are going to give out first. Just kneel on your pad instead, it’s more comfortable anyway. In our experience, we have found floor HH ratings above 1500mm are plenty for normal use. Keep in mind, the pressure estimates above assume you’re on completely saturated ground- we’re talking a literal puddle of water under the tent, in which case you may want to read up on proper campsite selection techniques. Ground that is quite wet but not saturated will require significantly more pressure than quoted above for water to penetrate the floor.
An example of water penetrating fabric in a HH test. The test normally ends before the water gets to this point, but water will penetrate all coated fabrics like this with enough pressure.
Is There A Downside To Higher HH Ratings?
So even if most fabrics are more waterproof than they need to be, is that a bad thing? The disadvantages are minor, but they are worth considering. The first is that coatings add weight. While this isn’t as much of a concern with larger expedition tents, on lightweight tents the weight difference of thicker coatings can be significant. Secondly, while adding more coating might make the coating itself last longer (it’s more durable, as there’s more coating to wear through), PU and PE coatings actually decrease the tear strength of a fabric. Again, not much of an issue on heavy expedition fabrics, which have plenty of tear strength to begin with, but potentially problematic on the gossamer fabrics used in ultralight tents. Silicone coatings increase the tear strength of a fabric, but achieving high HH ratings is difficult with silicone coatings, since multiple coats need to be applied and silicone is famously hard to stick things to.
Wind-driven rain hits the Portal 3 during product testing in Iceland. The wind in the video is about 30-35mph, which means it's only exerting about 18mmH2O of pressure on the fabric- far below the fabric's 1200mm rating.
Why Are Most HH Ratings So Much Higher Than Necessary?
If 1200mm is enough to handle any wind-driven rain, why do some companies advertise flysheet HH numbers that are so much higher? There are a few reasons:
1. Marketing
In an increasingly online-centric outdoor marketplace where folks often don’t see and touch their tents before making a purchase, marketers (and by extension consumers) have become increasingly focused on specs. Hydrostatic head is a concrete number that people can point to. Every marketing department with an impatient VC firm breathing down their necks has done what they do best and reduced hydrostatic head to “BIG NUMBER GOOD”, with marketing that exploits customers’ fears to sell them fabrics with HH ratings high enough to withstand a tornado, even if they’re only car camping in the middle of the summer. Here’s an example from a UK tent company’s blog that a customer referred me to along with their question about HH ratings:
“A 3,000mm, 5,000mm, or 7,000mm rating can mean a cosy night or waking up in a puddle… Planning a family trip in changing UK weather? You need reliable waterproofing. Look for tents with a 4,000mm–5,000mm rating.”
In other words, “buy our tent or your family will suffer”. In addition to being factually incorrect, this kind of blatant fear-mongering is designed to bait under-informed consumers into buying more tent than they need. It also doesn’t make camping sound very fun, if you ask me.
2. Fabric coating
Marketing aside, there is a functional angle to high HH measurements. The type of coating used on your fabrics has major implications for HH. Without going into too much detail about fabric coatings (I do that here), it’s important to know that PU fabrics require higher HH ratings to function well in extended wet use. This is because PU coatings are actually hydrophilic, meaning they absorb water over time. It’s counterintuitive, because PU is used for waterproofing, but PU-coated fabrics will eventually absorb water and wet out. The higher the HH, the thicker the coating, and the longer this process takes. With PU-coated fabrics, the HH reflects how long the fabric will take to soak through. Silicone, on the other hand, is hydrophobic (so is PE, which we use in our floors), so it doesn’t wet out like PU coatings. This means a lower HH on a silicone or PE-coated fabric will keep you dry longer than the same HH on a PU-coated fabric. This is also why PU-coated fabrics lean on DWR coatings to prevent water from soaking in, while silicone and PE-coated fabrics do not. But PU coatings are cheaper and can be seam taped, unlike silicone coatings (which have to be seam sealed), so most tent companies continue to use PU coatings despite their many shortcomings.
3. Weathering
Finally, another reason some fabrics have much higher HH ratings is because as fabrics wear over time, the coating loses waterproofness. All fabrics are subject to UV degradation. I go much deeper into this in our article on UV and tent fabrics, but for now all you need to know is that silicone-coated fabrics are much more UV-resistant than PU and PE (this is why we only use PE fabrics in our floors, where prolonged UV exposure isn’t an issue). In only one season of use, PU-coated fabrics can lose a significant amount of waterproofness from UV exposure alone. To compensate for this, fabrics coated with PU are often made with high HH ratings with the expectation that they will lose waterproofness quickly with use. Silicone-coated fabrics also lose waterproofness with prolonged UV exposure, but at a much slower rate. This means that the HH ratings do not need to be inflated as much to compensate for UV damage. Additionally, PU-coated fabrics lose waterproofing regardless of whether or not the tent is being used because PU reacts with moisture in the air and breaks down (a reaction called hydrolysis), leading to a sticky, smelly mess. Boosting the HH increases the time it takes for the fabric to reach non-waterproof levels.
We once did a small sewing repair on a SplitWing tarp (10D silicone coated fabric) that had seen more than 6000 trail miles over several years. We did a HH test on it and we were impressed to find that the HH was still about 800mm. While this is technically under the “waterproof” threshold, remember that 700mm=1psi=200mph winds. Not bad for a tent that had already completed several national scenic trails. We finished our repair and that tarp is still out there and kicking.
Is HH even a useful spec?
If you did enough research to find this article, chances are you probably don’t need to factor HH into your buying decisions. The companies you’re shopping from probably all have perfectly adequate HH ratings on their fabrics. Any company making halfway decent tents will be have HH ratings that can withstand the conditions the tents are designed to handle. In my experience, it’s actually difficult to source fabrics in the sub-1000mm range unless they’re breathable fabrics that aren’t designed for use as a waterproof layer.
The types of fabric coatings in your tent have a much larger impact on your tent’s performance than the hydrostatic head rating. Look for tents without any PU coatings, and specifically seek out two-side silicone-coated (SIL/SIL) fabrics, especially in the fly. If the type of fabric coating isn’t listed, it’s probably PU. If the fabric says “siliconized” or “silicone treated”, it’s probably a light silicone finish over a PU coating. Even some fabrics advertised as “silnylon” and especially “silpoly” are not two side silicone-coated. Because pure silicone coatings are higher performance and more expensive, companies who use them will usually highlight them explicitly. Hopefully the information in this article will help you cut through the technical jargon and marketing mumbo-jumbo to find a tent that will actually suit your needs.
Stay dry out there!