HYDROSTATIC HEAD RATINGS (HH)

Hydrostatic Head, commonly referred to as the HH rating by manufacturers and retailers, indicates how waterproof a fabric is—whether it’s for a tent, outdoor jacket, or even groundsheets. This rating is typically listed on the product, packaging, or in the website description. It helps to understand what the numbers mean—and when and where lower waterproof ratings can actually be better for a particular tent or shelter.

Hydrostatic Head indicates how much water pressure a tent can handle before it starts to leak. Imagine placing a plastic tube over your tent and filling it with water. If your tent is rated 5000 HH, it will begin to leak when the water reaches 5000 mm in the tube. This rating reflects the pressure the fabric can withstand rather than its overall waterproofness. The higher the HH number, the more pressure the tent can endure.

Manufacturers use these tests and ratings to differentiate between various levels of waterproofing. While "waterproof" used to be a one-size-fits-all term, HH testing now allows us to distinguish between products and garments that are highly waterproof and those that offer only minimal protection. 

Clothing needs a higher HH rating for effective waterproofing due to the additional pressure exerted on the fabric from backpacks and contact with other surfaces.

Groundsheets usually have a higher hydrostatic head rating than the tent material itself, as they come into the most contact with moisture. This higher rating helps prevent water ingress. However, be aware that groundsheets can wear out over time due to close and regular contact with rocks and sharp objects, making them more prone to leaks if they become damaged.

For typical UK weather, which includes light to moderate rain and sustained showers, a tent with an HH rating of 1500 mm to 3000 mm is usually sufficient to keep you dry under standard camping conditions. For harsher rain or snowy climates, you’ll need a tent with a higher HH rating. Most polyester tents have a minimum HH rating of 1000 mm.

HH Ratings

FACTORS INFLUENCING HYDROSTATIC HEAD PERFORMANCE 

Even if your tent boasts an impressive hydrostatic head rating, water can still seep through if the seams are not properly sealed. A tent with sealed seams will be significantly more effective at keeping you dry. It's important to note that tent manufacturers will often state whether the tent has been seam sealed or not. 

While the Hydrostatic Head ratings offer a fundamental understanding of a fabric’s water resistance, it is not the only factor determining a tent's waterproofing effectiveness. Elements such as the tent’s overall shape and design, seam sealing, and zipper quality are also crucial. A tent with a lower HH rating but well-sealed seams and effective zipper covers can outperform a one with a higher HH rating but poor construction. 

HOW MANY MILLIMETRES OF WATERPROOFING DO YOU ACTUALLY NEED?

Surprisingly, not always as much as you might think. Take an umbrella, for example. You’d expect it to offer solid waterproof protection, right? But in a hydrostatic head test, umbrella fabric only rated at 420 mmH20. This shows that a higher number isn’t always necessary to keep you dry. So why do tents have waterproof ratings between 1,000 and 10,000 mmH20?

The answer partly lies in durability. Higher-rated, thicker waterproof coatings tend to offer better durability. Unlike tents, an umbrella doesn’t experience much abrasion since it’s suspended above you. On the other hand, a tent floor sees a lot of wear, which is why tents need more robust coatings. Tarps, facing less abrasion, can get away with lower ratings while still keeping you dry.

CAN I INCREASE THE HYDROSTATIC RATING ON MY TENT ?

Although you can't change your tent’s hydrostatic head rating (since it measures the fabric's inherent water resistance), you can enhance its water resistance and performance with several steps. Apply a waterproofing spray designed for tents to enhance fabric repellency, and reapply seam sealer to prevent leaks through the seams. Use a groundsheet or footprint to guard against water from the ground, and check zippers for proper function, applying lubricant and using covers or rain flaps as needed. Pitch your tent tautly to allow water to run off, and consider adding a tarp for extra protection in severe weather. Regular maintenance, including cleaning, applying UV coating, and following care guidelines, will help preserve your tent's performance.

Improve Water Resistance When Camping

Some tents feature an extra layer of waterproof coating that helps water bead up and slide off the material, directing it away from the tent and into the ground. This additional layer provides enhanced resistance to water, helping to keep the inside of the tent as dry as possible.

The best time to reproof your tent is in the spring, just before camping season begins. Ideally, you should reproof your tent every 12-24 months, depending on how often it's used. If you camp up to three weeks per year, reproofing every 24 months is usually enough. Think of it like servicing your car—it needs regular care to stay in top condition! You've invested time and money in choosing the right tent, so give it the care it needs to last.

While all the above steps can enhance your tent's waterproofing, they won’t alter the fabric’s HH rating, which measures its resistance to water pressure.

ARE HIGHER WATERPROOFING RATINGS ALWAYS BETTER? 

Not necessarily. A higher waterproof rating doesn’t automatically mean more durability. In fact, adding more coating can make the fabric heavier and stiffer, and beyond a certain point, it can even increase the risk of tearing. This is especially true for rainfly fabrics, which need to remain flexible to handle the constant forces of wind and tension from guy lines. Making the fabric too rigid with excessive waterproof coating can compromise its ability to withstand these stresses.

METHODS USED TO MEASURE THE HYDROSTATIC HEAD

  • Water Column - A fabric sample of the material being evaluated is clamped or sealed into a testing apparatus such as a cylindrical tube or a test cell. A vertical column of water is gradually added above the fabric. As the height of the water column increases, so does the water pressure exerted on the fabric. Technicians observe the fabric closely for any signs of water penetration or leakage. The height of the water column (in millimetres) at which 3 drops of water are able to pass through the fabric is recorded as the HH rating. For example, a 1500 mmH20 rating means the fabric can sustain 1,500 mm of water on top of the fabric before it can leak.
  • Schmerber Test - A fabric sample is placed on a test apparatus with a hollow cylinder, stretched tightly over one end. The open end of the cylinder is submerged in water, creating pressure on the fabric. Water is then drawn into the fabric under suction pressure. The HH rating is determined by measuring the height of water drawn into the fabric before it begins to penetrate.
  • Static Water Pressure - In this method, a water column with a known flow rate is used. The fabric sample is positioned below the water column, and pressure is applied. Technicians check the fabric for any water leakage. The HH rating is determined by noting the height of the water column at which the fabric starts to let water through.

For some thicker and more waterproof materials, such as leather or rubber, HH may be assessed through manual inspection. However, traditional methods can vary in precision and reliability compared to modern laboratory tests. Advanced techniques, like hydraulic testing machines, are now commonly used in quality control and laboratories to provide more accurate and consistent HH measurements, ensuring precise evaluation of material waterproofness.