Tent hydrostatic head: what 1,500, 3,000 or 5,000 mm really means

Take a piece of your flysheet, stretch it across the bottom of a tube, and fill the tube with water to a height of 3 m, well above your living room ceiling. If the fabric holds, the spec sheet can say 3,000 mm. That is the entire promise of hydrostatic headThe water pressure, expressed in millimetres of water column, that a fabric withstands before water gets through.: a pressure, on a small piece of fabric. It says nothing about a stormy night, a poorly finished seam or a zip left slightly open.

So this guide reads just that one line of the spec sheet, the one our article on choosing a wild camping tent only skims. It backs up the shelter step in our guide to your first wild camp, and it answers the question you ask yourself in front of two tents: how many millimetres before you can sleep easy?

What hydrostatic head measures

Decathlon defines it as a fabric’s resistance to water pressure, expressed in millimetres, from a test based on the ISO 811 standard [1]. The French say ‘colonne d’eau’ and the Germans ‘Wassersäule’, a word you’ll meet on German spec sheets. A 2,000 mm fabric withstands the pressure exerted by 2,000 mm of water [1].

Vaude, a German brand, describes the test: the fabric is stretched beneath a water-filled cylinder 10 cm in diameter, and the figure recorded is the point at which water starts to come through drop by drop [2]. At the American brand MSR, the pressure rises until 3 drops get through [3]. ISO 811, whose title refers to resistance to water penetration and a hydrostatic pressure test, is also a reference for the camping tent standard, ISO 5912 [4]. ISO 811 is a paid-for document: these details come from the brands that use it, not from its text.

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These aren’t the millimetres from the weather forecast. A 2,000 mm tent won’t hold out against 2,000 mm of rain, as Which? points out [5], and the retailer Scandinavian Outdoor sets the record straight in the same way [6]. To give an idea of scale, MSR put umbrella fabric through its tester: 420 mm [3]. Your umbrella keeps you dry with far less than a flysheet, though it has never had to rub against a stone.

1,500, 3,000, 5,000 mm: what the brands say

None of our sources gives a common scale. Vaude and the German magazine OUTDOOR cite 1,300 mm as the DIN threshold for calling a fabric waterproof [2, 7], without saying which DIN standard (DIN is Germany’s standards institute), and the Swedish brand Hilleberg notes the lack of a test method shared across the industry, which makes comparisons harder [8]. So everyone sets their own benchmarks, and sellers don’t all have the same tents to defend.

The gap goes further than you might think. MSR stands by 1,200 mm flysheets [3, 10] and refuses to hold to a single value, as each tent has its own balance of waterproofing, durability and weight [3]. Decathlon sticks to 2,000 mm on its trekking tents and tarps [1], Nordisk to 2,000 mm on its LW models [11]. And OUTDOOR takes the view that the figure only really counts on polyurethane-coated groundsheets, not on the flysheet [7].

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Why the groundsheet needs more than the flysheet

The flysheet takes the rain. The groundsheetThe waterproof part of the tent that forms the floor of the inner tent., by contrast, gets squashed against wet earth, and Vaude describes it as exposed to high pressure [13]. Bergzeit offers a calculation: a person weighing 83 kg, in EU size 43 shoes, exerts about 0.22 bar standing up, or 2,200 mm, and more than 4,000 mm crouching on the balls of the feet [12]. A 3,000 mm groundsheet would no longer hold, it concludes [12]. That’s a seller’s example, not a standardised test: the pressure depends on the contact area, and nobody rummages through a rucksack in that position all night.

MSR gives another reason: abrasion. On the Access, the 3,000 mm groundsheet makes up for wear, and tent floors generally get a thicker coating than flysheets [3, 10]. The two explanations go together: a groundsheet has to cope with both pressure and wear.

Independent tests look at this too. For the Swiss consumer programme Kassensturz, in 2022, a German laboratory examined ten two-person tents: for four of them, the head of testing expected moisture to come through the floor after about three days on wet ground [14]. OUTDOOR, for its part, wants more than 4,000 mm on a polyurethane-coated groundsheet [7], but finds the figure says little about a polyethylene or silicone-coated one, which it prefers to put through its paces on a flooded surface, kneeling and jumping [7].

What wear, coatings and hydrolysis do to the figure

A tent fabric’s waterproofing comes from its coatingThe layer of polyurethane or silicone applied to a fabric to make it waterproof., MSR reminds us [10]. And the figure on the spec sheet describes new fabric. Hilleberg offers an example: until 2017, the brand required at least 3,000 mm after mechanical rubbing; since then, it has published figures for the fabric as delivered, like most manufacturers [16]. Its published figures then leapt, because they were no longer measuring the same thing.

HydrolysisThe chemical breakdown of a coating caused by water, heat and time. is the enemy of polyurethane. Time, moisture and heat chemically break down the coating, MSR explains [10], in use as well as in storage [17]. The brand adds that coatings last better on thicker fabrics [10], and promotes its own Xtreme Shield coating as three times more durable, a manufacturer’s claim [17].

The sun is another matter. According to Hilleberg, UV attacks tear strength first, then water repellencyThe property that makes water form beads and roll off a fabric’s surface.: water stops beading, without the fabric being any less waterproof [18]. Decathlon describes the same repellency, which fades with UV and rubbing and can be revived with a spray [19]. Don’t mix the two up: a treatment makes water roll off, and Hilleberg points out that it doesn’t give back tear strength [18]. None of these brands promises that a spray will return the millimetres to a damaged coating.

Why a very high figure doesn’t guarantee a dry night

Which? sees it in its tests: two tents with similarly high hydrostatic heads can end up one dry and the other with a puddle, because most leaks come through the seams and zips, not the fabric [5]. MSR adds that a higher figure comes at the cost of more coating: the fabric gets heavier and stiffer and, past a certain point, tears more easily [3], which does a flysheet in the wind no favours.

• Seams. The needle punches through the fabric: Decathlon closes these holes with heat-sealed tape [19], MSR with a thread that swells when wet on its tapeless models [10]. Hilleberg admits that a little moisture can get through a seam during prolonged rain, and then advises a seam sealant [20].
• Zips and openings. Nordisk calls its PU tents waterproof, except for the zips and openings [11]. Which? warns against doors that open inwards: the water they hold falls into the tent [5].
• Tension. Nordisk wants the fabric pulled taut for strength and water repellency [11], and MSR counts on a well-tensioned tent to shed water to the ground [21].
• Contact. Pressing against a wet wall can let water through at that point, Which? warns [5].

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The link with condensation

A more waterproof fabric doesn’t mean a fabric that gets less wet from the inside. Hilleberg points out that its flysheets are neither breathable nor air-permeable, so vapour condenses on them [22]. Decathlon takes the reasoning further: a fabric made perfectly waterproof keeps the sleeper’s vapour inside [19]. So the figure on the spec sheet promises nothing against condensation, which comes down to ventilation and where you pitch.

The two do meet in a downpour, though: raindrops hitting the flysheet knock off the condensation clinging to its underside, and the rain seems to come through, Hilleberg explains [22]. Before you blame your 3,000 mm, look at where the water is landing. Erik Gauden lived this distinction during his three days of rain in the Monts d’Arrée: nothing through the groundsheet, condensation under the flysheet.

Which hydrostatic head to choose for your tent

None of our sources makes this decision for you, and I’m not about to hand you a magic number. Here is how I would read a spec sheet, using what this guide’s sources say:

A good habit to get into: read two figures rather than one, the flysheet’s and the groundsheet’s, then ask yourself where the most exposed seam is. And when you get home, take the tent out of its stuff sack to dry it, because the cupboard is where the millimetres go missing.

Worth watching

Mountain Warehouse demonstrates the test on squares of its own fabric: three minutes to watch what this guide describes, the water column rising until the fabric gives way.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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