Hydrostatic head explained

What is hydrostatic head?

Hydrostatic head is a measure of how much water pressure a fabric can take before it leaks, stated as the height of a water column in millimetres. A flysheet rated at 2,000mm held back a 2m column of water in the test.

It is the number printed on almost every tent's specification, often shortened to HH. It lets you compare the waterproofing of fabrics on paper, before you ever pitch the tent in the rain.

How is hydrostatic head tested?

Hydrostatic head is tested by clamping a sample of fabric over a water chamber and raising the pressure steadily until three drops come through. The pressure at that moment, expressed in millimetres of water, is the rating.

The standard method is ISO 811, published in Britain as BS EN ISO 811. It allows more than one rate of pressure rise, and makers do not always say which they used, so two figures from different brands are not always strictly comparable. A difference of a few hundred millimetres between two tents tells you less than it seems to.

For a sense of scale, each millimetre of water is a pressure of about 9.8 pascals, so 1,500mm is roughly 15 kilopascals, about a seventh of the air pressure at sea level.

What hydrostatic head does a tent need?

A tent flysheet needs a hydrostatic head high enough to shed wind-driven rain, and many makers treat 1,500mm as the lowest figure at which they call a fabric waterproof. UK backpacking flysheets are commonly rated somewhere between that and a few thousand millimetres.

Rain falling straight down puts little pressure on a sloped flysheet. Strong wind drives it at the fabric harder, and a flysheet sagging onto the inner tent can let water wick through where the two touch. A well-pitched tent with a modest figure often stays drier than a badly pitched one with a high figure.

Check the stated hydrostatic head on each tent in our one man tents and 2 man tents ranges. The camping kit hub covers the other numbers worth comparing.

Why do groundsheets have a higher hydrostatic head?

Groundsheets have a higher hydrostatic head than flysheets because your weight presses the floor into wet ground. A knee or an elbow concentrates that weight on a small area, which raises the water pressure against the fabric far beyond anything rain does.

The arithmetic shows why. Pressure is force divided by area, so the same body weight on a knee pushes much harder per square centimetre than it does spread across your back. That is why tent floors are usually rated well above their flysheets, and why a footprint under the groundsheet helps on wet or stony pitches.

Does a higher hydrostatic head always mean a better tent?

No. A higher hydrostatic head means the fabric resists more pressure, and that is only one part of a dry night. Seams, zips, pitch and ventilation matter as much.

Every needle hole in a seam is a gap, so seams need to be taped or sealed. Zips need a storm flap or a waterproof coating. And a fully waterproof flysheet traps the moisture you breathe out, which condenses on the inside and drips. Many people who think their tent is leaking are looking at condensation, and the fix is ventilation.

Coating matters too. Polyurethane coatings reach high figures and break down slowly with age, heat and damp storage. Silicone-coated fabrics are often lighter and shed water well, sometimes with a lower quoted figure than a polyurethane fabric that performs no better in rain.

Does hydrostatic head drop over time?

Yes. The hydrostatic head of a tent fabric falls as the coating wears, flexes and ages, so an old tent is rarely as waterproof as its label says.

Packing a tent away wet speeds this up, because damp and warmth break down polyurethane coatings. Dry the tent fully at home after every trip, store it loosely, and reproof or reseal seams when water starts beading less well than it did.

What about bivvy bags and waterproof clothing?

Bivvy bags and waterproof jackets are measured with the same hydrostatic head test, and their fabrics also carry a breathability figure because they sit close to the body.

For a bivvy bag, breathability matters almost as much as the waterproof rating, since you breathe and sweat inside it all night. What is a bivvy bag? explains the trade-off, and our wild camping kit list shows where a shelter fits in the rest of your kit.