What is Hydrostatic Head? We explain

If you’re looking to buy a tent that uses a polyester-based material, then you’ll likely come across the term Hydrostatic Head. But what does it mean?

So, let’s recap and expand on the video above.

Hydrostatic Head is the measure of how water resistant your tent material is.

It measures how tall a column of water the fabric can hold before water seeps through the weave.

A Hydrostatic Head of 5000mm means that a tent fabric could hold a column of water that is 5000mm tall.

5000mm of water exerts more pressure on the material than 3000mm. Hence, a more accurate description of Hydrostatic Head is the measure of water pressure equivalent to a column of water that is 5000mm tall.

The higher the Hydrostatic Head value, the more water-resistant fabric is, and the more water pressure it can withstand before leaks.

What’s a good Hydrostatic Head value for a tent?

The larger the number, the better – but that’s not the whole story regarding how dry your tent will be.

A tent made from 3000mm HH (Hydrostatic Head) can keep you perfectly dry for most camping in the UK. You could buy a tent with 10,000mm HH but still get wet.

Hydrostatic Head is only one factor in a tent’s design.

The seams where the tent fabric has been stitched together must be good. That means double stitching for strength and taping over and sealing so that no water can enter via the stitching holes or between the two sheets of fabric.

Zips should also have protection. A good tent will often cover the zips so that water keeps away from them. Some tents even come with a few zip covers, and you can also get tents where the doors are protected with a small porch or ‘storm entrance’ as it is sometimes called.

Any entry point to the tent is a weak point in waterproofing, but a good design around the doors can help keep the tent interior dry.

Tents may come with a waterproof and UV coating in addition to the Hydrostatic Head of the material.

The additional waterproof coating will help water bead off your tent. The UV coating will protect the polyester-based fabric from degrading in the sun, reducing both the loss of colour and the degradation of the waterproofing.

UV and waterproof coatings can deteriorate over time and get damaged. Oils and even simple washing up liquid (and yes, kids blowing bubbles) could damage the coating.

You can get waterproofing sprays and seam sealers to repair patches yourself.

What about the groundsheet?

Groundsheets will not usually have a Hydrostatic Head rating as they are made from fully waterproof PU material.

With groundsheets, the thickness of the material is important to avoid wear and tear and damage from sharp objects on the ground – always check for twigs, sharp stones (and, unfortunately, even broken glass on some campsites) before you pitch your tent.

Can I increase the Hydrostatic Head of my tent?

No, but you can improve water resistance.

Additional waterproofing sprays and seam sealers can improve water resistance. Still, the Hydrostatic Head (how much water pressure the fabric can hold) is related to the physical properties of the fabric, such as how small the holes are between the weave.

If you are stuck with an extreme downpour or heavy wind-driven rain, an additional tarp over the tent may help. You may only need to cover one area of the tent, such as the side most exposed to the oncoming wind and rain.

In this picture, you can see the tarp we fitted to the side of the tent to provide some extra protection. We were pitched on a hilltop facing the sea during a bad storm. Here the rain was more horizontal (and even from below sometimes!), and the wind had shifted to this side of the tent where the side door was.

Unfortunately, some water started to wick through the zip, even though the zip was well protected – the gale force wind was lifting the zip’s rain covers! So a tarp came to the rescue, took the worst of the weather, and solved our problem. So, we did effectively increase the overall Hydrostatic Head of the tent.

We always recommend that you take at least one tarp when camping.

Conclusion

You should now be a little wiser about Hydrostatic Head and that it’s just a guide when choosing your tent, as many other factors can determine how dry your tent will be.