Graham Thompson
GT – Trail’s technical editor for over 25 years – is our walking gear guru.
Why is the hydrostatic head (a measurement of waterproofing) figure so much lower on tent flysheets than on waterproof jackets? Steven Murphy, via Facebook
The minimum for a fabric to be called ‘waterproof’ is an ability to withstand water at a pressure of 1.4 pounds per square inch (PSI), which is equivalent to a hydrostatic head (HH) of 1m, or 1000mm. In theory this is enough to withstand rain driven by a wind of 35 miles per hour. With jackets we often see fabrics rated as having a 10,000mm or 20,000mm HH and we know they can still leak, so how tent manufacturers can use materials with a much lower HH (down to 1,200mm) is a logical question.
Firstly, tents are not used every day, so suffer less abrasion during their lifetime. There are also fewer folds of material, putting less stress on the material compared to a waterproof jacket. If we made tents of the same material as jackets they would be heavy and expensive, but because they’re not exposed to the same levels of wear and tear as clothing, they can be made from lighter, less waterproof material. But the key factor is that a tent keeps off the rain through a different mechanism to a jacket. If well designed with minimal seams and a tight flysheet, the fabric causes the rain to bounce off the surface and roll down the material to the ground more effectively than on a jacket.
There are also different treatments used on tents (such as silicone) that encourage the rain to run off the material more rapidly. Because these coatings can’t be seam-taped they’re not suitable for use in waterproof jackets, but in a tent with few seams they are ideal.
In a waterproof jacket that is worn every day, UV degradation from sunlight will also have a greater impact – whereas a tent is often only erected for short periods of time during daylight hours. As such, most lightweight tents successfully stay waterproof with flysheets rated as having a hydrostatic head of around 1500mm, with higher HH fabrics reserved for the groundsheets which are subjected to more pressure. The combination of HH, UV protection, coatings and tent design plays an important role in water resistance, so basing choice of tent fabric only on HH is not really the best way to compare models as there are so many other factors involved in staying dry.