TECHNICAL: What does waterproof mean?

Rain has returned to the Lakes. Within minutes the email and Trail forum was buzzing with the question every walker wants to know about. How to stay dry and why am I getting wet in my waterproof jacket!

To stay dry on the hills you need clothing made from fabric that can withstand the driving force of rain. To be called waterproof a fabric must be able to withstand 2.1 pounds per square inch (psi) to conform to British Standards for low activity use such as golfing and fashion. For high activity such as hill walking and skiing the fabric be able to withstand 7.1 psi.
To give you some idea of what this means, 1 psi is roughly equivalent to rain hitting the jacket at 35mph, while it is possible to exert a pressure of 15psi on the fabric when kneeling. Sadly very few manufacturers state how waterproof their fabric is and those that do use so many different testing methods that it is difficult to compare fabrics.

Levels of waterproofness
So Trail uses a league table to compare levels of waterproofness as follows:

Water Resistant - means ‘water resistant’ and will only resist light rain
Waterproof - meets the ‘low activity’ British Standard to be called waterproof
Very waterproof - meets the ‘high activity’ British Standard.
Extremely waterproof - means it can withstand a pressure of 25psi, so it offers a very durable level of waterproofness

To complicate the story, Paramo garments are not waterproof when using the terms described above, as water can be forced through them quite easily if you lean on a wet area of ground. But when used for walking they keep you dry as body heat pumps the sweat away from the inside of the jacket and as the garment comprises of two very water resistant layers, driving rain does not pass through the garment. So although they are not truly waterproof, you’ll stay dry when wearing them for walking in driving rain.

Design effects on waterproofness
While a jacket may be made from waterproof fabric, its design can easily allow water to leak in. The seams should be seam sealed, but on lower priced jackets the sealing tape is sometimes poorly applied and allows water though.
The front zip can leak easily if it is not protected by a system of storm flaps. There are now many jackets with unprotected water resistant zip fitted. These leak quite easily, especially when the zip is stretched of flexed in a convex manner to expose the teeth, which tends to happen when a waist belt of a rucksack is buckled tightly against the waist area of a jacket.
Pockets with openings that are not protected by a double storm flap can easily fill with water and if they are mesh lined this water will leak into the jacket.
Water will always follow gravity along a route of least resistance, so it tends to follow creases and folds in the fabric, it follows channels of fabric like water flowing down a ravine so not surprisingly water easily finds it ways down the neck of a jacket through a front zip not fully zipped up and down the sleeves when climbing.
Some designs of jackets actively encourage water to enter the jacket by having designs that catch the water and encourage it towards zips and other openings. For example if a single storm flap on a chest pockets of a jacket lies diagonally or vertically and faces towards the front of the jacket it will collect water when walking into the rain. In high winds the water will be forced through the zips. So look for storm flaps that face down the jacket so that they don’t collect water even in driving wind and instead force running water to flow away from zips rather than towards them.
In fact it it’s amazing that any waterproofs actually keep us dry in the hills. But on the whole they do a good job if looked after properly.

Durability
The durability of the waterproofness varies. A jacket with minimal levels of waterproofness will tend to leak sooner than a jacket with the highest levels of waterproofness. This is because the waterproofing treatments applied to the materials tend to wear off with use.

In tests carried out by Cumbria Trading Standards in 1998 on jackets priced between £15 and £50 pounds, only the Craghoppers Pakka Jacket passed the British Standard for Water Repellency (BSEN 20811). This suggests that jackets that meet the lowest levels of waterproofness may lose some waterproofness through manufacturing due to quality control problems for example.

In tests carried out in 1999 by George Fisher of Keswick (George Fisher Online) the majority of waterproofs costing under £80 started to leak inside 3 years.
Jackets costing between £80 and £100 performed much better and it was only after three years that they started to find more failures than non-failures, with many jackets in this group maintaining waterproofness for 9 or 10 years.
Jackets priced at £150 and over using fabrics such as Gore-Tex, Sympatex, Triplepoint, Rab Downpour and H2No for example performed the best with only 1% of them leaking inside 3 years and over 80% of them maintaining their waterproofness for up to 6 years or more.

So it appears that higher priced jackets are generally made from more waterproof fabrics and that these fabrics remain waterproof for longer. In other words you get what you pay for.

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Reviewed by Graham Thompson