There are two basic methods to waterproofing a fabric, membranes and coatings. Both of these methods are applied to the inside of the outer fabric of a jacket (or overtrousers).
Sometimes called laminates, membranes are fine films that are bonded to the back of the outer fabric. These layers are incredibly thin, at Rab we use membranes that range between 7 and 30 microns in thickness – for reference, a strand of human hair is around 100 microns. Finer membranes will allow for greater breathability but regular use can lead to a loss of waterproofing. To avoid this we use laminates of the highest possible quality.
Most of our waterproofs use one of four fabric technologies, GORE-TEX®, Pertex Shield®, Pertex Shield®+ and Proflex™. Each technology is different both in terms of membrane composition and how they are manufactured.
Generally speaking, this is the simplest way to waterproof a jacket. We simply apply a liquid coating to the back of the outer fabric in the same way that you would apply paint with a roller. When the liquid dries it forms a solid (but breathable) barrier. The more layers that are applied, the more durable the waterproof coating will be.