Water-repellent fabric is designed to shed water, but will soak through in a heavy downpour. This fabric is both tightly woven and specially treated with a finish, such as a silicone spray, that causes the water to bead on the fabric's surface rather than permeate it. Over time or with chemical exposure, such as during dry cleaning, the finish can wear off.
Waterproof fabric is designed to keep you dry even in the heaviest rains. Usually a nylon-polyester blend, waterproof fabric is extremely tightly woven and nonporous, with tightly sealed seams and welded or covered zippers. The best and most expensive waterproof fabric is breathable, which means it allows air in and perspiration out, but not water through.
A fabric's assigned waterproof rating depends on how much moisture can permeate the material under extreme conditions. Permeation is measured in millimeters (mm) per 24 hours, or pounds per square inch (psi). The mm/24 hours rating indicates the amount of rainfall a fabric can tolerate in 24 hours without saturating. For example, a 10,000 mm waterproof rating is higher than 5,000 mm. Likewise, a waterproof rating of 5 psi is not as protective as a rating of 10 psi. Water-repellent and water-resistant fabrics typically keep you dry up to 5 psi water pressure, while completely waterproof fabrics can tolerate up to 40 psi water pressure, reports the "Waterproof Gear Guide" at sierratradingpost.com.
