Waterproof fabric is a natural or synthetic fabric that has been coated with a substance to repel water. Common waterproofing substances include polyurethane, polyvinyl chloride, silicone, and wax. The term "waterproof" applies to the fabric only, not the entire garment. Even if a fabric is entirely waterproof, the seams must be sealed or taped and the zippers must have storm flaps, or the garment is merely water-resistant.
In most cases, waterproof fabric is subject to laboratory testing and must conform to industry standards. A waterproof garment will often be labeled with a rating that denotes how much liquid the garment can be subjected to in 24 hours before the wearer gets wet. For example, a rain jacket rated "20K" can withstand 20,000 millimeters of rain before the waterproofing fails. A rating of 20K is considered excellent, and is usually expensive. Ratings of 5K and 10K are more common, especially for non-specialized, inexpensive rain gear.
Woven fabrics with a waterproof coating have millions of tiny pores, about 1/20th the size of a rain drop but much larger than a molecule of water vapor. These pores allow body heat and moisture to escape while keeping liquid water like rain out. This breathability can also be rated, in grams of water vapor per square meter per 24 hours. For example, a breathability rating of 25K means a garment can allow 25,000 grams of water vapor to pass through a square meter of the fabric in a 24-hour period.
Polyvinyl chloride and polyurethane can be made into clothing themselves, without fabric backing. Such garments are waterproof, but they do not allow the skin to breathe. This is often remedied by incorporating vents or armpit zippers into the design of the garment.
Silicone and Teflon® are considered very effective coatings for waterproof fabric. They form an effective moisture barrier without changing the look or feel of the original fabric, and they also preserve the moisture-wicking capabilities of the material. Rubber and wax are rarely used to coat waterproof fabric because they change the look and feel of the material, and exposure to the elements can cause them to crack and melt.
Once damaged, the waterproof membrane of a fabric cannot be repaired. Higher-quality waterproof garments also have a "durable water repellent" (DWR) finish that tends to wear out long before the membrane breaks, but there are sprays available that will restore the finish and allow water to bead rather than soak in. These same sprays have also been used by crafters to waterproof material at home with varying degrees of success.