Whether you are looking for waterproof fabric to make your own raincoat, or to protect an item like patio furniture from the elements, it is important to understand the different options available. There are many different fabrics that can be described as "waterproof", but the only way to ensure that a fabric is truly waterproof is to test it under certain conditions and with specific equipment. Waterproof fabric is designed to withstand a large amount of water pressure, so it will not let moisture in or out. The most common method of testing for this is to use a hydrostatic pressure tester.
There are several types of materials that can be used to create waterproof clothing, such as PUL or PU (polyurethane) and olefin/neoprene (a flexible synthetic rubber). These fabrics are often used in items like jackets, pants, tents, boots and shoes. Other fabrics that are considered waterproof are oilcloth, which is traditionally made by coating cotton with a layer of oil, and vinyl, a synthetic material that is very resistant to wear and tear. Some people also look for waterproof fabric when making cloth diapers and incontinence pads.
Waterproof fabric is generally tightly woven,
and the spacing between warp and weft (the threads that run horizontally and vertically in a woven fabric) is very small. This makes the fabric almost impermeable when dry. The fabric is then treated with a water repellent, such as paraffin wax or soy-based oils, which reduces the pore size of the fabric to prevent further penetration by liquid water. The fabric may also be laminated or coated with a nonporous substance to prevent water vapor from passing through it.
Some waterproof breathable fabrics are also directional, such as Nikwax Analogy or FurTech, and can be used to direct both water vapour and liquid water away from the body. These fabrics would fail a strict hydrostatic head test, but they do keep perspiration from cooling the wearer and provide more comfort under heavy use.
Other methods for making fabric waterproof are to laminate it with a polymer
or to coat it with an elastomer or polyvinyl chloride (PVC) compound that gives the fabric stretch, while keeping the water out. This could be done either during manufacture or after production, as in the case of a finished product such as a jacket. Other methods include the use of a spray that can be applied to the fabric, or rubbing a wax or petroleum-based substance onto it. Regardless of the type of fabric used, it is important to allow the waterproofing treatment to fully dry before using the item. This can take a few hours, depending on the treatment and the fabric.