The material chosen for waterproofing and the thickness needed will be determined by the severity of the hydrostatic pressure and environmental conditions to be expected for the life of the structure. For a material to prove satisfactory for a waterproofing liner it must be impermeable, durable, flexible and extensible, amenable to wide range of temperatures, and able to resist soil water and algae. The need for an impermeable material is self-evident; any material which would permit the passage of water or water vapor would defeat its primary purpose. Because waterproofing generally is expected to last the lifetime of a structure- 30 to 50 years or more- it must be able to withstand the rigors of the exposure to which it will be subjected. A waterproofing sheet must also be flexible and extensible to conform to the irregular surfaces against which it will be placed. Natural and synthetic rubber have been used for waterproofing sheeting materials with good results. They provide all the necessary properties for such an application with outstanding ability to resist ripping and puncturing. Neoprene synthetic rubber is another popular material for this use and has been used for a number of large and small jobs. Polyethylene by itself is suitable for use as a barrier to water vapor and water, especially in slab on grade construction. Asphalt reinforced with cores of glass fibers and other materials or bonded to high strength backings is another popular waterproofing sheet. Installation procedures will vary with the type of flexible sheet used; manufacturers' instructions are ordinarily explicit and should be followed carefully to achieve a truly watertight result.