British Standards set out to ensure that reinforced concrete structures provide the degree of protection, internal environment and functionality required by owners or developers. The term ‘Waterproof’ therefore conjures up a whole range of ideas. If the structure in question is to retain water, say a water storage tank, then the designer may simply work to the design requirements of the water retaining code, BS 8007, which sets out to limit crack width. This may be achieved by utilizing a good quality, ordinary concrete, close attention to joint details and providing extra crack control reinforcement; dampness being acceptable. If on the other hand the structure is habitable or designed to house sensitive electronic equipment or valuable archives, then simply designing to the water retaining code is not sufficient. This is especially so given that ordinary good quality concrete provides little resistance to the passage of water vapour. There are several degrees of waterproofing to be considered, all loosely defined under 4 categories in BS 8102.