ISO 21004:2006 specifies the general requirements and the performance requirements for multilayer pipes based on thermoplastics intended to be used for water supply outside buildings, for buried water mains and services and for water supplies above ground.

ISO 21004:2006 gives guidance for the design of piping systems consisting of multilayer pipes based on thermoplastics or, for which at least 60 % of the wall thickness is polymeric material, and the inner layer in contact with water is made of polymeric material.

The polymeric layers used for the stress-bearing layers are selected from polybutylene (PB), polyethylene (PE), crosslinked polyethylene (PE-X), polypropylene (PP), chlorinated poly(vinyl chloride) (PVC-C) and unplasticized poly(vinyl chloride) (PVC-U).

ISO 21004:2006 is applicable to piping systems used for the conveyance under pressure of cold water (up to approximately 20 °C) for drinking. It is also applicable to piping systems for the conveyance of water (up to and including 40 °C) for general purposes.