Glass Reinforced Plastic (GRP) pipes are widely used as an alternative to metallic piping in challenging environments, such as concentrated acids. However, compatibility with the environment is typically described only in qualitative terms, and non-destructive inspection techniques do not provide data that can be used to accurately quantify in-service degradation. Consequently, remnant life prediction as the basis for maintenance and replacement of non-metallic piping presents a considerable challenge. The ISO-14692 standard provides a methodology for qualification of GRP piping that uses data from tests carried out with potable water, which can be scaled using a factor for other service fluids, although a default value of 1.0 is commonly used. This paper extends that design practice specified in ISO-14692 by incorporating the chemical compatibility factors as described in EN 13121 for design of GRP tanks and vessels. This procedure is used to calculate a chemical de-rating factor for long-term hoop stress, in addition to the standard design lifetime factor defined in ISO-14692. The combined de-rating relationship is repurposed to evaluate the amount of degradation under the actual service conditions experienced by the piping. Examples of this approach are provided for non-metallic piping carrying a range of service fluids in an operating plant.