Geomembranes used in separation liners between old and new waste in vertical expansions of landfills may be subject to strain caused by differential settlement. Therefore, it is important to define a maximum allowable strain (MAS) that any given geomembrane can tolerate without compromising its required service life. In a number of instances some very low values have been proposed; as low as 0.25% to 1.0% for high density polyethylene (HDPE). Such low numbers are likely based on German regulations for HDPE landfill liners that require a maximum allowable global strain of 3% and a limiting local strain due, for example, to protruding drainage stones, of 0.25%. The low allowable strain values adopted by the German regulators have been based on products and practices utilized in the 1980s and do not reflect current conditions, nor do they address geomembranes other than HDPE. This paper presents the background and reasoning for updating the MAS values for geomembranes.