Requirement of a geotextile underlayment (under the liner) with our BTL and PPL liners is relatively rare. It is always a judgment call, but we don't require the underlayment unless it is impossible or impractical (cost ineffective) to achieve an acceptable subgrade any other way. The "other way" is usually to haul in and spread clean fill over the entire area to be lined. Often it is cheaper and faster to place an underlayment over angular rock subgrades than the "other way". Underlayment is also sometimes used as a means to allow the escape of gasses, such as methane, so it is not trapped under the liner. Use of the geotextile as an overlayment (over the liner) is normally to achieve a separation between the liner and any type of fill, rock, gravel, rip-rap, etc., that is to be placed on top of the liner for various reasons. As an option, we often sell a "ribbon" of geotextile overlayment (typically 8 oz. per square yard non-woven needle-punched polypropylene, whew!), usually simply referred to as "geo", which is installed over the liner on the upper perimeter and keyed into the anchor trench or shelf along with the liner itself. This "ribbon" is normally the roll width of either 12.5 or 15 feet. Prior to filling a pond, some minimal amount of fill must be placed over the entire ribbon so that it is not prone to floating. Use of this "ribbon" is an option and not mandatory. However, it is becoming increasingly popular for a number of reasons. We know from experience that any form of physical damage (from deer, kids with rocks, mechanical equipment, UV degradation, etc.) is by far most likely to occur on the upper edges of the pond liner, not out in the depths of the pond. Also, an added degree of safety is introduced with this option. The upper edges are made less slippery so kids, dogs, deer, etc. can get out more easily. So while not "necessary", this option is a relatively cheap form of "insurance" in a number of ways.