Pond Underlayment provides additional protection for flexible rubber or plastic liners. Since patching systems require a clean and dry surface on which to bond, a puncture to the liner typically means draining the pond for a repair. Water is easy to move and replace, but beautiful fish and other aquatic creatures are often not cooperative.
The warranty provided with a pond liner varies according the material. PVC liners, which are among the least expensive but most flexible liner materials, generally carry only a one year warranty. Firestone EPDM PondGard is considered one of the best materials for backyard ponds, because it is also quite flexible and comes with a 20 year warranty. For large ponds, polyethylene liners are usually used. Although they are less flexible than EPDM, they carry the same 20 year warranty and are less expensive.
Warranties are the manufacturers’ guarantees for product performance against temperature variations and ultraviolet light. They do not address punctures or tears caused by sharp-edged rocks, constantly growing tree roots, or inquisitive burrowing rodents. Underlayment protects against these hazards and should be a routine part of all pond construction.
Many DIY pond enthusiasts seek to minimize construction costs by skimping on the underlayment. The use of everything from recycled carpet to newspapers and cardboard has been recommended in DIY online pond articles. The truth is that pond liner underlayment is not expensive when compared to the cost of the liner, and the extra peace of mind it provides is well worth the additional cost it incurs. Using materials not especially designed to be underlayment is a false economy, because these materials will decompose under the liner, any puncture protections they initially provide will be lost, and they are not particularly resistant to punctures in the first place.
Geotextile is specifically designed to be landscaping, soil stabilization, and pond underlayment material. They are highly puncture resistant and do not deteriorate when exposed to water. Because they do not break down in the water, they can be used as excellent padding material under large rocks placed inside the pond on top of the liner. They are breathable materials, and allow gas and water to pass through them when used as underlayment.
Woven geotextile fabric has the consistency of burlap. Nonwoven geotextiles are more like a thick sheet of felt, created by a mechanical needle-punch process that entangles fibers using thousands of serrated needles. The dense fabric formed by this process is then stabilized through thermal or adhesive treatment. Both types can be used as underlayment, but the nonwoven type is generally used.