One of the most common questions we receive is whether it is okay to use roofing liner for ponds instead of fish-safe liner. I see the draw to using roofing liner due to the reduced cost but it is not worth the risk. Let me explain the difference between the two and why it is wise to only use Fish-Safe pond liner.
The main difference between roofing liner and Fish-Safe pond liner is the formulation of ingredients. In either case, certain ingredients are interchangeable but will result in the same final product. The use of one ingredient over another is determined by current market price of the ingredients. However, one or more of these ingredients may be toxic to fish in the form of leachates.Fish-safe pond liner cannot contain any of these ingredients. This means stricter quality control and the manufacturing being limited to the use of only those ingredients which have proven to be safe.
Another difference between roofing liner and pond liner is that roofing liner is treated with antibacterial compounds and the pond liner is not. The roofing liner has the treatments necessary to keep fungi and other pollutants from growing on the roof surface breaking the liner down prematurely whereas the pond liner needs to promote bacterial growth as a benefit to the pond.
This in no way means that a piece of roofing EPDM is unsafe for fish, but it does mean that there is no guarantee that it IS safe. The use of roofing liner in a pond that will house fish is a crap shoot that could get expensive, in more ways than one, if all your koi and goldfish get sick or even die.Furthermore, tearing the pond apart and replacing with a fish-safe liner is going to cost much more than doing it right the first time.