Good pond liner can be expensive, and that can be a high barrier to entry for a lot of prospective pond owners. Luckily, the same damp-proof material used to waterproof concrete can also be used to hold water.
DPM is short for damp-proof membrane, a class of waterproofing often used in construction to prevent water damage. All membranous pond liner (Polyetheline, tarp, etc.) are types of DPM. The best kind of DPM to use in pond lining are those produced to be used as pond liner, especially if your pond will have fish in it.
So what kinds of DPM are out there, and are there any kinds that definitely shouldn’t be used as pond liner?
DPMs With Algaecides
Algaecides are a double whammy for fish that have been introduced into your pond. Not only are algaecides poison for fish, but they also target the main food source for many fish, which is microscopic algae.
Because of this, any DPM that is treated with chemical algaecides should never be used if your pond will have fish in it.
This is the reason that pool liners make bad pond liners. People don’t generally like to have algae living in their swimming pools, so most commercial pool liners come treated with algaecides to prevent this.
While pool liner will be fine for fountains and other fixtures that you may not want to keep living creatures in, they are generally bad for ponds that will have fish in them.
DPMs That Break Down Easily
Most commercially produced plastic membranes will eventually break down if left out in the sun for too long.
Black painter’s tarp may last for a year or two, but it will need to be replaced eventually if you use it in your pond. Tarps that aren’t UV resistant fall under this same category, as they will also usually break down over time.
This being said, some tarps actually can make great pond liners. In particular, vinyl tarps that have a good underlay separating them from the ground make fantastic pond liners if you have them handy and can certainly last a long enough time.
Just make sure that what you have isn’t going to leak or melt away in the summer sun.
DPMs That Just Aren’t For Ponds
The EPDM liners that are used for roofs are not safe for fish. They’ll work for ponds that won’t have anything living in them or for fountains, but they just aren’t meant to be used to hold living things.
DPMs That Make Great Pond Liners
With all that out of the way, let’s get to the good stuff: Things that can definitely be pond liners.
Polyethylene comes in all sorts of forms, but most of them are well suited to use in domestic ponds. The only kinds that may be harmful to your fish are those that are meant to be used for waterproofing concrete.
Contact an expert if you’re worried that the specific liner you want to use might be bad for your fish.
I would recommend LLDPE, low-density polyethylene, for use in ponds. It’s cheaper than other polyethylene-based liners, isn’t toxic to fish, and should last around 36 years if you treat it well.
EPDM that has been produced specifically for pond lining is a very good pond liner. It won’t be cheap, since EPDM liner tends to be incredibly heavy, but it also isn’t toxic to fish like the kind used to line roofs and it’s extremely durable even in extreme temperatures. It’s also extremely difficult to tear, which is certainly a plus.
This makes it ideal in a lot of ponds. If you can afford it, it’s a pretty good choice.
Butyl is a similar substance to EPDM in a lot of ways. It is the most popular pond liner in the UK, and very cheap in the United States. It has a lot of the good qualities of EPDM, although it doesn’t have the same tensile strength.
This makes it easy to tear. It also doesn’t last quite as long depending on where you buy it.
Butyl is a solid budget choice that should do a good enough job for any small-scale pond without hurting your fish.
Vinyl tarps are both waterproof and resistant to UV rays. I’m not aware of any vinyl tarps that could be dangerous to fish, but just in case you may want to check the type of tarp you’re using before you use it.
That being said, if you have a lot of tarp lying around or if you just like the aesthetic of it, it’s a great choice for a pond liner.
Alternatives To DPMs
DPMs aren’t the only player in the game of pond liners. There are a few particularly strong challengers that you could use instead of the idea of putting a plastic sheet at the bottom of your nice-looking fish pool.
PVC is a very common pond liner in the United States. It is both versatile and relatively cheap, which is great if you’re on a budget. Most people can probably install a PVC liner on their own.
Unfortunately, PVC needs to be plasticized with chemicals in order for it to be installed. These chemicals are almost always toxic to fish, meaning that you probably can’t keep fish in a PVC-lined pond.
Clay is a great natural alternative to the various synthetic pond liners on the market. Clay is as waterproof as anything else, and most certainly not toxic to fish. It’s also comparatively cheap to purchase and install. The only problem is that you’ll need a lot of it.
It will take 8 to 12 inches worth of clay to do the job of only a centimeter of DPM, meaning that the hole for our pond will need to be much wider than the pond itself will be.
One way to solve this problem is to install a layer of lime just underneath a thick clay covering. The clay is important, as the lime could be damaging to any fish in your pond if it isn’t covered. However, used together the two may be able to save you some money compared to other liners.
Read our article on pond liner alternatives for more options.