Black Waterfall Foam 500ml - Expanding Pond Foam for Sealing Rocks

Building a waterfall or a rocky stream over a pond liner? This black expanding waterfall foam is the material that makes the water run over your stones instead of disappearing underneath them. Sold as a 500ml aerosol can, singly or as a pack of three, it is a black polyurethane foam that you spray into the gaps behind, between and under the rocks once they are in place on the liner. It expands to fill the voids and its black colour sinks into the shadows of the rockwork instead of standing out like yellow builder's foam. If you have planned a waterfall or stream with our pond planner, this is the finishing material it needs.

Why it works

It closes the hidden channels

On a newly built waterfall most of the water often slips through the gaps between and beneath the stones, so the face you built looks dry while the pump runs. Expanding foam fills those cavities and holes, so the flow is pushed up and over the lip of each rock where you can see and hear it.

Black blends into the rockwork

The foam is black, which is why it is sold for landscaping and waterfalls. Black reads as shadow between stones, and it takes a dressing of gravel or sand pressed into its surface while wet, so the finished joint looks like part of the rock bed rather than a repair.

Built for outdoor water features

The source describes this foam as weatherproof, UV resistant and fish and plant safe, and states that it bonds to both porous and non-porous rock, stone, cement, wood and cloth. A detachable nozzle comes with the can to help you place the foam accurately in tight joints.

• Black expanding polyurethane foam in a 500ml can
• Fills gaps and holes so water runs over the stones, not under them
• Weatherproof and UV resistant, as published

Where it's used

Garden pond waterfalls and cascades, rocky streams and rills over a flexible liner, header pools, spillway stones and rock edging. It is also useful when renovating an older waterfall that has started losing water behind the rocks.

Specifications

Choosing the right option

A single can suits a small cascade, a short run of rock edging or patching a leaking waterfall. Coverage per can is not published for this line, so judge the quantity by the number and size of the gaps rather than by the length of the watercourse. Allow generously: deep voids behind large stones can take a surprising amount of foam before the joint is closed. The pack of three is the better buy for a new waterfall with several steps, a stream of any length, or a build where you would rather not stop halfway through to reorder: three cans cost noticeably less than three single cans bought separately.

Installation, use and care

Lay and settle the liner first, then place your rocks and run water over them to see where it escapes. Turn the pump off, shake the can and foam the gaps behind and under the rocks, working from the bottom step upwards and filling in short passes, because the foam keeps expanding after it leaves the nozzle. Press gravel or sand into the wet foam to hide it. Let it set before running water; the cure time is not published, so follow the instructions on the can. Wear gloves and old clothes, as uncured foam is very hard to remove from skin. This is a sealing and gap-filling foam, not a structural adhesive: bed heavy stones securely on their own and never rely on foam to hold their weight.

Frequently asked questions

Will the foam stick to my pond liner?

The published bonding list covers rock, stone, cement, wood and cloth; liner is not listed. In practice the foam's job is to fill the gaps between stones sitting on the liner, so test a small area first if you need it to grip the liner surface itself.

Is it safe for fish?

It is described as fish and plant safe. Let the foam set fully before water runs over it, and keep overspray out of the pond while you work.

How much does one can cover?

Coverage is not published for this line. It depends heavily on gap size, so for a multi-step waterfall or a stream the pack of three is the safer choice.

Can I trim it once it has set?

Yes, set foam can be cut back with a sharp knife, but cut faces look less natural than a gravel-dressed surface, so apply in short passes.

Ordering and delivery

Choose a single can or the pack of three and add it to your basket alongside your liner, underlay and pump. Delivery charges and estimated delivery dates are shown at checkout before you pay. Unsure how much you need? Contact us with the size of your build.