The way to Use Foam in Waterfall Construction
When you are stacking rocks to build a waterfall cascade, it’s important to do it in such a manner that water flows on top of them, instead of underneath. Because it’s not possible to stack stones tightly enough to guarantee this, you want a sealant to close the gaps between stones. Black waterfall foam is such a sealant. Like foam insulation, waterfall foam is a polyurethane product which expands when you spray it, sticks to virtually anything and dries hard. Not only does it prevent water from seeping under the stones, it binds the stones and keeps the structure together.
Construct a berm or incline for your waterfall and cover it with a waterproof liner. Stack rocks on the liner to obtain the configuration which makes the best waterfall. Use stones which fit together tightly to minimize the total amount of space you have to fill with the foam.
Remove the top layer of stones so it is possible to seal around the base of the stones in contact with the liner. Filling large gaps with small stones will help preserve foam.
Pull the cap off of a can of polyurethane and then screw to the plastic nozzle that comes with it. Push the plastic tube, which can be supplied, in the nozzle. Alternatively, screw the can onto a foam applicator gun, available where you buy the foam.
Shake the can and point the tip of the tube or gun at the bottom of one of the stones and pull the trigger. As the foam begins to come out, then move the tip along the base of this rock to the next rock to form a continuous bead. Maintain the tip stationary in massive spaces until the spaces fill with foam. When you are done, then all joints between stones along with the liner should be filled.
Wait for the foam to enlarge, and lay on the stones that form the upper layer of the cascade, working them in the foam whenever possible. Apply more foam as required to maintain the stones in place and maintain water flowing on top of them.
Camouflage the polyurethane by covering it with stones and small rocks while it is still moist.