Building Water Features: Expanding Foams and Fillers

Great Stuff (tm) polyurethane foam

Expanding foams and fillers are extremely popular in the construction of DIY  waterfalls, and you can rest assured that they are incredibly handy in some cases. When building outdoor water features, it’s important to understand the difference between latex and polyurethane foam.

Differences Between Latex and Polyurethane Foam Sealant

There are two key differences between latex foam and polyurethane foam sealant:

1. Latex foams are typically “open cell” and, as a result, can take on water. In fact, the same properties that allow you to wash latex foam off your hands with water also mean that the cured foam can absorb water. This can cause wood rot or deterioration in areas where wet latex foam is next to wood, such as a window frame. In contrast, polyurethane foam is a closed-cell foam. It forms a water-resistant outer skin when cured.
2. Latex foam does not expand. Polyurethane foam expands to thoroughly fill all voids and cavities making it an ideal air sealant.

Polyurethane foam can be shaped and molded to help direct water flow over and around rocks in a waterfall, rather than under and in between.  It can even be used to set and secure the cap stones in a retaining wall. Once cured they are water resistant and rot-proof, they provide some heat insulation.

Health Precautions

The EPA warns that spray foams pose a health hazard if not used with proper health protection (respirators and gloves) and work site ventilation. Spray polyurethane foams contain diisocyanates, which are potent lung and skin sensitizers (or allergens) and irritants.

How to use spray foams

Piston-type cans are available which enable the can to be used at any angle to reach difficult areas. It’s quite handy having a can which shoots out foam when you press the nozzle, but you need to remember that even once the foam is in place it will continue expanding. This often ends up with the foam becoming visible, but of course you can disguise it by throwing some fine gravel onto it before it sets.

Foam is of course ideal for using under a fall, particularly if you need something in place quickly. Using foam will also prevent water from being able to run underneath the stone. If you do use foams and fillers, make absolutely certain that your stones are in the correct place, bearing in mind that excess foam will make an unsightly mess on your rocks.

You should also make sure that you choose black foam, not white or yellow.  The black color allows it to blend among rocks and shadows and can fill, seal and adhere to other pieces without looking unnatural. Remember, if you use the common yellowish foam and it begins showing through between the rocks, your water feature is certainly not going to look as attractive as to what you had hoped for.

There are many different brands on the market to choose from. A good polyurethane foam is GREAT STUFF™ Pond & Stone Waterfall Foam Filler.  Whatever product you choose, be sure that it is fish safe and recognized by the International Professional Pond Contractors Association.