Waterproofing a Flagstone Waterfall: Liner Is Essential

Building a flagstone waterfall to a pond using Quikrete mortar. Asks about waterproofing additives.

No Additive Makes Mortar Waterproof

Mortar is inherently porous. Water migrates through rapidly—especially at stone-mortar interfaces. You can lose a pond's volume in a day.

Required: Waterproof Membrane

Install a waterproof liner or membrane under the mortar, sloped toward the pond reservoir. Any water penetrating the mortar hits the liner and drains back. Sealing mortar helps but isn't sufficient alone. Professional standard: never build without a liner.