Reset Flagstone on Spa Waterfall

[MarkSD100]

Looking for best solution to repair flagstone that came off spa waterfall. It came off several years ago and I used a waterproof construction ahesive. But that only worked for a few years and the water always seeped under the flagstone. Should I use waterproofing motor instead?

I run the spa spillway each night which used to leave the water level with the top of the flagstone. With the seeping under the flagstone, the water line is always below the flagstone.

Thanks,
Mark

 

[JoyfulNoise]

Flagstone is a very porous stone material that really isn’t great in submerged or constantly wet applications. It basically acts like a sponge and soaks up water. In order to use it as a spillway surface, it really needs to be sealed with a waterproof sealing agent.

But to answer your specific question - yes, you need to use a waterproof mortar material, not adhesive. The stone backing and the underlying gunite shell needs to be cleaned of all old material and then the flagstone need to be reset on to a freshly laid mortar bed. After the mortar has set, I suggest finding a highly quality, penetrating stone sealer/densifier (something based on siloxane ) to waterproof the top surface. That should reduce the amount of moisture incursion and hopefully keep the stone in place. Make sure that any cleaned and exposed gunite pool shell had a layer or two of waterproofing barrier painted on to it. Sikaflex and Laticrete make several products for that application.