While it's technically possible to waterproof over existing waterproofing, it's not something we would recommend. As an alternative, we recommend that you invest in corrective waterproofing, such as those offered at Remedial Property Solutions. You can waterproof the new installation. After preparing the old tile, use the fine-setting mortar modified with high-strength polymers and apply a thin layer of mortar to the existing tile to obtain a flat surface.
Next, apply a liquid-applied waterproof membrane to the cured skim layer. Next, glue the new tile to the cured waterproof membrane. Make sure that the floor-to-wall transitions are waterproofed. It's best to climb the wall at least 3 inches.
All transition joints must be sealed with an ASTM C920 sealant. Wet-Seal Enviro-coat is a water-based, latex-hybridized polyurethane waterproofing membrane, with a single component and very durable. We do not recommend applying any type of coating to the existing ceramic tile to make the installation waterproof and non-slip. An experienced commercial roofer can perform all the necessary steps to properly repair the waterproofing membrane.
Theoretically, the only way to waterproof the roof with this illegitimate approach is if the shingle is actually waterproof and there are no other openings or adjacent areas that allow water to pass underneath the shingles. Fiberock fiber-reinforced, water-resistant gypsum backing board is water resistant, but they don't say it's waterproof. But if you install a mortar bed, you must apply a waterproof membrane over the mortar bed to minimize efflorescence. Often, waterproofing membranes have a “short” drying time, but this does not include the need to apply several layers during the waterproofing process.
It's important to remember that knowing how to properly waterproof surfaces can help you find the best way to protect them. The right thing to do is to remove the stone and reinstall it correctly with a properly installed waterproof membrane. I bought this house about 5 years ago and recently found out that no waterproofing had been done on this house. If you do, it can't be with the Laticrete Hydro Ban waterproof membrane, as you have to cover it with a tile or a thin-layer mortar.
Using an ASTM C920 sealant (sealant) alone will not replace the lack of a suitable waterproof membrane under the roof. One method that, in theory, can waterproof a glazed tile installation after installation is to remove the cementitious grout and replace it with an ASTM C920 sealant, for example, a 100% silicone sealant. Technically, it doesn't make the tile installation waterproof, but in fact it could work or not. It must have a primary waterproof membrane that is a layer of tar with a hot mop or a sheet of material that is properly applied to the inclined subfloor.