It is normally best to remove the tiles, mortar bed and defective membrane together with any render up say 100mm and re-waterproof structural elements.
It may however be worth trying a clear sealer.
Water normally penetrates through the grout and especially via cracks at the perimeters. If there are any cracks present, then sealers will not be effective unless the cracks are ground out and sealed with Joint Urethane Sealant.
Since sealers will encapsulate any dirt, mould or stains, these should be removed with a Gerni high-pressure water jet, possibly after soaking with bleach, to provide a clean surface.
We offer these possible solutions:
1. Wrimco Clear solvent based acrylic sealer (hydrophilic) - Gloss finish
Brush grout and then roll over entire area. Repeat when dry. Leaves a gloss finish that highlights the dark colours in the tiles and hence must be used over the entire area for uniformity of appearance. If tiles are glazed, only apply to the grout.
2. S900 Clear solvent based siloxane sealer (hydrophobic) - Matt finish
Leaves a matt finish with little effect on colour. Mostly used on brickwork or porous tiles, including shower floors. If tiles are glazed, only apply to the grout.
Penetrates more than the above and hence provides protection even after surface wear due to traffic. Glazed tiles should be tested for adhesion.
3. Re do the waterproofing over the tiles with liquid membrane.
4. Torch applied sheet membrane colour light grey - Wrimco SBS Granule. Suitable when the pleasing appearance of tiles is not a priority and the deck is little used. Attention to perimeter detailing is required but is not normally a problem. No need to fix cracks due to the elastic properties of the 3mm thick SBS membrane with 1mm of ceramic chips.