Wall tiling in my shower ?

Hi, question about wall tiling in my shower/bath.
Is the waterproofing done underneath the tiles, or is it done over the surface of tiles?
The reason I ask -- the wall tiles in my shower seems to have lost its seal. I'm trying to figure out if I must tear out all the tile, apply a seal, then retile, or do I simply apply some type of sealant on top of the existing tile?
I'm open to any suggestions... simpler the better.
Thanks
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You have to be more specific to get the right advice.
If the tiles are typical glazed ceramic, they should be fine. In other words the tiles should not lose their seal. The next thing to check is the grout, is it sound, or cracked and broken? If its sound, you may just need to reseal the grout. Visit your tile shop or home center for good quality grout sealer.

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Martin Lynch pitifully trolled:

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