We used standard grout for our flat, tiled roof. After about a yeas, heavy rain would come through ib patched and on close inspection there were tiny cracks between grout and tile in a few places. We used a common waterproofing sealant (Wilkinsons but other makes available here). This is a wax suspended in white spirit, like Waxoyl for cars. Painted on 2 coats into the grout lines and no further problems in 5 years since. Also effective on hairline cracks in monocap or rendered walls.