If floor tiles are laid over a flat RCC roof, will they prevent water seepage permanently?

Floor tiles wont prevent water seepage, yet it will reduce it a lot especially when provided with quit good slope.

I don't know about your site preferences, but in general the tile are placed in following way,

Adding waterproofing agent to the slurry is a very bad practice. It will not stop water penetration. The waterproofing coat has to be applied on the RCC roof and then covered by screed. Recommend you not to suggest such practice.

• The lean mix( mix of cement and sand with low cement content) is spread over the levelled area.

• Then a slurry of cement(water+cement) is poured on this lean mix and the tiles are directly placed over it.

So at our sites we simply mix water proofing agent(preferably Dr.Fixit) to the cement slurry to avoid water seepage.

And as per my experience it worked quit well for our sites even for bathroom tiles.

Hope this helped…

Comment for further discussion…

Edit :

This not the proper / standard procedure of waterproofing and not recommended as per the IS Code.

Yet the above solution is kind of desi / jugaad type solution which works for small/unimportant sites.