how to seal roof tiles?

[S S]

I have a flat roof with floor tiles (sunroof) Whats best way to seal them, I dont want the expense of retiling. As yet there are no leaks but would like to seal as a precaution.

thanks

[Goedjn]

I was under the impression that the tiles in that situation were only there to protect the waterproof membrane underneath. Are they supposed to be sealed?