Seal natural stone window sills from the inside?

Hello everyone,

I am new here and hope for an answer to my question, if possible also with a justification.

Tomorrow is the day that we will install the interior window sills made of natural stone (Steel Grey). The slabs are 2cm thick and are to be fixed with cement mortar. My question: do the window sills have to be fitted with a sealing tape? I have heard something about that, but it somehow doesn’t seem plausible to me.

Best regards Micha

Where is this supposed to go? Our FB have a silicone joint to the window, subtle in clear silicone. The wall connections are sealed with acrylic. Spackled from below. I see no need for a tape there.

No, of course not. They are normally only set into the mortar bed.

Thank you very much for your answers, we have omitted the sealing.