ᐅ Installation of Slate Window Sills Indoors / Drywall Construction

Hello everyone,

For our new timber frame house, I am planning to install the interior window sills made of slate soon. This should, of course, be done before plastering and sanding so that everything can be finished neatly in the end.

However, I’m unsure about the best approach for this. That’s why I asked both the site manager and the supplier of the window sills for advice – but they gave me different recommendations.

The site manager thinks the window sills can be aligned using small shims and then the cavities can be filled with foam. On the other hand, the specialist company for slate recommends against using PU foam with natural stone and instead suggests using pozzolan cement or special window sill mortar designed for natural stone.

As you can see in the pictures, I don’t have a continuous flat surface to apply the mortar directly. There is a cavity between the drywall and the window frame. Should this cavity be completely filled up to the underside of the future window sill? If so, does the drywall on the inside need to be covered with foil or another protection beforehand to prevent moisture damage?

I’ve uploaded 2 pictures attached.

Best regards