On the inside of my slate roof?

[slowlybutsurely]

Hope you can see the photo.

View media item 82501

Old house (100 yrs plus). Slate roof.

On the back-side of the slates, between the rafters is this white plastery stuff. In parts, the white stuff has dropped off.

What's this stuff called? It looks useful - how do I fix it please?

Thanks

 

[Nige F]

Looks like lime/sand/horsehair mix - the process is called torching just stops the draughts not really serving any purpose ,I`d leave it because daughts in roofspaces are good