Replacing Rendering on Stone Walled Outbuildings Information, help and solutions to many issues encountered when converting barns and older buildings

[Thehighlandpiper]

I have two small outbuildings with stone walls, stone slab floors and stone tiled roof.
They are about 120 yrs old and were the original wash houses. They still have some of the original rendering on the walls and I would like to strip and replace this. I have no idea with what or how?
I am totally new to all this so can anyone advise?
Is it possible to make the roof watertight without having to remove and redo it?

[welsh brickie]

You can use a club hammer and bolster to remove the render, or hire a jack hammer,
As for the roof, if the timbers are ok and you don't want to remove the stone tiles, then sprayfoam is the only way that I can think of to make it watertight