I have an old property and the lime mortar that is on the inside of my tiles in the roof is wearing away and am considering removing it and replacing it with Polyurethane spray foam. Is it a risk to spray it direct onto tiles from the inside as I understand when the foam expands it could dislodge the tiles and make them loose, is that right?

What kind of tiles, and where in the world are you?

They are slate tiles and am in Cumbria.

If it is anything like the material in this video you certainly wouldn't want to use it.
Polyurethane foam expansion - YouTube (http://www.youtube.com/watch?v=xjap74m4228)
Also, the resistance to ultra violet rays would have to be checked into.
Could you provide some pictures which might give someone an idea and how long was the lime mortar in there-- is it still possibly the best material?