405 TD Estate wrote:
My roof has no felt under the tiles - maybe it never did.

I assume it should have something. What is the best stuff to use, what
does it actually do (tiles are supposed to keep water out right?)

Your roof may vary.

In Scotland, for example, it's almost unheard of for this to be the
case, there is a skin of 18mm or so boards covering the roof, covered
with felt, on which are battens that carry the tiles.