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?)

What is does is to stop high winds lifting the tiles actually.

Its there to prevent massive pressure surges under the slates or tiles.

It should have no impact on waterproofing at all.

In high wind areas, board is better.