timegoesby wrote:
I have looked at a terraced house and there is no felt under the tiles.
A couple of slates need replacing and others renailing. Does felt
have to be there? Would a building society refuse a mortgage on the
place if no felt under the tiles?

The felt does not have to be there, and it's not so long ago that
it wasn't used. The B.S. or bank will be mostly interested in
whether the house is worth the money, so that their loan is safe.
They may make it a condition of the loan that you repair the roof,
so that their investment doesn't deteriorate.

It sounds as if the roof is well on the way to needing re-doing.
For a terraced house, this shouldn't be very expensive, make sure
that particular attention is paid to flashings and secret gutters
(if any). You'll get felt with the new roof, if you have it done.
Nails should be copper or stainless. You may find that some of
the slates are re-usable - I'd consider slates as a strongly
desirable choice for re-roofing, rather than tiles, at first call.