I want to remove the bottom row of cement ties on my pitched roof, in order
to lay a strip of damp proof membrane under the lower edge of the felt
which has disintegrated where it projects from under the tiles.

The tiles are fairly larhge (I guess about 40cm x 50 cm or so). Each tile
is nailed, at the top, to a betten. How do I extract the nail, when it is
covered by the tile above? And how do I replace the nail when restoring the
tile to its original position, after the felt work is done?

Many thanks,

Al