Mintnick

i have been advised to replace the bedding of the coping stones and pointing etc
to cure my leaking roof, which got my purlins very wet. i have heard of many ways to do the work and one way to do the work is to use a damp course membrane on the party walls and place on top the coping stones - what damp course membrane/grade should i use?

Help wet purlins and brick work and chimneys are wet too? Flashings gone or pointing failed?