hi
this is about a 1920s brick-built flat with slate roof. the whole building comprises 4 flats, (two ground floor, two upper). the slate roof is divided (above the party wall) by a small wall with coping stones, sealed along the sides with cement "skews".
recently rainwater has been leaking into both upper flats, affecting the corresponding rooms on either side of the party wall.
this has been diagnosed by quotes from two different roofers as being a problem with the skews (i think the diagnosis was based on their sounding a bit hollow when tapped). the solution is to remove the old skews cementing, replace any slates that get broken along the way, and build it back up again.
but one roofer has suggested a cheaper alternative would be to paint two or three coats of flexacryl.
i'm worried in case this might be a false or short-term economy: what's the expert opinion about using flexacryl for this...?
many thanks for your help!