Fairly big lumps of cement are detaching from below the ridge tiles. To put this right, would I be better to build it out from the bedding a bit at a time, allowing overnight drying in between, or simply do the whole cement fill at the same time? Would PVA help in this situation? The reason I ask is, I believe it's been repaired before and has, perhaps, been filled all in one go. The other issue may be the cement mix, it would help if I had some idea for the build out (if appropriate) and if the final pointing mix, if different? It would seem, for whatever reason, ridge tiles are usually bedded one day then pointed another day! Thanks.