If the exterior is sandstone then yes, they should use lime mortar, reason being (as I’m sure you’ve read if you’ve done a bit of googling)is that in order to prevent damage to the stone from the growth of minerals in any water inside them, the lime mortar acts as a more permeable medium, so water is drawn to that, rather than the face of the stone. Part of our house consists of an old sandstone church hall, and the previous owners repointed part of it using normal sand/cement, which has caused the front of the masonry to blow, elsewhere, where lime mortar has been used it’s sound. Lime mortar tends to have a mroe graded (bigger bits) of sand than modern stuff, pluss is generally lighter in colour.
Also, I’ve been using ‘Stormdry Masonry Protection Cream’ to coat the exterior of the hall to weatherproof it and attempt to stop ongoing damage to the badly repaired bits (googe it for more info/suppliers). It’s worked very well so far, and has been used on plenty of other historic buildings to no ill effect (I did a load of research before hand), whilst cheaper generic stuff (i.e. Thompsons Water Seal) can do bad things to masonry. Downside is it’s expensive, but it’s meant to last a long time so figure it’s worth it.
