TIP 1: USE LIME
The number one tip for any work to a traditional building (the generally accepted definition for which is those built pre-1919) is to use lime, and never cement. All conservation books will highlight this fact, so there is no need to go into great scientific detail other than to say that lime is a fundamental material for building conservation. It allows the building to perform as it was originally intended; to ‘breathe’ by allowing moisture to both permeate through walls and to evaporate out.
Cement does the very opposite, as it is intended to keep moisture out. Typically, problems occur when existing moisture is sealed inside and therefore has nowhere to escape. This will cause salts to migrate through the walls and cause any gypsum plaster to crack and fall off in time. The tell-tale signs of salts will be white efflorescence. Often this creates a vicious cycle, as the usual remedy is to call in damp treatment companies, most of whom will recommend chemical damp-proof injection which will not treat the root cause and instead will exacerbate the problem (see Tip 2).
As highlighted in Tip 4, most contractors do not understand why cement causes so many problems with traditional buildings, which is partly because college courses do not explain this in their curriculums. Many may pay lip service to the conservation approach by adding some lime, usually the most common hydrated form, to cement, but again this will not solve the problem and just serves to demonstrate why they should not be working on such properties.
The fact that they may say they have been working in this way on such properties for many years just means that they have been causing problems for this time, so beware, and always ensure that you use a contractor who truly understands how traditional buildings work.