It is possibly the most significant element to maintaining the 'health' of the wall and enhancing the appearance. The correct repointing can assist in waterproofing and stabilizing the walls of the structure, extend the life of the wall and individual stones and enhance the visual qualities of natural stone walls. The importance of this cannot be over-emphasised. Incorrect pointing can seriously reduce the performance of a wall and by implication a building. Water can become trapped behind dense mortar and not evaporate out, physical damage to stone and brick can result from poor work.

If the house is built using lime mortar, it is most likely that the walls are solid - they won't have a cavity. These walls need to be treated very differently to ones with a cavity. These walls need to breathe - the stone and brick from which they are built is harder, and less absorbent than the lime mortar, and any moisture in the wall escapes through the mortar joints. This can be moisture from condensation on the inside of the walls, or water which falls on the outside of the wall from rainfall. If the mortar joints are left to breathe, your walls will stay dry. The lime mortar gradually 'self sacrifices' over many years, and recedes into the joint, so that eventually, there is a slot between the bricks or stone.

The most common problem with old houses is flaking brick and spalling stone, which can almost always be traced back to when the joints were raked out, and cement mortar used to re-point them. If cement mortar is used, water will not escape the walls, and you will have damp problems inside the house as well. This is one of the common causes of so-called rising damp - it is nothing to do with rising damp, and the solution is never an injection damp proof course!

On an originally lime mortar pointed property, any cement pointing needs to be raked out. Stone masonry repairs are far more costly than re-pointing works; it's worth the investment to avoid larger costs at a later date. Although lime mortar is generally thought of as a much softer substance than cement, it can be mixed with a pozzolan to improve its compressive strength and help bolster it against weathering damage. Examples of pozzolans would be volcanic ash, heat treated clay, silica flume etc. 

Another issue which you need to understand is that these houses were built to 'move' - the lime mortar is flexible. It actually moves and self heals - if a small crack develops because the house settles a tiny bit, or brickwork moves, the crack heals - lime has this amazing property of actually taking in water and carbon dioxide from the atmosphere, and growing tiny calcite crystals which span the cavity of the crack, and tie it together again. Using cement based compounds is an absolute no-no - this introduces rigidity, and in an old house which needs to move, cement just cracks and falls off within a short period of time - it is useless, and a completely inappropriate material to use.