Or so I was always told...but it's not good manners which present problems when it comes  to re-pointing, it's the inappropriate use of modern(ish) materials, such as Ordinary Portland Cement, which, while it has excellent qualities of its own, was not the original material of choice for sticking things together.

Since people  first started putting one on two and  two on one, they've been using lime based mortars of various mixes and strengths to ensure the walls they were building not only stood upright but also fulfilled other, equally important, design functions.

These  included preventing rainwater from invading the inside spaces and  also  allowing the building to 'breathe', so that any moisture produced by the occupants inside  the structure escaped to the outside.

It was the properties of lime that allowed these things to happen, and the modern preference for the use of Ordinary Portland Cement (OPC), can be a major contributory factor in a building's inability to function as it was intended.

Take a look at the drawing above: It represents a stone wall that originally had lime mortar beds between the individual stones, and was also pointed with a lime based mortar.  The top half, as depicted, has been re-pointed using sand and OPC  mortar .

The inside of the wall could either have its original internal finish, which would have been of lime plaster, or it could have been re-plastered using a gypsum based, or other coating.

The properties of lime and cement mortars are quite different when it comes to the way they handle moisture and this is demonstrated in the drawing.  A strong cement mortar is almost impervious to moisture transmission; while a lime mortar isn't.  Lime mortar's more open structure, whilst not allowing water to soak through like a sponge, nevertheless has the abiity to allow it to pass, whilst keeping its (mortar's) form and other characteristics.  It also has better flexibility and, because it's less dense, better insulation qualities than an OPC based mortar.  Finally, a lime based mix is invariably weaker than the surrounding stonework whereas some cement based pointing isn't.

As mentioned, the drawing shows the three stones to the top half of the drawing (above the dotted line) have been re-pointed using OPC and the bottom three have been re-pointed using lime mortar.  In the case of the OPC mortar, when rainwater strikes the stone, some of which is invariably porous, it will soak into it and migrate downwards towards the the joints between the stones.  At this point it meets the sand and cement mortar joint which, because this joint is impervious to water, will deflect it inwards.  Now; every pointer worth his salt will have ensured that his mix is pushed as firmly into the joint as possible and to make sure this is the case, he'll no doubt have run a shaping iron over it prior to 'clipping' it off. This will almost always mean that the new pointing is  in contact with the original lime mortar bed.  Result: moisture will begin to soak inwards, via the original lime mortar bed, towards the plasterwork and eventually show up as a damp patch on the internal wall.

Now, take a look at the lower half of the wall.  Here the re-pointing has been done using a lime based mortar which allows the water to soak outwards, away from the original mortar bed and thus away from the inside surface.  When it stops raining, evaporation will dry out any remaining moisture from the wall

With both methods of pointing you're unikely to get a water-tight joint between all of the pointing material and the stonework.  It may be pretty good when it's first applied but, as time goes by, the joints will gradually separate and water will soak in.  This applies more to the OPC joints than the lime joints because of the inflexibiity of the OPC.

Inflexibility means it has less chance of coping with the stresses caused by expansion and contraction due to thermal factors and is much more likely to part company with the stonework.  Take a look at even relatively new OPC pointing and see how many small cracks have appeared in it, even after only a short time; or perhaps contemplate why it is that buildings constructed with lime mortar don't need the expansion joints that modern ones do.  Lime mortar provides the flexibiity necessary to absorb contraction and expansion and, to a degree, it's also self repairing when small cracks do occur.

Where the OPC pointing has adhered thoroughly to the stonework then moisture can readily build up at these points (because the ponting won't let it out) and this can result in spalling or a build up of salts which can cause damage or decay to the stonework itself, especially if the pointing mix is too strong. Many old buildings have been seriously damaged by injudicious use of OPC pointing because of this very reason, particularly in the middle years of the twentieth century.

Even if an OPC pointed building escapes the worst aspects of water ingression, there's always the problem of groundwater and also moisture production from the inside.

An average stone built house has  walls which are probably in the region of eighteen to twenty feet in height and this means, in each external wall, they will have about forty courses of stonework plus several more (diminishing ones) in the gables.  Between each course is a pointed gap averaging approximately 25mm in depth. That adds up to a total depth of approximately one  metre of pointing which, if it was originally lime based, used to allow the evaporation of moisture from the inside the building but, when replaced with OPC pointing, will prevent that very thing from happening.  As far as  evaporation is concerned, it's akin to putting an impervious band one metre in depth all around a house!

The consequences for condensation and other problems should not be underestimated, especially at the lower levels of houses where there may also be moisture from ground water.  The ground water moisture used to start to evaporate from the first joint it met but, because of the OPC pointing, it can't do that anymore.  The consequences of this are often seen on the lower reaches of inside walls and 'rising damp' is usually diagnosed.  Strictly speaking this is correct; but it's only there because someone removed a natural evaporation path and replaced it with an impervious plug!
 
Most people involved with re-pointing never consider the consequences of their well intentioned actions but, in the long term, re-pointing with OPC can cause more problems than it was intended to cure; especially if the mix is too strong.  The same problems can occur with rendering of course, but more about that elsewhere.

This short article isn't intended as a thesis on pointing or the attributes of lime, it's here to alert you to the dangers of using modern materials to repair older buidings. Sometimes these methods work out just fine, other times they don't.  If you'd like to read more on lime based materials, try this site.   There are lot's of others too, just do a Google on 'lime mortar' or something similar.