Seven Properties, One Problem
The Need to Maintain Moisture Equilibrium in Old Buildings
I have been lucky enough to survey some quite fabulous old buildings over the last two weeks; they all form part of the same estate and without exception they have all suffered damage from the use of incompatible materials in their historic maintenance regime. Specifically, repointing has been done with Ordinary Portland Cement rather than the correct lime mortar. All these buildings were constructed using lime mortar, with some of the buildings being thin joint construction built to incredible tolerances. What this means is that all these buildings are built on the ‘overcoat’ principle, which is to say that they are meant to absorb moisture and will go through seasonal wet/dry cycles. So long as we do not impede the buildings ability to breathe and therefore manage moisture, then these buildings will manage moisture perfectly well. As a general principle, the mortar joints should always be weaker than then the masonry units it holds together and this rule applies to ordinary portland cement as well as lime. However, once cement mortar cracks, it stays cracked, it has no ability to move with the building, whereas lime is much softer and has the ability to self heal. More importantly, it is sacrificial and its ability to breathe focusses moisture management or evaporation at the mortar joints. It will naturally erode and so long as the building is repointed with lime then the buildings ability to manage moisture will be maintained. As for these buildings, the qualities of lime mortar was forgotten for quite some time and I think that those responsible for maintaining the buildings genuinely thought that OP cement was a better option. It is certainly more favoured by modern bricklayers due to its ease of use and workability and I generally find that bricklayers just do not like using lime mortar.
Aside from moisture management we need to recognise the consequential damage caused by using ordinary portland cement on old buildings and the last slides below highlight that damage perfectly. Once buildings are repointed with OP mortar then moisture is diverted into the masonry unit where the higher moisture levels then leave the masonry susceptible to increased levels of spalling due to hydraulic freeze/thaw action. Bricks come out of the kiln with a dense outer ‘fireskin’ that has increased weather resistance, once the fireskin is lost then the underlying material is even softer and more susceptible to increased levels of spalling, the process is self perpetuating. Stonework is also affected by this problem and we’ve all encountered stone buildings where the cement mortar joints are standing proud of the stonework.
There are some general principles for working with lime that are not dissimilar to the principles encountered for OP cement and these are:
1. The slower the lime dries out, the better and more durable the end result. You should always wet down the surface being worked on to prevent moisture being drawn from the lime too quickly. You should also protect against direct sun and stiff breezes when drying out.
2. As for portland cement, frost can be very damaging to lime, and as lime takes longer to go off the threat from frost is even greater. This is really a timing issue and the use of anti-freeze additives should be avoided entirely.
3. The more you mix lime, the softer and more workable it becomes. Too much water increases natural shrinkage and as for Portland cement mortar, you should use as little water as possible. The water/cement ratio is as critical for lime as it for ordinary portland cement and the mix should be used as dry as possible and pressed firmly into place.
I am grateful to have made a positive impact on this estate now because the use of lime mortar is now fully understood and it is used as a rule rather than the exception. There will no doubt be some long term maintenance to replace spalled brickwork caused by the use of Portland cement but we have at least limited the potential for future consequential damage to these beautiful old buildings.
Seven Properties, One problem.
Thin joint construction
Still a working farm
Retrofit cementitious plinth also causes problems.
One of the worst affected buildings
I'm guessing that this one was built in 1877
OPC mortar repointing has caused spalling of brickwork
Widespread spalling and mortar standing proud of brickwork.
Plastic repairs are supposed to be colour matched to brickwork.
An old slate DPC is clearly still fully functional
Cement mortar buttered over original lime mortar