As you gather quotes from building restoration specialists, the type of mortar they propose for the pointing speaks volumes. Anyone suggesting the use of cement mortar for your heritage building should be discounted since it is wholly inappropriate.
Masonry walls rely on the fact that moisture entering the wall can escape as both water and water vapour. The use of cement based mortar prevents this as it has a closed pore structure. The retained moisture may find a ‘bridge’, and track into the dry interior creating damp patches and the associated health issues and problems related to internal damp issues.
Cement mortar is often the least permeable part of the wall, so retained moisture tries to find its way out through stones and brick. During the winter, as this moisture freezes, it expands causing damage to the stone or brick as well as the mortar – repointing and renovation is then the only solution.
Lime Mortar offers many advantages over cement mortar, the main one being that it ‘breathes’. Moisture within the solid stone or brick can therefore escape through the pointing instead of the stone or brick. This allows the wall to dry out, ensuring that the stonework or brickwork is not damaged.
It also has the advantage of being more flexible than cement mortar so it moves with the building. It is also ‘self-healing’, so if the mortar develops micro-cracks due to building movement, these will gradually repair themselves. A lot of the replacement Limes are also naturally hydraulic absorbing moisture and stiffening in the process.
- The replacement of existing mortar
- Existing joints have weathered and the mortar requires replacing.
However, to the untrained eye, lime mortar can appear to have reached a point where it needs replacing and this may not necessarily be the case. Many historic lime mortars will break or crumble much more easily than cement based mortar. This gives the impression that it needs replacing. In fact, softer mortars are more likely to have higher rates of moisture vapour permeability, protecting your historic stone or brick work.
As mentioned earlier the Lime Mortar ‘chat’, or Lime specified in the Estimate may speak volumes about your choice of contractor. The use of inappropriate strengths of Lime, additives and quick set solutions could spell disaster for the stone, brick and in the long term, your Bank Account. It is not simply enough to say,’ We use Lime’, it’s the Which, Why and How that counts.
- Was the type, density and strength of the stone being pointed discussed?
- Was it explained that an NHL 5 Lime mortar may be less appropriate than an NHL 3 if your stone is softer and more porous?
- Were coloured mortars discussed? Whilst mostly a matter of personal aesthetic preference, if colour choices were offered was information about the source and compatibility of the colorant being mixed with the Lime offered? Was it explained that if a red coloured lime mortar is used to point red sandstone or brick, when the mortar is expelling moisture vapour and drying out, traces of white salts may appear on the red mortar, making the pointing look unsightly?
- How is the existing mortar to be removed, what is the damage assessment to the host stone during the mortar removal process?
- What depth of mortar in the joint will be replaced?
- Was it explained that in certain cases where there has been no moisture penetration or damp issues on the interior of the property it is sometimes prudent to leave the cement pointing in place as the process of removing it may do more damage to the stone the benefit of replacing it with Lime Mortar at that time?
- In the case of a Listed or Historic Wall or Building was a mortar matching service offered?
- Was there any discussion about how long after the works that protective hessian tarpaulins may have to stay in place?