Golden rule

In the restoration of historical buildings, construction materials supplier Afrimat argues that going ‘back to basics’ is the best approach

This is a country that is rich in history. A great testament to SA’s heritage is the numerous heritage sites and unique historical buildings scattered around our beautiful country – monuments that paint the picture of this precious history. As such, is it imperative that we are able to effectively preserve these structures for their historical value and cultural offerings.

Many historic South African buildings have been restored over the years, and iconic sites such as the Castle of Good Hope and Robben Island are prime examples of the importance of authentic and accurate restoration work

Historic Building Repair and Restoration
There are many features in the construction aspect of authentic historical renovations – carpentry, joinery and roofing to name but a few.

A common problem and focal point in the renovation of historic buildings is the issue of damp in walls, and the apparent poor durability of plaster and mortar over time. The question is, why do we see this recurring issue on so many historic buildings in SA?

The problem is not unique to this country – indeed, in many European countries, there has been intense debate on this issue.

What is happening to SA’s OLD buildings?
After considerable European-based research and investigation, it was found that the cause of what appears to be durability issues on plaster and mortar is in fact mostly damp related. Some first-hand experience and exposure to historical buildings such as Mooikelder in Paarl, Robben Island, Wuppertal and Tulbagh’s Church Street, to name a few, has revealed the same causal issue.

Lime is a superior product to cement in many broad-reaching construction applications

At each of these sites, the symptoms of damp are apparent in the form of ‘bubbling’ paint and plaster detaching from walls. In many old buildings, particularly those that were constructed before the turn of the century, the traditional building materials that were used are considered ‘softer’ than those used in modern construction.

As per European literature, the root cause of all the recurring damp is the use of incompatible ‘modern’ materials on the traditional materials during previous renovation work.

Raw materials in plaster and mortar
At the turn of the 19th century, cement was invented, and it was used to replace lime as a binder in construction and building work. With its fast-setting and high-strength benefits, cement rapidly received more attention in the construction industry and turned the sector on its head.

Cement was widely used instead of lime in construction work, specifically in the renovation of historical buildings. Unfortunately, the effects of using this material, which isn’t compatible with older materials such as lime, were not known at the time. It is only decades later and after thorough investigation that restorers have realised that the use of cement is a major cause of damp-related problems.

The British Cultural & Heritage Association has since banned the use of cement in heritage restoration work and now allows only ‘cement-free’ lime-based mortar and plaster.

One of the most important concepts in heritage buildings is breathability and avoiding trapped moisture. On almost all of the historical building sites in which Afrimat is involved, damp issues are caused by moisture trapped in walls due to the previous use of non-breathable paints, or renovation work done with cement-based plaster and mortar.

The general correlation for mortar and plaster is, the higher strength, the lower the vapour permeability. Cement creates non-breathable, high-strength mortar and plaster, thus affecting the original masonry work in historical buildings through trapped moisture and decay of original material. Unfortunately, in SA there is limited knowledge about heritage restoration, and widespread durability and damp issues are often misunderstood. Many renovators of historical buildings use modern techniques to cure damp issues – methods and materials that are usually costly and, ultimately, ineffective.

Afrimat used building lime in a recent restoration project, which is best suited for heritage work

Afrimat has witnessed many instances whereby buildings that were originally built using quicklime made from seashells have been renovated using cement-based mortar, after which seashells were stuck into the mortar in an effort to match the original appearance and look authentic. In these cases, future damp and durability issues are inevitable. The best and most straightforward, cost-effective solution, is to follow the example of the UK and switch entirely from the use of modern cement back to lime-based mortar and plaster.

With historical renovation work, going ‘back to basics’ is the golden rule – lower-strength lime-based mortar and plaster has been proven to work the best for heritage work due to their superior breathability capabilities. It must also be emphasised that although lime-based mortar is initially lower in strength compared to cement-based mortar, it hardens over time and ultimately exceeds the strength of its cement-based counterpart.

Afrimat’s most recent involvement in a large-scale heritage renovation project in the missionary village of Wuppertal (in the Western Cape’s Cederberg) has re-affirmed the fact that lime sets more quickly than is perceived. No cement is allowed on-site, and all heritage renovation and rebuilding work is lime specified.

It is often forgotten that building lime, which is mostly used as a plasticiser in modern construction, is also a material that hardens through a process of CO2 absorption. This environmentally friendly feature, paired with effective breathability in mortar and plaster, arguably deems lime a superior product compared to cement in many broad-reaching construction applications, especially in heritage restoration construction.

PO Box 5278, Tyger Valley, 7536
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