Advice & Guidance

Masonry

The UK has a diverse range of building stones ranging from tough Cornish granites through to soft, easily weathered chalk. Limestones, sandstones, slate and flint contribute much to the character of our places of worship as do manufactured materials such as brick and terracotta.

We may also find that our ancient walls contain reused materials such as Roman tiles or early grave markers. The variety in colour, texture and pattern is very importance and for this reason we must take extra care to treat our walls with sensitivity when we maintain and repair them.

New stone following original profile

As different building materials have a range of characteristics and decay patterns we must take time to understand the materials our buildings are made of before we contemplate their repair. We also need to understand how walls are constructed and be alert to the early signs of structural problems.

In the main, the masonry elements of the building e.g. the brick, limestone, sandstone, granite or flint will stand up well against the elements and will have a very slow rate of decay.

Weathered stoneworkHowever, problems with other parts of the building fabric, such as the rainwater goods, may increase their rate of decay. We also know that the presence of incorrectly specified mortars containing cement can have a damaging effect so it is good practice to examine the wall surfaces from time to time.

Architectural features such as string courses, cornices and hood moulds above windows and doors are designed to throw water clear of the face of the wall. If these features are damaged water will run down the face of the wall increasing the rate of decay. You should therefore pay close attention to these elements. 

Where masonry is severely damaged and is thought to be in need of repair there are a range of approaches that your professional advisor might take.  The starting point is usually the principle of minimum intervention which aims to retain as much as possible of the existing fabric. 

Stone with characteristic patterningThis may mean carrying out repairs using lime mortar or perhaps pinning fractured masonry with stainless steel rather than replacing old stones with new masonry. When new stones are required it is vitally important that the replacement stone is a close match with the original in terms of its geological type, texture, porosity and hardness. Colour is important too but bear in mind that stones change colour as they weather.

As part of your annual fabric inspection it is a good idea to have a close look at the condition of the masonry and to report any concerns to your professional adviser. In order to gauge the rate of decay you might like to photograph a representative part of the structure so that you can compare the condition of the stone over the coming years.  This will give you some idea of how quickly (or slowly!) the stone is weathering and may help you make sound judgements about the need for future repairs or replacement.

Partially replaced hoodmould

When looking at the walls you should also take time to consider the condition of the mortar in the joints. Note areas where the mortar is deeply recessed, very crumbly (friable), loose or missing. Your architect or surveyor will be able to advise whether there is a need for some localised repointing.

Decayed joints between the stoneworkThey will also be able to specify a suitable mix for the repointing mortar as it is very important to make sure that the mortar allows any moisture present in the wall to evaporate through the joints rather than through the face of the stone.

Pollution may affect the appearance of buildings in urban areas over time. However, unless the soiling is heavy and is actively harming the masonry, cleaning is usually inadvisable. You should also avoid using colourless water-repellent coatings on masonry. These have a limited life, rarely provide a cure and can often cause more harm than good.

Further information

The following articles may be of interest and can be found on the Building Conservation Directory website:

  • Building Stones of the United Kingdom' by Francis Dimes
  • Stone Replacement: To do? Or what to do? That is the question' by Malcolm Coulson
  • The Search for Stone: Identifying, Sourcing and Matching Britain's Building Stones' by Graham Lott
  • New Stone for Old: Techniques for Matching Historic Stone Finishes' by Jamie Vans

© SPAB 2012