Clair Shields – LEADER Small Scale Enhancements Co-ordinator & Planning Officer (Building Conservation)
A couple of weeks ago a team of us from the National Park Authority’s Planning and Conservation Departments attended an educational (and enjoyable) lime training event to better understand the use of traditional lime mortars in pointing and plastering work. The practical training was delivered by Nigel Copsey from the Earth Stone & Lime Company, at his headquarters in Thornton le Dale. Everyone got to try hands-on re-pointing work as well as having a go at both earth and lime plastering, using a large panel of timber laths, which was the type of construction used for interior partitions and ceilings from the eighteenth century into the twentieth century. The purpose of the day was to make us all better aware of the benefits of using lime over cement and to spread the word to residents and builders in order to try and preserve and improve the built heritage of the National Park.
The reason for using lime mortars in preference to cement mortars is that cement is an impervious material which is almost always harder than the stone or brick that it surrounds. This means that moisture in the wall (from rain penetration, rising damp or faulty gutters) has to evaporate through the masonry rather than the harder mortar, and through freeze-thaw action this process erodes the surface of the masonry over time, sometimes leading to severe erosion and even a ‘laddering’ effect where the cement survives but the masonry has disappeared!
By contrast, lime mortars are breathable, permeable mortars which will generally be softer than the stone or brickwork. The lime wicks moisture from the masonry, allowing it to evaporate without harm to the masonry-surface. A lime mortar acts as a sacrificial material which protects the more valuable stone or brick – it’s a lot cheaper to re-point every century or two than to replace eroded masonry!