Cement and Lime Mortars at Blackmore Farm Cement and Lime Mortars at Blackmore Farm Cement and Lime Mortars at Blackmore Farm

Lime mortar and heritage building conservation

These photographs show the replacement of lime mortar at the Blackmore farm near to Cannington in Somerset, England. The manor house and attached chapel were largely built in the 15-16th centuries, but the manor was first cited in the Domesday book. It is Grade I listed and generally well preserved. It is mainly constructed from roughly dressed sandstone with lime mortar joints. However, some previous repair and conservation work was performed using ordinary Portland cement mortar, which damages the surrounding stone.

Due to its unique hygroscopic properties traditional lime mortar should, where possible (and if appropriate), always be used for the conservation of medieval buildings. The word 'hygroscopic' refers to the ability of the mortar to absorb water.

By comparison ordinary Portland cement mortar is non-hygroscopic and will exclude water, forcing it to find ways around the mortar, which damages adjacent stone-work. Cement mortar is cheaper and less labour intensive than its lime equivalent as lime mortar pointing requires not just one application, but ongoing work. Ordinary Portland cement mortar is stronger than lime mortar, but does not allow for breathing. Being impermeable it is usually the best solution for modern buildings with central heating, and concrete or brick construction. Lime is ideal for older buildings with open hearth fires and thick walls, as it allows both water and air to pass through it. There may be some problems with damp in old buildings as water will evaporate on the surface, but by re-pointing damaged joints the integrity of the main structural materials (stone, brick and timber) can be preserved, although this can be costly and time consuming. Problems with damp in old building will depend upon a complex variety of factors such as the use of renderings, paints and inclusion of an impermeable damp-proof-course, or the attachment of stud walls as an internal barrier to the damp. In many cases lime mortar may not be appropriate because of later additions or changes to the fabric.

Qualified experts can provide advice, but the kind of advice given will depend on the purpose. The most 'authentic' conservationists may wish to retain as much of the original fabric as possible, following the anti-scrape philosophy of William Morris and SPAB, whereas chartered surveyors working for estate agents may wish to see a house ready for sale as quickly as possible, with no obvious problems, so there will be a certain degree of partisanship that should be considered when advice is sought.

If the decision has been made to use lime mortar for re-pointing an old property it must be borne in mind that this will be a long and costly process. Lime mortar must be kept damp whilst it is bedding-in, and after setting it will require the outer layers scraping out ('raking' out) and pointing. This inevitably costs far more than simply using one application of cement mortar, and also the skills needed to work with lime mortar have often been lost except for especially trained conservation masons and bricklayers. English Heritage runs a certification scheme for stonemasons to ensure that conservation work is up to standard. After pointing with lime mortar it must be kept fairly damp overnight, hence the wet hessian attached to the wall shown in the inset photograph. Hot weather will dry out the mortar before the internal parts have set properly, much like a cake in an oven which is set at too high a temperature, cooking the cake on the outside, but with the middle still runny. The mix for the mortar will depend upon local conditions and be specific for the job. In this instance a red sand aggregate has been used to blend in with the colour of the stone and historic mortar.

It is worth noting that the strength of cementacious products such as cement mortar is also directly related the degree of control over loss of water from the product after placing. Ideally cement mortar and concrete should be kept surface-damp for at least a week after placement.

With thanks to Richard Morley, stonemason, for technical advice.

AKL 5/5/2012

 

Further information:
Blackmore Farm and listed status,
http://www.blackmorefarm.co.uk/about/farm-history
http://www.britishlistedbuildings.co.uk/en-268848-blackmoor-farmhouse-cannington

 

Lime mortar:
http://www.buildinglime.co.uk/damp_historic.htm

Pointing walls at Blackmore Farm