When starting out on the journey of designing the most appropriate mortar for repointing a Grade I Listed medieval house project in the Cotswolds, I went to Rory Young.
Rory is an expert conservator of historic buildings and knows everything to do with lime mortars and renders. He is also a leading sculptor with his most recent work being the stone statues in St Albans Cathedral. Rory also happens to live a stone’s throw from my office in Cirencester!
I met Rory having attended the RICS & SPAB Building Conservation Summer School in Cirencester, where his knowledge and attention to detail shone through. Having known and worked with Rory on a number of historic conservation projects and leant from his expert advice, I knew he was the right person for this new project.
The project in question was a large Grade I Listed Manor house. The existing mortar was a mix of cementitious mortars. The existing mortars were hard and impermeable, causing extensive and considerable damage to the existing stone. The building was also damp, with moisture being trapped by the mortar.
The project before we started work with the existing cementitious pointing can be seen below:
We decided that re pointing the entire house with a breathable and pourus lime mortar would be the best remedy. It was though not clear what mortar would be the most appropriate, ie a lime putty or a natural hrydraulic lime mortar, and what was the best aggregates to use.
Rory and I therefore set out to construct a series of sample panels that we could review and judge the performance of the lime mortars and renders.
We also carried out an extensive academic reveiw of lime mortars to help inform us on the most appropriate specification.
It was important to select aggregates that not only were the right colour, but had an even particulate grade. This would ensure that the mortar would be stable and not crack and would also look right within the context of the building. We visited local quarries and obtained a large collection of samples that we sieved to establish the particulate sizes.
The mortar mixes we used can be seen below:
Hot Lime, Natural Hydraulic Lime, or Lime Putty?
We reviewed and debated the appropriateness of each type of lime. We concluded that using a hot lime mortar would be the most historically accurate in recreating the original bedding mortars. We could understand this as we looked at the origional bedding mortars and saw lime inclusions within the mortar, indicating a hot lime mix was origionally used 600 years ago.
Hot lime was also thought to be a better performing mortar than Natural Hydraulic Lime (NHL) as it doesnt over harden, which is thought to occur with NHL. Hot lime is also thought to be more breathable and easier to work with than other mortars.
We both also looked at the success of hot lime mortars projects being undertaken by Nigel Copsy and the Building Lime Forum and were encouraged by this.
The Sample Panels
Rory and I built a large comprehensive sample close to the building to ensure the sample would be meaningful and representative. We spent time reviewing the aggregate and lime ratio’s, making sure we recorded the method and finish accurately so they could be easily re-created by the contractor.
See below for the selection of tools used to properly point lime mortar with:
The success of our sample panels gave us the confidence to proceed with re-pointing the entire house with our hot lime mixes. It was useful to monitor and review the mixes over time. In one particular case, one of the mortars tended to crack and shrink back. We concluded that this was because the proportion of stone dust in the mix was too great, which meant that the particulate grade of the aggregate was too narrow. We ultimately learnt from our mistakes on the small scale to ensure that when we undertook the work on a large scale it was undertaken without mistakes being made.
The repointing and stone repairs was undertaken by Vitruvius Conservation who were meticulous and thorough in their work. The work was procured on a cost plus basis, which incentivised the team to take their time and carry out the work to the highest quality, efficiently.
Nearly four years since undertaking these sample panels the re-pointing work is now nearing completion. The mortars we used in the sample panels have been used throughout the site in a variety of different settings. We have gauged the hot lime mortars with NHL limes with success on areas of the building more exposed to the elements.
Photos of the detail of the completed repointing and rendering: