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Old Farmhouse with Cob Walls

Cob Plastering Challenges and Solutions
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Overview of the Project

This 250 year old farm building had thick cob walls built on a stone base. In recent decades the building has been rendered with cement and internally plastered with a sand and cement plaster. The current owner decided to restore the building as much as possible to its original condition.

The fabric has been stripped from all its modern materials, taking everything back to stone and cob. Several layers of cement had to be removed as over the years they tried to fight the dampness by making the farm house more and more waterproof.

New drainage has been installed. The floor has been replaced with a limecrete floor with an underfloor heating system. During these works the walls have been left bare allowing them to dry. 

To retain the breathability of the fabric, the walls have been replastered with a good quality lime plaster. However, despite having adequate heating and ventilation, the lime plaster did not dry. Six months later, the lime plaster was still not dry.

cob walls in old farmhouse
6 month later: the lime plaster was still damp, did not dry

The Problem of Salts in Cob

The building owner, who was also a very skilled DIY person, started searching for answers to understand the factors that can prevent a lime plaster not to dry. He found the answers on the Core Conservation web site when reading about the effect of salts.

The reason why the plaster did not dry was the excessive amount of salts inside the plaster. Salts were present in the cob wall from several sources:

  • The cob itself: cob, a natural soil-based building material, natively contains many minerals including ground salts.
  • From rising damp: porous masonries draw up moisture and salts from the ground over time. As the building ages an increasing quantity of salts are being depositing into the masonry (nitrates, chlorides, sulphates).
  • From the chimney: one wall contained an old decommissioned chimney. Chimney soot contains a high concentration of salts (sulphates) which migrates into the wall fabric and plaster - a known problem for chimney walls.
  • From cement: cement as a building material contains many additives. One of these additives is gypsum, which is a salt (sulphates). Salts act as a retardant, keeping cement plasters workable for longer. Applying cement plasters on old walls adds salts into the fabric, contributing to its crumbling and long-term deterioration.

When the fresh lime plaster was applied, the salts migrated into the lime plaster, absorbing humidity from the environment, keeping the lime plaster damp, preventing it to dry. The lime plaster would not dry due to the high quantity of salts. Some salt crystallization also became visible on the surface of the 40-50 mm thick lime plaster built up in several coats.

Solving the Problem - Dealing with the Salts

On request, Core Conservation did a holistic assessment of the situation. Salt measurements have been taken, which indeed confirmed a very high amount of nitrate and sulphate salts in the cob and the lime plaster. Because the salts have now contaminated the lime plaster, that had to be hacked off and the renovation had to be started fresh.

To prevent future salt problems, the Rinzaffo MGN Roman salt-resistant lime base coat has been applied before the reapplication of the main lime coat. This Roman plaster being imperious to all types of salts acts as a breathable salt filter, protecting subsequent lime coats from the destructive effect of salts, increasing the longevity of any lime plasetering significantly.

lime-after-core-conservation
After the application of the Roman base coat the plastering dried without problems.

The Roman lime base coat has been applied on the cob wall and dried beautifully. This has been followed by the same hairy lime plaster used earlier, which has also dried as expected. There were no problems with the lime plastering since.

The photo gallery below shows more details about this cob building renovation project.

Solutions Implemented

These solutions were implemented during the project:

Products Used

Here are the products used during this project to resolve the problem:

More Information

Here are some related pages with more technical information related ot this project.

Any Questions?

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Photo Galleries

Here are some photos demonstrating this solution. Click on any image to open the photo gallery.

Farmhouse with Cob Walls

Overcoming replastering challenges and dampness problems of a 250-year old farm house with thick cob walls after the applied traditional lime plaster failed to dry.