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Walls Refurbishment 1.0

Walls Refurbishment 1.0

In this article, we explore the process and phases behind the refurbishment of the old stone walls through the use of similar materials and methods that were employed during the original construction work. To do this, we have based our work on recent research showing that lime-based mortars are more effective in restoration works because of their compatibility with existing walls especially in “ancient” buildings.

Why refurbishing?

We contemplated the refurbishment as a necessary step to reinforce the original stone walls so that these can support the wooden structure of the new green roof, to take our urban experimentation one step further. Apart from being economical, refurbishing vis-à-vis demolition-reconstruction also offers an efficient solution to reduce environmental costs and negative impacts associated with the latter process. Adding to this, and perhaps more on the cultural and aesthetic levels, we consider old stone walls an important heritage that should be enhanced as much as possible.

Figure 1: Prototype of the beam used to support the new wood structure of the roof.

In line with Critical Concrete’s body of thinking and modus operandi, the construction of our green roof aims to mostly use techniques that can be both shared within and improved by our team, to then be diffused to the broader community as open source tools. In fact, this project gave us the opportunity to mix-match the skills of our team members with those of external professionals supporting us during the construction phases. In the specific, the supervision of this project is followed by a consultant engineer, a construction expert and a team of architects. The successful completion of the project would not be possible without effective (and essential!) communication between team members and professionals.

To reiterate the importance of this: if you plan to refurbish your walls, please ask for expert supervision before and throughout your refurbishing. You will appreciate their advice and be able to observe the benefits.

Phases and tools needed

The plan for the old stone walls’ refurbishment is structured along two preparatory phases, cleaning and preparation of the walls, and the actual refurbishing phase, which includes the production of lime mortar (argamassa de cal in Portuguese) to fill empty joints and bind recycled stones. Before going into our step-by-step analysis, learning from independent research and experimentation, we have put together a list of indispensable tools needed for each phase.

Figure 2: Original old stone wall, covered with cement, lime and plaster.
Figure 3: Cleaned wall, prepared for refurbishment.
Figure 4: Refurbished wall, with new joints filled with lime mortar and stones.

Preparatory phases:

  • Chisels and hammers – crucial tools that allow for the removal of old joints while avoiding damages to the remaining stones. The size of each tool should be based on the width of the joint;
  • Water compressors or Karchers – used for cleaning the joints of the walls with high pressure;
  • Brushes – used to further clean the walls from debris and fragmented stones.

Refurbishing phase:

  • Mortar/concrete mixer – necessary to mix the lime mortar (of course, you are welcome to stir it by hands…) ;
  • Trowels – used to apply the mortar into wider joints;
  • Wooden floats – useful for holding the mortar while you are handling the trowel;
  • Buckets – to store your lime mortar as well as stones;
  • Mash hammers – used to break up stones for pinnings;
  • *Cement injector – this is optional but effective for injecting the lime mix into deeper joints.

Throughout each phase, it’s important to protect your safety and good site practice. For these, you will need:

  • Scaffoldings;
  • Helmets;
  • Protective goggles;
  • Appropriate protective gloves;
  • Security shoes;
  • Reusable dust masks.

The refurbishing journey

Analysis – the theory behind:

The first step of the refurbishment of our lime and granite walls was identifying the walls’ weak points as well as ensuring that the existing walls we planned on using have solid foundations. This was made possible after consultation and advice of our external partners and professionals. Once again, we are thankful for their support and contribution!

It is possible to identify two different types of walls through this workshop, which define the history of the building:

  • Simple walls;
  • Double walls.

More than thickness, the critical difference between these two typologies is the way these are built. The simple wall, which is +/- 30cm thick is made by big stones fitting together, while the double wall, which is +/- 55cm thick, is composed by two layers of stones, also known as faces. The space and gaps behind and between these faces is filled by small stones and earth. To provide weight that helps tie the faces to the core, the heaviest rocks are used as cap stones for the final course.

All of these particulars may affect the undertaking of the refurbishment work as well as the planning of each step. Let’s take the above examples as an interesting window to learn how old edifices used to be built with granite stone in the region.

Action – a step by step guide

1.Cleaning of the walls

Figure 5: Old stone walls to be cleaned from dirt and unstable stones.

During the cleaning step, existing decayed or damaged joints are uncluttered and fragmented and unstable stones are removed. Following the typical demolition logic, the cleaning has to be made from the top to the bottom of the wall.

To keep the continuity while not dramatically impacting the structure of the wall during the refurbishment, it is necessary to work on it meter per meter during all phases. Because the cleaning of the lime joints might create a good amount of waste and non-reusable, non-recyclable materials, do not forget to anticipate the evacuation of trash from the construction site.

2.Preparation of the walls

When the cleaning is completed, cleared sections must be washed with a high pressure water spray. This is when old joints are pulled out, and dust as well as other debris removed. This step is fundamental as joints must be cleaned and humidified for the mortar to stick properly.

Figure 6: Removing dirt and unstable stones from the wall’s joints.

3.Preparation of the lime mix

As mentioned above, in order to prepare the lime mortar, you will need the mortar mix and water. It’s important to make sure that water proportions are adequate for a good lime mortar, keeping in mind that different compositions will lead to distinct uses.

Figure 7: Lime mortar produced by mixing the hydraulic natural lime base with water (Azichem, n.d.).

What is lime mortar? ***
In a way that respects the skills and knowledge accumulated from traditional building techniques, we felt the necessity of learning about the history and properties of lime mortar in the first place. If today, lime-based mortars are most used in the conservation of existing old buildings, or the creation of new ones using traditional practices, the method itself is at least 6000 years old, going back to when ancient Egyptians used lime to plaster pyramids! The use of lime mortar slowly declined with the introduction of Portland cement, during the 19th century. However, recent research has shown how profitable it is to use lime mortar instead of regular cement composts when working with materials such as natural stones due to its softness and porous properties. Indeed, regular cement doesn’t allow for a good evaporation of water, thereby risking to create damp issues which can bring surfaces to peel, pop out or even flake off on stone surfaces with consequent disintegration. On the contrary, lime possesses the ability to evaporate the damp, also known as the “breathable characteristic” of materials. There are a variety of types of lime mortar that can be found in the market, depending on their purposed uses. In our case, we sought the guidance of our engineer to find one with adapted structural properties, specific for refurbishment of old stone walls (NHL – natural hydraulic lime base or “base de cal hidraulica natural” in Portuguese).

4.Fill the joints with lime and stones

In the case of simple walls, the subsequent steps is quite straightforward: filling all empty joints with the lime mortar and fixing small-sized cleaned and humid stones in between spaces when possible.

Figure 8: Carefully filling joints with lime mortar and small stones.

In the case of double walls, some more difficulties were encountered in order to ensure cleaned joints and gaps were properly filled and stabilized. In fact, because of the nature of double walls, during the cleaning, we had to empty large quantities of earth and dirt, which left sizable holes to fill. To aid the process, we conducted some research that pointed us to use a “mortar injector” to enable us to fill gaps even when more profound. To optimize the use of this tool, we mixed the lime mortar to be slightly more liquid than usual (strictly following the instructions for the product to retain its structural properties). Another factor to consider within this process is, of course, the working site’s temperature and humidity level, which will affect how lime mortar solidifies and reacts.

Figure 9: Solidified lime mortar binding previously emptied joints.

So…What’s next?

While continuing with the refurbishment of the old stone walls, we are learning-by-doing, experimenting with innovative yet traditional, environmentally friendly techniques and materials, seeking to produce new knowledge and validate this with experts’ advice. Drawing on the many permaculture and urban agriculture initiatives from all around Portugal, the next step will see us researching and planning for the layering and system design of the green roof. Stay tuned for our next article (!), more of our learnings will be shared soon.


[1] The EngineShed (n.d.). “How to make lime mortar repairs”. [Online] available at: https://www.engineshed.scot/building-advice/common-problems/how-to-make-lime-mortar-repairs/#site-practice (last accessed: March 2019).

[2] Ecolime (2018). “What Is Lime Mortar?”. [Online] available at: https://ecolime.co.uk/what-is-lime-mortar (last accessed: March 2019).

[3] Bokan Bosiljkov, V. (2001). “The use of industrial and traditional limes for lime mortars” in: Historical Constructions, P.B. Lourenço, P. Roca (Eds.). Guimarães, 2001. Retrieved from: http://www.csarmento.uminho.pt/docs/ncr/historical_constructions/page%20343-352%20_26_.pdf.

*** To know more about mortars and walls refurbishment:

[4] Moulis, C. (2018). “Quelques Réflexions et Questions Autour des Mortiers : Pose, Plasticité, Inclusions, Représentations” (in French), Traverse, 26 March, 2018. [Online] available at:https://chantiers.hypotheses.org/1055 (last accessed: March 2019).

[5] Roman Concrete (n.d.). “The Roman Pantheon: The Triumph of Concrete”. [Online] available at: http://www.romanconcrete.com/ (last accessed: March 19).

[6] ARTE.Tv (2015). “Guédelon renaissance d’un château médiéval” (in French). Full documentary available from Youtube: https://www.youtube.com/watch?v=gbiVEY2l9cw (last accessed: March 2019).


Figure 7: Azichem (n.d.). “CALCESANA: Cal Hidráulica Natural” (in Portuguese). [Online] taken from: http://www.azichem.pt/produtos/calcesana/10/(last accessed: March 2019).

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