Replastering cob walls present specific challenges during renovation. Some of the important factors that need to be taken into account during cob wall replastering and renovation, are:
Contrary to common belief it's not moisture that damages the wall fabric or the plastering. It's the salts. High moisture content alone does not damage old buildings. Salts do.
Although water can cause damages to lime plasters through flooding, longstanding water ingress or frost, in the vast majority of cases, clean rainwater evaporates without a trace without causing any damages. Thus, dampness caused by clean fresh water is fully reversible.
The real problem, however, is when dampness is combined with minerals or salts. In the presence of salts everything changes. Salts are hard, solid minerals that in the presence of water temporarily go liquid. However, when humidity evaporates the salts re-crystallize and expand in volume 5-10 times. The crystallization or mechanical expansion of salt crystals is what makes old masonries or plasters crumble, salts being responsible for far more damages than water ingress and frost. This is a very important technical point that needs to be understood.
On real buildings the effect of salts can be very difficult to assess, as dissolved salts become invisible to the eye and touch, only dampness being obviously present. However, under controlled laboratory conditions the effect of salts can be easily demonstrated, measured and understood.
Here is a lab experiment: when two identical bricks - one salty, the other one not - have been subjected to high air humidity variations for an extended period of time, the salty brick has crumbled badly while the non-salty brick stayed completely intact in a high humidity environment.
So salts are the real problem, however it's water that carries them everywhere, contributing to their damaging effects.
In cob walls, salts can be present from several sources:
Due to the combined effect of multiple salt sources, applying a lime plaster on a cob wall can result in a relatively short life expectancy of these lime plasters, these acting sacrificially. In the presence of very high salt content of the cob, salts can keep the lime plaster permanently damp, preventing the lime plastering to dry. This can be a major problem in some renovation projects.
Dealing with high humidity and the destructive effect of salts requires special lime plasters. These are ancient plaster mixes initially used by the Phoenicians and Greeks, then rediscovered and perfected by the Romans throughout the centuries. Being outstanding architects and builders, the Romans have observed that adding to lime certain volcanic sands and ashes (also known as natural pozzolans - highly porous and breathable lava materials), these can significantly alter the properties of lime while retaining its breathability.
Through the skilful use of different types of volcanic sands and ashes, the Romans have created many types of lime mortars with outstanding properties - some of them becoming waterproof or able to set underwater, or becoming highly resistant to salts resisting seawater.
These volcanic lime mortars have been extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts and they have survived for many centuries. They have also been widely used in Venice, as they perform extremely well in the humid and aggressive environment of the Venetian lagoon.
Traditional pozzolanic Roman mortars differ significantly from today’s modern NHL (Natural hydraulic lime) mortars.
NHL mortars - a mix of lime and clay - need to be fired at higher temperatures than normal (air) limes for the lime and clay to react. The higher firing temperatures and more heat partially closes off the pores of NHL plasters, reducing their breathability. Thus, NHL plasters trade-off breathability for improved water resistance.
Roman mortars use lime and volcanic materials. These are mixed at room temperature without heat as the volcanic sands and ashes have already been pre-burnt by the volcano. This keeps the pores structure open resulting in high breathability. The volcanic ingredients make the plaster highly resistant to dampness and salts, making these Roman mortars perform extremely well in damp and salty environments.
The commercially available material today is the Rinzaffo MGN lime plaster. It comes in 25 kg bags with all necessary ingredients pre-mixed, requiring only water. Once it hardens it becomes waterproof, extremely resistant to all salts (chlorides, nitrates, sulphates), yet thanks to its porous volcanic lava ingredients, it stays breathable allowing the wall fabric to breathe.
This plaster - depending on where it's used - performs several important functions:
This lime plaster is made exclusively of natural materials. It contains no chemicals nor additives. All prime materials as well as the final product is certified.
Thus, the Rinzaffo MGN plaster is a lime plaster that acts as breathable water and salt filter. Due to its small pores it blocks liquid water. Due to its open pore structure allows the wall fabric to breathe. Having no salt content of its own, it does not introduce salts into the masonry. Being waterproof, it becomes impervious to salt penetration and crystallization, resulting in an extremely long service life even in very demanding and damp environments (e.g. basements).
The recommended lime plastering schedule for cob walls which "ticks all boxes" - breathability, water and salt resistance, moisture control, long life expectancy - consists of the application of the following plaster coats:
This concept is very similar to the current lime renovation concept, except it extends that by adding the Roman waterproof, salt-resistant base coat to the plastering schedule. This Roman coat has a very important role in the background, protecting the other lime coats (the second and finishing coats) from early decay from the combined effect of humidity and salts. Being a heritage-friendly material, it is extensively used in one of the most historic cities of the world - Venice - a World Heritage Site.
Here is a real-life renovation example of an old farm house with cob walls, from Devon, UK.
The necessary materials for the renovation concept are detailed below.
Here are the typical recommended materials / products for this solution. Other plaster variations are possible as we have different types of main coats (normal or thermal) and finishes (smoother, grainier, coloured etc.) depending on your needs or application. Please get in touch to discuss additional options.
Here are some of our projects using this solution:
Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.
Here are some photos demonstrating this solution. Click on any image to open the photo gallery.
If you have any questions about a project, a problem, a solution, or any of our plasters - please get in touch.
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Here are some key application points about the application of each plaster, as well as links to the full application guides. Additional documents can be found on the individual plaster pages.Â
The plaster application video below explains the concept in detail.