Replastering flooded walls of old, listed or historic buildings is not always an easy and obvious choice in terms of selecting the the right materials and technologies.
Most old building owners are aware of the importance of breathability and using lime as a building material. But there are additional technical aspects that must be considered. Let's look into this in more detail.Â
As a natural hazard, flooding can can cause significant damages to old buildings. It can affect the building structure, the condition of the plastering and finishing, the timber - virtually everything.Â
The biggest threat to the masonry and the fresh plastering after flooding is the effect of water soluble salts. These can be present in the masonry from several sources:Â
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.
Recovering after a flood can take lots of work. ‘Forced’ drying of the walls should generally be avoided - this can produce temporary and superficial results where surfaces seems dry but the core of the wall remains wet. Too fast drying can also result in excessive salt crystallization leading to spalling, crumbling and cracks in the lime plaster, as well as in the warping or cracking of the timber. So the building should be dried slowly, using natural ventilation, background heating and when required, dehumidifiers and extracting fans.1Historic England: Flooding and Historic Buildings (2015) https://uat.historicengland.org.uk/media/4569/heag017-flooding-and-historic-buildings.pdf
Very often after flooding no replastering work is necessary. Soft porous lime, if in good condition, usually recovers and dries out on its own. Lime plasters tend to be more resilient than modern gypsum (used for finishes and for plasterboards), which being a very sensitive material to dampness, tends to disintegrate and crumble, needing replacement.
However, if the old plaster is in poor condition and needs replacing, in the presence of a salt-laden masonry, the following plastering schedule is recommended:
This solution offers the following benefits:
For more information, please see below a list of recommended MGN lime plasters for the implementation of this solution, as well as technical pages, projects, image galleries and videos related to this solution and materials.
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 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.
This old cottage has been extended a few years ago, receiving a new extension. During flooding the waterproofing of the newbuilt extension did not hold, resulting in the flooding of the building. Using the Roman lime materials the waterproofing has been reinforced, preventing future flooding.Â
Here are some photos of different plaster types to demonstrate how they can be recognized.Â
Here are some videos related to this solution. Please unmute the videos when playing them.
If you have any questions about a project, a problem, a solution, or any of our plasters - please get in touch.
We understand that each project is unique. Using the contact form below feel free to ask us any questions. Give us as much detail as you can about your project so we can get back to you with more relevant answers.Â
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.