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Replastering Flooded Walls

Flood Renovation with Lime
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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. 

The Effect of Flooding onto Old Walls

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. 

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: 

  • Existing salts present in the masonry as a result of the general aging process. Salts can be present in old masonries from several sources, including the ground (rising damp), air (air pollution and and acid rain) or the proximity of the sea. The flood water dissolves and mobilizes many of these "dormant" masonry salts, moving them around in the fabric.
  • New salts added to the masonry by the flood water. The dirty flood water contains a lot of minerals (dissolves salts) that are deposited in the masonry by the flood. As a result, after flooding, the salt content of the masonry can increase significantly.

The Problem of Salts

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.

It's not moisture that damages old buildings or the plastering. It's the salts.

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Crystallizing salts under the microscope: the primary cause of masonry decay

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.

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Research experiment: salty bricks crumble, non-salty ones stay intact

Flooded Walls Replastering Schedule

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:

  1. Base coat: applying the Rinzaffo MGN Roman salt-resistant base coat is highly recommended. Being breathable, this plaster allows the gradual drying of damp walls affected by flooding. By regulating the evaporation, it prevents salt crystallization and subsequent damages to the historic masonry. Acting as a natural salt filter, it prevents the migration of salts into the fresh plastering, keeping the lime plastering intact for decades.   
  2. Main lime coat: this can be a good quality lime main coat such as Calcina Bianca MGN or for underground areas a Cocciopesto MGN plaster which is an excellent indoor humidity regulating plaster.
  3. Finishing coat: optionally, a decorative lime finish, with a smooth or gritty texture of your choice.
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The long-lasting replastering of flooded walls

Benefits

This solution offers the following benefits:

  • Faster replastering: no long wait is needed before replastering, once the flood water has receded, the walls can be replastered at once, if needed. Just because the walls are damp is not a problem as lime plasters are applied onto pre-wetted, damp surfaces anyway. As the whole plastering system is breathable, this solution allows the walls to naturally slowly dry out in the background. The Rinzaffo MGN plaster being salt resistant, the movement and crystallization of salts occurring during the dehydration phase of the wall fabric will not harm the plastering, keeping the decoration nice and intact.
  • No forced drying: although normal flood restoration processes should be followed, there is no need to force-dry the walls as this breathable solution allows the walls to dry out slowly and naturally.
  • Flood resistant plastering: both the base coat and main decorative lime coat is water and flood resistant. After the occurrence of another flood the plastering will highly likely stay intact.
  • Flood protection of critical areas: as the Rinzaffo MGN Roman base coat is waterproof, it can be used to reinforce and protect critical areas of the building from flooding. If the base coat is applied in a continuous coat on the walls and on the floor (application permitting), the area can be made waterproof, withstanding even water pressure until the flood recedes.    
  • Heritage friendly solution: this solution is listed building and heritage friendly, and it can be applied to any building, old or new.

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.

References

References

Recommended Products

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.

More Information

Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.

Photo Galleries

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

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Flood Proofing a Flooded Extension

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. 

Plaster Types - Lime, Cement, NHL

Here are some photos of different plaster types to demonstrate how they can be recognized. 

Videos

Here are some videos related to this solution. Please unmute the videos when playing them.

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Any Questions? Need Technical Advice?

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. 

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Plaster Application

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. 

Rinzaffo

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Base, Waterproofing

  • Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.
  • Must be the first coat: the Rinzaffo MGN plaster must be the first coat on any wall. It should not be applied on top of other parge coats or plasters because when those fail the performance of the Roman base coat will also likely to be affected.
  • No salt-inhibitors nor PVA bonding agents should be used before the application of the plaster. The plaster bonds well on its own.
  • Masonry paints should be removed (which occasionally can be challenging) so the plaster can firmly adhere to the underlying stones or bricks. Remove at least 50% of the existing paint – the more is removed, the better.
  • Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained. 
  • Wet the wall fabric abundantly before applying the plaster, as well as in-between each subsequent coat. Lime plasters need moisture as they set slowly in a damp environment. If the walls are already damp, there is no need to wait to become drier, you can proceed with the application of this plaster. 
  • Level uneven surface first: very uneven walls (e.g. stone walls or crumbling old brick walls) are recommended to be patched up and levelled first before the application of a continuous coat, to ensure the consistency and required thickness of the base coat.
  • Application: apply the plaster in 10 mm coats.
  • IMPORTANT: Respect the thickness: do not under-spec the material. Apply min 10 mm (1 coat) for above ground level walls. Apply min 20 mm thickness (in 2 coats) for underground or extremely damp or salty walls. If more than one coat is applied, embedding a 10 x 10 mm fibreglass mesh is recommended between the coats – a standard practice in the industry.
  • IMPORTANT: Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Attention should be paid to compact it on the wall closing off all pores, no matter how small, leaving no gaps or holes where salts or liquid water could come through. Once an area has been completed, recheck that here are no missed holes, not even small ones.

    The plaster application video below explains the concept in detail.

  • Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.
  • IMPORTANT: Dark patches. Allow the plaster to dry for 48 hours. Ideally, the whole surface should dry out uniformly to light brown, however you might notice some dark or damp-looking areas exhibiting surface condensation. Dark areas indicate insufficient thickness of the plaster in raport to the amount of moisture behind it. This occurs in areas where the underlying wall fabric is very damp, the intense evaporation causing surface condensation.
    The fix is easy: apply extra material over such dark areas increasing the thickness of the plaster, closing off all pores. The increased plaster volume dilutes the vapour flow allowing the surface to dry.
  • Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.
  • Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).
  • IMPORTANT: Please watch the plaster application video below before applying the material. Unmute the video if it plays without sound.

Cocciopesto Deumidificante

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Main

  • Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.
  • Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained. 
  • Application: apply the plaster in 10 mm coats.
  • Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.
  • Finish options: main lime coats can be optionally left without finishing. To have a finished surface any MGN finish can be applied: Calcina Fine MGN (white lime finish), Intonachino Arenino MGN (coloured lime finish) or Marmorino MGN (Venetian Marmorino finish).
  • Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).

Calcina Fine

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Finishing

  • Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained. 
  • Recommended thickness of the finish: 3-4 mm. For best results, the finish should be applied in two “half-coats” of 2 mm each, with a fine 3-4 mm fibreglass mesh embedded in-between. The mesh makes the finish more flexible, minimizing the appearance of fine cracks.
  • Various textured finishes can be achieved, depending on the finishing technique used:
    • Coarse finish: by finishing the surface with a sponge or wooden trowel.
    • Smooth finish: by using a stainless steel trowel, compressing and smoothing the semi-dry surface, the sand granules are pushed into the material, resulting in a smooth finish.
    • Washed finish: by finishing the surface with a sponge trowel. After the surface has hardened, the surface is washed with water and blotted with a sponge to bring out the aggregate, its specific texture and colour.
  • Painting: as this is a breathable lime plaster, wall surfaces should be painted with a breathable mineral paint. Wallpapers and modern emulsion petrol-based paints, with no or limited breathability, should be avoided.
  • Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).