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Waterproofing Basements, Cellars

The Roman Lime Waterproofing System
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Waterproofing old basements and cellars is a common way of extending a property either by gaining valuable storage space or getting that much needed extra living space. The conversion of basements and cellars, being situated partially or fully underground, subject to more or less ongoing moisture, can come with significant technical challenges, especially on the waterproofing front.

Let's explore this is more detail.

The Problem

The primary challenge in waterproofing basement and cellars, in converting underground areas into useful spaces is how to deal with moisture; how to keep water out from the building without causing long-term damages to the building fabric.

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Main moisture sources in cellars and basements

The main moisture sources affecting basements and cellars are:

  1. Penetrating dampness: being underground, cellar walls are in permanent contact with the damp soil. Water penetrates the wall fabric causing either liquid water penetration or ongoing evaporation on the inner face of the walls.
  2. Rising damp: old walls underground are also subject to rising dampness and salts from the ground.
  3. Water ingress: aging, dampness and ongoing salt crystallization break down the pointing and the building fabric, making old basements susceptible to water ingress and leaks.
  4. Condensation (and often mould): constant evaporation from the walls and floor [1, 2 & 3] results in high air humidity. This humidity – assisted by the hygroscopic effect of salts and inadequate ventilation – is recycled back into the walls, resulting in condensation. 
  5. Dampness caused by incorrect past renovations (e.g. old tanking): renovating old cellars with non-breathable materials (cement pointing, tanking, impermeable membranes etc.) makes moisture accumulate behind them, water often being “pushed up” in the building, creating further dampness problems.

Making cellars and basement watertight consists of addressing all of these points, one by one.

Modern Solutions - Waterproofing Basements

Usually, the dominating moisture source in a basement is sideways moisture penetration due to the presence of the soil and rainwater at the other side of the wall. The penetrating dampness problem can be solved in several ways:

1. Barrier Protection or Tanking

Barrier protection, tanking or Type-A waterproofing provides protection against ground water ingress by applying a waterproofing material (e.g. cementitious slurry, flexible waterproofing membrane (bitumen, rubber), bentonite clay liner etc.) to either the external and/or internal surface of the walls or floors. Once the waterproofing material hardens it makes the walls watertight on a permanent basis.

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Tanking a basement

If the waterproof barrier is applied from outside, the water is stopped at the outer face of the walls, allowing the walls to dry if the internal surface is left breathable. The installation of external barriers involves excavation, which is not always possible due to environmental constraints, limiting the options to a drained protection.

2. Drained Protection or Membrane Waterproofing

Drained protection or Type-C waterproofing is applied when liquid water penetration is present. It involves some sort of cavity drain membrane and sump pump combination to drain out liquid moisture. The penetrating ground is then from behind the studded cavity drain membranes via designated drainage channels, being routed to a sump pump for disposal.

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Membrane waterproofing

Old Buildings Require a Different Approach

As we can see, most solutions on the marketplace for waterproofing basements and cellars have been developed for newer buildings. As described earlier, they use modern waterproofing materials – cement, tar, plastic membranes etc. – that can block water, but they are non-breathabletrap humidity which can lead to the accumulation of moisture behind the waterproofing layer, damaging the fabric of old, historic buildings long-term.

Because the fabric of older buildings often contains a significant amount of moisture (kept in check by ongoing natural evaporation), the materials used for the renovation of these buildings must permit evaporation, i.e. must be breathable.

Many traditional materials – such as regular lime plasters – that can last for decades in a damp-free environment, deteriorate quickly in the presence of elevated humidity and crystallizing salts, thus not being suitable nor long-lasting enough for the waterproofing basements or old cellars.

The ideal waterproofing material for cellars of old and listed buildings would be a lime plaster that could satisfy the following, somewhat contradictory, requirements:

  • Be waterproof: to completely stop liquid water in order to keep underground areas dry.
  • Breathable: to let water vapours pass through freely to prevent the accumulation of moisture.
  • Resistant to salts: salts are the primary reason behind the premature breakdown of lime plasters. A good lime plaster should be able to cope with all salts without being damaged by them.
  • Not too hard: so it won’t damage the underlying softer building fabric but mechanically resistant enough to withstand water pressure and the effect of salts.
  • Long lasting: have a long service life.

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.

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Natural volcanic pozzolan under the microscope: reveals a highly porous, breathable pore structure

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:

  • Liquid moisture barrier: as a result of its special pore structure, this plaster acts as a selective moisture barrier, stopping liquid water while allowing the passage of vapours.
  • Salt barrier: this plaster acts a salt barrier, protecting subsequent lime coats from premature decay caused by dampness and salts. As a result the longevity of lime plastering increases by about 10 times.
  • Consolidates the frail, crumbly masonry, providing a solid, dry, breathable surface to build on. 
  • Internal humidity regulator: by stopping liquid water it regulates the internal humidity, creating a drier, healthier, warmer, more comfortable climate.
  • Improves the performance of thermal insulation by keeping any thermal plaster applied on it dry. A dry insulation performs thermally much better than a moist or damp insulation.

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).

Sympathetic Solution - Waterproofing Basements

The right basement waterproofing plastering schedule, that "ticks all boxes" - breathability, water and salt resistance, moisture control, long life expectancy - consists of the application of the following plaster coats:

  1. First coat: the Rinzaffo MGN Roman salt-resistant lime base coat.
  2. Second coat: a lime-cocciopesto plaster. This is also available as a thermal plaster variant.
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The building-friendly Roman lime waterproofing solution

What is Cocciopesto?

In ancient Rome, a particular type of mortar called "cocciopesto", a mix of lime and brick dust or crushed terracotta fragments (coccio = pottery fragments; pesto = crushed) was widely used as:

  • A plaster to protect wall structures from moisture. It has been the plaster of choice for high-humidity environments such as Roman baths, basements and cellars.
  • A flooring material, both as a floor base and floor finish, due to their excellent longevity and moisture-regulating properties. 
  • A finishing plaster to create impermeable linings for cisterns, wells and aqueducts (ancient water pipes), or today for bathroom or shower room walls

Cocciopesto plasters are special lime plasters because they are:

  • Extremely breathable
  • Resist moisture and humidity
  • A warm material with high thermal mass, naturally preventing condensation even in high-humidity environments
  • Outstanding hygro-thermal regulators, creating a pleasant, healthy, low-humidity indoor environment

Cocciopesto plasters, given the wide accessibility of their prime materials and their wide range of application, spread rapidly to every corner of the Roman empire, and they are still widely used today.

The renovation concept is detailed below.

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The building-friendly Roman lime waterproofing solution

Other Recommended Actions

Because cellars are usually affected by several moisture sources, the following factors also must be taken into account and if needed, any irregularities rectified, if possible.

  • Drainage: the presence of drainage is very important for cellar projects. After all, it's easier and less costly to channel away the water as much water as possible from the building than to fight it. In lack of proper drainage, rainwater doesn't flow away but tends to pool around the basement walls, resulting in a more abundant sideways moisture penetration. Seasonal high water table problems can also be controlled with adequate drainage, depending on the location and specifics of the building.
  • Ventilation: there is a lot of invisible vapour humidity in basements, that needs to be ventilated out.
    Thus a working ventilation system is an integral part of a basement conversion project and it should not be overlooked.

For more information, please see below a list of recommended products for the implementation of this solution, as well as a list of technical pages, projects, image galleries and videos related to this solution.

Technical Guide - Download

Download the technical guide of this solution in PDF format. 

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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.

Completed Projects

Here are some of our projects using this solution:

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.

Showing items: 1 - 3 of 3 total.
The Crypt Waterproofing Project

Waterproofing the service area of an old crypt with traditional Roman lime waterproofing and Cocciopesto plasters. On some parts of the room the MGN Lime-Pozzolanic Tanking Slurry System has also been applied to reinforce the waterproofing in critical areas.

15th Century Listed Manor House Cellar

The cellar of this 15th listed century manor house cellar has been waterproofed and renovated with Roman lime plasters in a sympathetic way. 

Project - High Street Bank Basement Waterproofing

Long-term ongoing gutter leaks and lack of maintenance have ruined several walls of this building. Being a Victorian building, the waterproofing of the basement has been resolved with breathable lime-based materials.

Videos

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

Showing videos: 1 - 6 of 8 total.

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. 

Are you in the process of buying / selling the property?
Preferred time for the survey? (tick all that applies)

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).