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 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.
The main moisture sources affecting basements and cellars are:
Making cellars and basement watertight consists of addressing all of these points, one by one.
According to BS 8102:2022 - Code of practice for protection of below ground structures against water from the ground there are 3 different solution types for waterproofing basements.
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.
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.
Type B waterproofing deals with the moisture by building the basement as an integral shell of concrete. The reinforced or low permeability concrete walls provide the protection from water ingress. Construction joints commonly associated with leaks need to be particularly well sealed.
This solution is usually applied when the water table is permanently below the underside of the lowest floor slab. Variable water tables present less of a problem, unless the water table stays high for a long time.
Type C or drained protection waterproofing involves some sort of cavity drain membrane and sump pump combination to drain out liquid moisture.
This solution is not considered tanking, but a water management system. It is designed to control and manage seepage into a structure by adequately disposing of the water through a drainage system either by gravity or using a sump pump. Certain variations of the system only use the membrane without the pump.
There are 3 grades (levels) of waterproofing standards:
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-breathable, trap 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:
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 heritage-friendly basement waterproofing system, that "ticks all boxes" - breathability, water and salt resistance, moisture control, long life expectancy - consists of the application of the following plaster coats:
The renovation concept is detailed below.
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:
Cocciopesto plasters are special lime plasters because they are:
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.
Here is a video showing the lime waterproofing system in action, being applied behind an old crypt.
Here are the main benefits of the Roman lime waterproofing system:
It is important to understand that British Standards and Building Control Standards are all about modern buildings. All materials featured in the standards are modern non-breathable materials, which are generally not compatible with historic buildings. Current building standards make no distinction between newer and older (e.g. historic, heritage) buildings and the fact that older buildings need to be treated differently from newer ones - using different renovation materials, approaches and technologies.
This creates an inherent conflict between modern Building Standards and Conservation Policies, forcing conservation professionals to either specify modern materials in old buildings or use heritage solutions that do not conform to modern building standards.
The Roman lime waterproofing system, due to its robustness and tried-and-tested nature, can satisfy both sets of regulations, providing a low-risk long-lasting solution that is also building-friendly.
The Roman lime waterproofing system can satisfy all conservation and listed building requirements, being a lime-based breathable solution. In terms of performance:
Here is how the Roman lime waterproofing system fits into modern building standards:
If building control requires a combined waterproofing system - a combination of two different waterproofing systems for double safety - the (Type A) Roman lime waterproofing system can be combined with a Type C cavity drain membrane and sump pump system, offering a combined (Type A / Type C) waterproofing solution that satisfies building regulations.
Here is how the Roman waterproofing system supports and complements a Type C cavity drain system:
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.
Applying the waterproofing lime plaster on underground wall sections inside the drainage channel prevents the sideways penetration of liquid moisture into the walls, contributing to a drier wall base. The plaster can also reinforce mechanically frail wall sections, stabilizing the wall.
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.
Download the technical guide of this solution in PDF format.
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.
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.
The cellar of this 15th listed century manor house cellar has been waterproofed and renovated with Roman lime plasters in a sympathetic way.
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.
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.
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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.
The plaster application video below explains the concept in detail.
