Solving Rainwater Splashback

A Building-Friendly Approach

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The Problem of Rainwater Splashback

Rainwater bouncing back from the ground or the plinth is another source of moisture that affects all buildings, especially older buildings built of more porous, water-absorbent materials.

This can be recognized by the green discoloration around areas where water splashback is prevalent. The green discoloration is green moss, thriving in wet areas, fed by the abundance of rainwater splashing back onto the wall.

This problem can occur in several scenarios:

Rainwater splashes back from hard surfaces (pavement, pebbles etc.) near the base of the walls
Passing cars splashing groundwater and road salts onto roadside walls
Leaky gutters causing splashback near the base of some wall or window sill

The splashback water not only keeps the wall wet but also carries or dissolves salts from the wall fabric, reinforcing the rising damp, making it rise higher. This keeps the walls in a more damp state than they otherwise would be.

Modern, Non-Sympathetic Solutions

Due to their properties, regular lime plasters do not provide a robust enough, long-lasting solution. Regular lime plasters are broken down by excess humidity and salts and they do not last in these extreme environments.

As a result, the base of old buildings is commonly replastered with cement in an attempt to protect the base of the walls from water splashback and salts. Although cement plasters perform apparently well in keeping liquid water out, when used in older or listed buildings designed to breathe, they also come with important drawbacks or trade-offs.

Some of the reasons why cement-based renders should not be used in older buildings are:

Chemical incompatibility with old buildings: cement contains many chemical additives and salts (up to 10%). In the presence of moisture, these salts from the cement continuously migrate into the masonry¹, steadily increasing its salinity, leading to spalling, crumbling of the underlying wall fabric and the detachment of the cement render. A hollow-sound render, when tapped, indicates this detachment.
Non-breathable: they don’t let moisture freely evaporate from the underlying (damp) wall fabric, resulting in an excessive build-up of moisture leading to long-term dampness problems.
Hard and brittle: being dense, rigid and inflexible materials, ongoing vibrations and building movement make the cement crack. Once their waterproofing ability gets compromised, cementicious materials let rainwater in, leading to additional dampness problems over time.
Poor thermal performance: being a dense material, cement plasters in general are very poor thermal insulators, causing condensation and mould problems.

Lime-based breathable waterproofing plaster alternatives exist that can solve all water penetration problems fully in a building-friendly way with no drawbacks, keeping the wall fabric breathable long-term.

The Building-Friendly Solution

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.

coreconservation solutions natural pozzolan01 — 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).

Rebuilding the plinth with the Rinzaffo MGN Roman waterproofing mortar - instead of using cement - will protect the wall fabric long term while letting the wall fabric to breathe.

The recommended plastering schedule is the following:

Rinzaffo MGN: the Roman lime waterproofing coat. This protects the wall from water and salt penetration, while allowing the underlying fabric to breathe.
Sanacolor 2000 MGN (optional): an optional second decorative coat. Although the Rinzaffo MGN waterproofing coat can be left on its own, it looks nicer with a decorative coat. While the waterproofing coat has a single brown colour, using an optional decorative coat - which comes in 24 colours including white - offers more decorative options for the wall base.

solving rainwater splashback — Lime plinth that replaced a former cement plinth

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

1
Vecchio F: Evaluation of mortar performance for maintenance, conservation and restoration interventions - Master's thesis, Polytechnic of Turin, April 2019. https://webthesis.biblio.polito.it/10439

References

1
Vecchio F: Evaluation of mortar performance for maintenance, conservation and restoration interventions - Master's thesis, Polytechnic of Turin, April 2019. https://webthesis.biblio.polito.it/10439

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.

Rinzaffo

Roman Waterproofing Lime Base Coat

Base

Waterproofing

Sanacolor 2000

White / Coloured Decorative Lime Render for External Use

Main

Videos

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

Breathable Lime Plinth on Listed Building – A Traditional Alternative to Cement

Solutions

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

Base, Waterproofing

Application Guide

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.

Sanacolor 2000

Main

Application Guide

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.

Recommended thickness: the recommended application thickness is about 20 mm.

Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.

Finishing: apply the last 5 mm of plaster as finish, creating the desired texture. Alternatively a fine plaster can also be applied, if needed (Rasante B2007 MGN) is recommended.

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 coloured plaster, it does not need painting. However, it can 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).