Solving Rainwater Splashback

Building a Lime Plinth

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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 — Volcanic lava sand (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 - require higher firing temperatures than normal (air) limes to ensure proper reaction between ingredients. The higher temperatures partially close off the pores, making NHL plasters less breathable than air limes. As a result, NHL plasters gain water resistance at the expense of breathability.

Ancient Roman mortars combine lime and a variety of volcanic sands and ashes. These are mixed to the lime at room temperature (without heat) as the volcanic sands and ashes have already been pre-burnt by the volcano. The lack of industrial heating retains the open pore structure of the mix resulting in high breathability. Furthermore, in Rinzaffo MGN, a portion of conventional sand is replaced with volcanic sands. Unlike dense quartz sands resembling glass, being impermeable to water vapours, volcanic sands are highly porous, contributing to the plaster's breathability.

The skilful combination of lime and volcanic ingredients can make these plasters highly resistant to dampness and salts, resulting in outstanding performance and longevity 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 clean tap 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 is 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 mechanically 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 much better thermally than a moist or damp insulation.

This lime plaster is made exclusively of natural materials. It contains no cement, 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

The application of the plinth can be plinth can be supplemented by additional drainage to channel the water away from the base of the building. In that case, the Rinzaffo MGN Roman plaster can be applied inside the drainage channel.

Applying the Rinzaffo MGN waterproofing lime plaster on underground wall sections inside the drainage channel prevents the seepage of liquid water into the walls, contributing to a drier wall base. The Roman plaster also reinforces the damp, frail wall sections mechanically, consolidating the old fabric.

baement-2024-core-conservation — Applying the Roman base coat inside the drainage channel can help keeping liquid water away

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 product variations are possible as we have different types of plasters and paints, depending on your needs or application. Please get in touch to discuss additional options.

Rinzaffo

Roman Salt-Resistant Waterproof Lime Plaster

Base

Waterproofing

Sanacolor 2000

White / Coloured Decorative Lime Render for External Use

Second coat

Work Specifications

Here are the recommended work specifications for this solution.

Base, Waterproofing

Application Guide

Rinzaffo Controspinta MGN

[15-20 mm] ABOVE GROUND: Lime base coat, applied as:

█ [5-10 mm] Levelling / Repair: level, repair and point uneven walls first. The material consumption here can vary, depending on substrate condition. Very uneven walls (e.g. after cement plaster removal) can take up more material.
█ [10 mm] Protective base coat: apply a 10 mm coat.

[25-30 mm] UNDER GROUND: Lime waterproofing coat or for very damp wall sections, applied as:

█ [5-10 mm] Levelling / Repair: level, repair and point uneven walls first. The material consumption here can vary, depending on substrate condition. Very uneven walls (e.g. after cement plaster removal) can take up more material.
█ [10 mm] Waterproofing coat 1: apply a first 10 mm coat.

▒ [Mesh] Fibreglass mesh (10 x 10 mm): embed a mesh into the plaster for added flexibility.

█ [10 mm] Waterproofing coat 2: apply a second 10 mm coat to complete the waterproofing.

Second coat

Application Guide

Sanacolor 2000 MGN

[20 mm] Lime Render coat, applied as:

█ [15 mm] Render base, in a single coat.
█ [5 mm] Render finish – smooth or textured – from the same material.

Plaster Application

Here are some key application points about the application of each plaster, as well as links to the full application guides. Please have your plasterer read / watch the application instructions before applying the product. If any questions, please contact us, happy to help.

Base, Waterproofing

Application Guide

Rinzaffo

VERY IMPORTANT POINTS

There are a few very important points that MUST be respected for the correct application of this plaster. These are highlighted in red.

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. The easiest way to achiever his is to level (dub out) the wall first, then apply your required thickness. 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.

Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Cover the whole wall surface thoroughly leaving no holes or gaps behind, no matter how small. Smoothen the surface, giving it a light key with a damp brush.

Reinforce dark patches. The thickness of the plaster must be adapted to the (damp) condition of the underlying wall. Damp wall sections do evaporate out a lot of moisture, creating dark plaster areas as a result of condensation. These areas need extra thickness, more material. Apply an extra coat over darker areas.

– – – – –

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.

Cement backgrounds: the Rinzaffo MGN base coat can be applied and will bond well to cementicios backgrounds. The removal of cement plasters is recommended for the vast majority of cases to restore the breathability of the wall fabric. However, in some (rare) cases, when the removal of cement would be too invasive, too costly or would not beneficial the underlying masonry, if the cement backing is stable, the Rinzaffo MGN can be applied over it for waterproofing purposes (e.g. to waterproof cement floor bases).

Salt-inhibitors or PVA bonding agents are NOT needed and should not be used before the application of this plaster. This natural lime plaster bonds very 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.
IMPORTANT: Dry to wet mixing: plasters with hydraulic ingredients should always be mixed from dry to wet (workable) state. Add your powder to a bit of water, start mixing and gradually add more water till the right consistency is reached. This is the right way. Mixing the other way – wet to dry mixing – by starting with lots of water then gradually adding powder to the right consistency, is incorrect. Reason: excessive wetting weakens the plaster, alters its hydraulic set resulting in less performant, more friable and shorter lasting plasters.

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.

Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.

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

Second coat

Application Guide

Sanacolor 2000

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.
IMPORTANT: Dry to wet mixing: plasters with hydraulic ingredients should always be mixed from dry to wet (workable) state. Add your powder to a bit of water, start mixing and gradually add more water till the right consistency is reached. This is the right way. Mixing the other way – wet to dry mixing – by starting with lots of water then gradually adding powder to the right consistency, is incorrect. Reason: excessive wetting weakens the plaster, alters its hydraulic set resulting in less performant, more friable and shorter lasting plasters.

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).