Stone erosion and deterioration is widespread in Scotland. Lithomex is a lime based restoration mortar often used as a cheap repair and as an alternative to stone replacement. "Cheap" is the principal attraction for many but it may be a false economy.

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I am increasing encountering problems on building surveys, where the stonework has been repaired with the use of Lithomex or some other proprietary product that has been marketed as a “restoration” mortar.

Since it first appeared in the 1990s, it has been quickly adopted and used for many building repair jobs across Scotland. It has a predicted lifespan of 30 years if used correctly and well maintained. As the product is lime based and can be purchased in a wide range of colours to suit a buildings appearance, it has often (wrongly) been assumed to be;

• An invisible repair
• A safe repair, compatible with the historic materials.

Restoration mortars are intended to be used as a coating across the surface of damaged stone only. (Not across a large surface area as a often find). Because the mortar is pliable it can be moulded to the shape of the stone being repaired and tooled to create the same texture. The technique is termed “Plastic Repair”. The name is derived from the product malleability and ability to be moulded – it doesn’t mean that it contains hydrocarbons used for the plastic that we encounter in bags and packaging. The lime content suggests that the product is highly breathable and a high level of compatibility with the stone that it is applied to in the repair process. However, as a result of poor specification, poor workmanship and a lack of understanding of the materials, problems are encountered. This is particularly the case around the Inverness area where many buildings use a relatively soft red sandstone.

The biggest problems is that restorations mortars is that they are often used for cosmetic, patch repairs that mask damage to the stone without addressing the root cause of the damage in the first place.

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In these instances, the Lithomex rather ironically does ends up acting like a plastic bag.

Like cement, the restoration mortar seals in moisture, preventing the stonework from breathing. Moisture trapped in the wall core, tries to migrate outwards and pulls minerals and salts with it but gets blocked at the interface between the lithomex and the stone. Moisture is subject to repeated freeze/thaw action while salts chrystalise and dissolve. Over time, the mortar repairs becomes detached from the stone and when it comes away there is nothing but sand left behind. Meanwhile, those areas that haven’t been treated become saturated and stressed, as they have to deal with the increased evaporation load and moisture transmission. This takes a small problem and turns it into a very big one.

Key lessons to take away;

1. Find out the root cause of deterioration and fix that before you do anything else to your building.
2. If you can, always replace damaged and eroded stone with a matching/compatible stone based on petrographic analysis.
3. If there are situations where you have to use Lithomex or any other restoration mortar, use it properly, ensure it is compatible with the damaged stone, don’t use where the stone is in an exposed / vulnerable location. Avoid covering large areas of wall.
4. Understand that this is only a temporary measure and you are only delaying the inevitable, eventually the lithomex will deteriorate and will probably take the stone background with it, resulting in the replacement of the stone anyway.
5. If someone suggests Lithomex as a "Cheaper option", politely tell them "away and boil yer heed".

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MAAC studio are accredited conservation architects and principal designers working with traditional buildings throughout Scotland. 

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