Work started on conservation work at St Johns Chapel on the Black Isle, removing the vegetation to reveal the enclosure walls, and are raking out the loose mortar ready for repair work to begin.

The question of which lime mortar mix to use has been the “hot” topic of the week.

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We regularly highlight the dangers of using cement on stone masonry and the importance of using lime mortar, but there is no such thing as a standard or generic lime mortar. This is something we stress and stress again in our building inspections when we discuss the repairs that are needed.

The most commonly used binder is Naturally Hydraulic Lime or NHL. It comes in 3 flavours, 2, 3.5 and 5 that corresponding to their relative strength. These appear to be the most straightforward to use but looks can be deceptive and mistakes are easily made. First of all, the most common mistake is to automatically go for the NHL 5, since it is the strongest binder and mix it up with builder’s sand in a 1:3 mix.

Not all NHL 5’s are the same. The colour, strength and consistency vary from one manufacturer to the next. It is suitable for some exposed situations, but more often it is still too strong a mix and you really ought to be looking at the weaker binders. Mix ratios are also important. The bulk density of lime binders will change from one manufacturer to another and could end up giving you a different mix from the one you were trying to create. Varying the mix between a 1:2, 1:2.5 or 1:3 will affect strength, durability, frost resistance, breathibility.

When you are planning to use lime mortar, it is because you understand that there is a need to protect the stonework of a building, so it is important to do it right. Even the simplest Lime Mortars require some expertise and bit of care and attention in selection and preparation. Its not just a case of heading down to your builder’s merchant and loading up the truck with a few bags of NHL 5.

At St Johns Chapel, we are using a gauged hot lime mix, this is more sophisticated than the NHL binders. It offers a better match for the historic mortar and offering improved flexibility and durability which is more compatible with the soft sandstone. The mix that we are trialling on a sample panel has been the result of the dialogue between ourselves as architect, the stone masons and is based on analysis of the existing mortar that was undertaken in the Lab at the Scottish Lime Centre. If we like to way the sample panel turns out, we are good to go.

The key thing here to work with people who know the material, think about carefully about the work that is to be done and the appropriate mix for your situation and not to rush in.

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

If you need independent advice give us a call ?