How should I treat cracking plaster on a 19th-century house?

I am in charge of an old parish house called Millbank in Trillick, Co Tyrone. It has been unoccupied for some time but kept heated. The external plaster has extensive cracking and flaking paint. It appears to have been plastered with sand and cement, probably in the 1940s or 1950s. I have two questions. What do you imagine would have been used to plaster the walls when the house was built? And is leaving the walls without plaster an option?

I’m assuming from the information provided that Millbank House is, at a guess, circa 150 years old, and why I estimate this is that the house was probably at least 100 years old when it was refurbished in the 1940s.

These period houses are plentiful around the country, but sadly a lot of them are neglected and fall into dereliction.

Having inspected numerous similar type homes over the years, I have a real appreciation for such properties and I generally take a moment to think of the skilled tradespeople who worked on their construction and the people who resided in them. Coincidently, I am currently working on a church which was restored in the 1950s.

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It was common when restoring old buildings in the 1940s and 1950s for the wall renders to be replaced with hard cement-based materials.

To come back to your question as to the type of wall render that I think would have been originally used on Millbank House, it is my opinion that a lime-based render was used on the construction of this house. The use of lime-based materials in the construction of older buildings is one of the main reasons why they have survived so well to this day.

Flexibility

Lime-based mortars and renders allow the walls to breathe, allow damp to penetrate into walls while also allowing the walls to dry out easily. Lime-based mortars and renders have a degree of flexibility and can withstand structural movement without causing cracking.

On the other hand, the use of hard cement-based mortars and renders do not allow walls to breathe, these renders crack easily as they are less flexible and the presence of cement-based renders traps moisture in walls causing damage to the wall structure.

Period buildings from the Millbank era were generally constructed in two forms:

1 Buildings incorporating cut-stone construction – such as churches, castles etc – and these structures were designed and built using good quality stone. These stone walls were left exposed to the elements as designed.

2 Buildings incorporating stone and brickwork construction which have a render finish such as traditional buildings – houses, farm buildings, etc. These buildings were designed and constructed so that the walls would be finished with a lime-based render.

The purpose of a wall render on an old building is generally twofold (apart from aesthetics). First to reduce the risk of damp penetrations and, second, to act as an insulative layer on the walls.

Damp ingress

The removal of a wall render from an old house will expose the stone and brick work walls to the elements and this can result in damp ingress and deterioration of the walls. The majority of these old rendered wall structures were never intended to have their render removed. But sadly I see it all too often where renders are removed to expose stone walls, etc.

Without seeing this property, it is my opinion that the external walls should be finished with a lime-based render which will allow the walls to breathe and will reduce the risk of damp penetrations. The application of such renders should be undertaken by skilled tradespeople.

It is most likely that your property is classified as a "listed building" (similar to the "protected structure" classification in the Republic). In Northern Ireland, there are about 9,000 listed buildings and structures. You are advised to contact the local authority for guidance and seek the services of a suitably qualified professional (building surveyor, conservation architect, etc) before carrying out any proposed works. It is also possible that the project may be eligible for grant funding/tax relief.

[ Society of Chartered Surveyors Ireland ]