Repairing St Albans’ historic houses: Top tips for using lime plaster
PUBLISHED: 11:02 18 July 2019 | UPDATED: 11:02 18 July 2019
Interested in using historic plaster to repair your listed home? Franki Berry got hands-on to find out how.
With more than 800 listed buildings in St Albans, it's no secret that this district is brimming with history.
This ranges from Grade I sites of exceptional interest, for example St Albans Clock Tower and Rothamsted Manor, to Grade II* sites such as the former Town Hall on St Peter's Street.
But what does this mean for homeowners? Buying a beautiful Tudor townhouse or Victorian flat may seem like a good idea at the time, but it comes with strings attached.
Listed building consent needs to be sought from St Albans district council (SADC) for any significant material changes to the building - and that includes repairs.
The consent may require that suitable materials are used, such as historic limestone plaster.
According to limestone products supplier Graymont, the earliest documented use of lime as a construction material was in Egypt on the pyramids around 4,000 BC. Later, the Romans used lime mortars extensively.
It was used right until the 20th century, when it was superseded by modern materials like cement.
So what? Well, if your house is made with historic lime plaster and cracks (literally) start to show, getting an expert in will be expensive and difficult, especially if it is only a small repair.
I went on a Traditional Building Skills, Conservation Courses and Lectures Day at Hatfield House, delivered by Place Services at Essex County Council and run by Roy Cafferty from Traditional Plastering, to find out what to do.
The day-long session is designed for homeowners who want to repair a small patch.
Lime plastering is making a comeback, Roy said: "It's a realisation of the problems [with modern materials] and, to be fair, legislation - that is what really got it going.
"Builders dragged their feet around not wanting to use this or that material. There has been so much damage done to old fabric, there really has, from using modern materials on them.
"Virtually every timber frame house, you strip off the plaster and the timber is rotten because of cement and gypsum plaster because they don't breathe. It is so bad."
So how to go about using lime plaster? Although I am still far from being an expert, here are some of the top tips I learnt on the course:
- Hair is key
Without hair or fibres, lime plaster will disintegrate, shrink, and crack. Horse, yak, (even human) - the lime plaster must have a decent spread of about 5cm strands throughout.
Scraping up the lime mix with a trowel should reveal a "thick beard", in Roy's words, to strengthen the mixture.
He added: "You can make a good plaster mix without hair that will be rubbish, or you can make a rubbish plaster but with hair, and it will be okay."
-Mixing and crushing
The lime mixture, of about two parts sand and one part putty, needs to completely cover the sand and the hair. Roy acknowledges that the conventional ratio is three to one, but he says two to one makes a better blend.
Use a trowel to blend using a baking-esque figure of eight, and the small end of a stick to smash it together. Its colour will depend on the type of sand, but 'sharp' seems to be a good option.
Also, wear protective glasses for this bit. I was not mixing particularly viscously (my arms are not strong enough for that), but the mixture was flying everywhere. I was picking plaster from my hair for hours afterwards. I imagine getting this stuff in your eyes is not pleasant - better safe than sorry.
- Keep the surface damp
Water helps the plaster to attach and stick, and the top coat to even out to a nice sheen. When in doubt, splash a little on.
- Apply pressure
Lime plaster attaches to the wall by hooking onto wooden slats beneath. Push your plaster through these gaps to make sure your ceiling or wall is not going to come crumbling down around you in a few years' time.
- Three's the charm
When filling a patch, there should be a base, middle and top coat. The bottom layer should be thick with hair and the top should lie flat to the existing wall or ceiling, without holes or bumps. Use a polystyrene block to rub out any imperfections while the plaster is still drying, and always start at the edges.
Use a handy stick to shimmy along the plaster surface and scrape off the bumps. Any creamy mix left on the stick can fill holes that have probably appeared. Simple.
Find out more about the courses at www.placeservices.co.uk