Back to tradition – lime plaster

We’re using lime plaster to finish the outside walls. Lime is a fairly recent revival as a building material, and people are re-learning the old trades as they go, so there are still a lot of disagreements about the best way to do things. It’s not yet Lime Wars, but there are regular skirmishes.

As we’ve noted before, the Essex way (as taught on a course we attended near Braintree) was to use lime and sand as a hard render on the outside of a wattle and daub house. We repaired a wall like that, only to be told off a year later by a local Suffolk expert: the true vernacular coating in Suffolk was a plaster made from chalk, hair and lime, which is tougher and more flexible.

We couldn’t find much support at the time for this in publications of the Society for the Protection of Ancient Buildings, or indeed at a seminar of theirs we attended. The focus was on plasters inside and sand renders outside. But we found later that chalk lime plaster was very much backed as an external material by one of our local suppliers, Anglia Lime, when we went to see them at Belchamp Walter, which happens to be on the Esssex side of the border (but is not much more than a rifle shot from Suffolk).  Anglia Lime makes strong claims for the durability of their FibreChalk Mix (using artificial fibres) and Haired Chalk Mix (using animal fibres) on outside walls. We used their products for ceiling and internal wall plastering in the old building, and a neighbour says he is very happy with their fibre mix on his outside walls.

When we removed the cement render on the gable end of the old part of our house, Alan Wilkins and Rory Sumerling used a chalk lime plaster with pig hair, mixed on site, to finish it (see earlier posts). Alan first suggested artificial fibres, but we wanted to stick to animal hair on the old building.

Alan couldn’t take on the new job, so Terry Booty introduced us to Keith Southey, another Suffolk lime plaster specialist.  He supplies a chalk mix using artificial fibres called Fibrelime, made from lime produced by the Lincolshire company Singleton Birch.  Suffice it to say that Keith didn’t see eye to eye with Anglia Lime on fibre and chalk mixes or with Alan Wilkins on pig hair, let alone with our one-time tutors in Essex. But we let Keith’s team get on with it (which they did very speedily), and the end result was a good, clean lime coating on the walls.

Why, you may wonder, did we opt for a traditional breathable lime finish on a modern watertight building? Surely a more durable modern cement-based render would be better, and more waterproof?

In fact, traditional lime makes a lot of sense on a timber-frame building, which is bound to show some movement. Lime plaster is much more flexible than a hard render, so less likely to crack. It also makes for a much more interesting surface texture that will be closer to that of the old building.

I have one more reason for preferring it: in ordinary British weather it is almost impossible to complete a building in the dry, and to the extent that some moisture will be trapped in the fabric, a coating of breathable  lime on breathable Savolit boards will allow it to escape. After the building is finished, lime will absorb more moisture than cement would, but it will also expel it again quickly in dry weather, rather than trap it (which is what happens when moisture gets through microscopic cracks in cement render).