Using Renovating Plaster in Wet Buildings

The farm, formerly part of the Badminton Estate, is privately owned and has two traditional Cotswold barns with outbuildings and farmyard.

Turning what was a working farm just three year ago into a large family home, presented architect Nick Hare with a whole host of challenges. The early Victorian barns are grade 2 listed buildings and were subject to a multitude of planning restrictions that required sympathetic development in all phases.

The main house has been developed from the largest barn, a traditional Cotswold threshing barn that has two large entrances opening into a full height threshing hall, overlooked by first floor galleries. The second barn has been converted into a cottage with stables.

Internally, it was decided to have a self-coloured white plaster with similar properties to a traditional lime, haired plaster and Tarmac’s Limelite Renovating Plaster was selected. Conservationists accept Limelite renovating plaster as a direct replacement for lime, haired plaster. It is a lightweight backing plaster that is designed to control dampness passing through the walls of old properties making it ideal for renovation work as it can be applied direct to damp walls.

Following installation of a damp proofing system, the exposed stonework was sandblasted in preparation for plastering. Slade Plastering of Chippenham then brush-applied a coat of Limelite Easy-Bond sulphate-resisting bonding agent before applying a scratch coat to flush out joints. Limelite Renovating Plaster was then applied as part of a two-coat system with Tarmac High Impact Finishing Plaster. More than 10 tonnes of Limelite renovating plaster was applied, much of which was freehand work to form soft angles. By using a wet sponge, Slade Plastering created a slightly texture surface finish that was in keeping with the style of the property.

In response we wrote to the magazine:

Dear Sirs,

A recent issue included a feature in which pre-packed lime and cement based renovating plaster was used on a barn conversion, and because of past experience with such properties and products, I offer the following advice.

While we all accept that this type of formulation is more robust than ordinary Gypsum based plaster, in theory allowing substrates to dry out gradually via movement of vapour, while preventing or limiting movement of salts, and providing a relatively porous face that discourages condensation, not all properties offer risk free substrates, particularly barns where animals were once housed, or indeed many ordinary properties in a wet condition prior to repair or refurbishment.

Having spent thirty years solving problems of dampness, I have seen not an insignificant number requiring very extensive post completion repair where direct applied ‘plaster’, be it a supplied formulation or one mixed on site, including lime or not, have not been robust enough to withhold latent dampness and hygroscopic salts, resulting in variations between the need to remove isolated areas of plaster to large scale wholesale replacement.

Now, what do I actually advise, or what should one put in writing to caution the client and limit liabilities? Well, firstly in respect of barns, if conservation requirements call for  direct applied plaster, the least one must do if direct applied products of any type are proposed, is test for ammonia salts, because if present, even the most hardy ‘tanking’ formulation is at risk. We have all seen barns where the Farmer has rendered the low level walls, I expect commonly incorporating a waterproofing additive, but even this does not always prevent the render degrading in time, so it is important to explain to a client the basic science of the subject.

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