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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Well, when farm animals are housed in barns and shippons, they do not necessary remove themselves to a planned urinal, instead spraying where they stand, with ‘spray’ being the operative word, resulting in the ‘ground floor’ walls being wetted and contaminated by hygroscopic ammonia salts to a considerable height, and because such salts are particularly aggressive, it is common to find symptoms of rising dampness, even if not True Rising Dampness, and more accurately Hygroscopic Dampness, to that considerable height.

In dealing with a barn refurbishment, one can offer various options, ideally giving the client cost against risk scenarios so that informed choices can be made. This may include the following and take into consideration several factors:-

(a) leaving existing dense and well adhered render in place where removal may cause unacceptable structural damage to the substrate brickwork, this a particular problem with porous soft bricks;

(b) because direct applied plaster is at high risk where animals were known to be housed, limiting specialist liability in the event of any direct applied plaster being chosen, no matter what the formulation is, this by warning the client that if hygroscopic dampness does later occur, one reserves the right to undertake charged repair, and in extreme cases to advise a different remedial specification rather than attempting risky patch repair using the initial method.;

(c) emphasizing that the safest action would be to line the walls, this also including variations of (i) using internal block walls, an option commonly taken so that upper floors can be supported off or within them, and to assist in stabilizing defective existing walls, (ii) lining with cavity drainage membranes, dry lined over, or (iii) covering the walls with a proprietary dry lining system, the latter specifically ensuring that no plaster based product, be it the board or finishing plaster, be allowed to make contact with the walls;

(d) specifically advising that no dabs of any kind be used, this of course being the most economical option at the time of refurbishment, but most costly in the long term when cold bridging and associated hygroscopic dampness causes damp spots on internal finishes, dampening at times of high humidity, calling for large scale post completion repair that inevitably involves a different method;

(e) where dry lining is used without a vapour barrier, warning that in extreme cases there can be post completion movement of vapour, allowing escape of unpleasant ammonia based odours, this more commonly where solid external walls may be subject to rain-water penetration or less than perfect damp proof courses, and (f) warning the client that an injected remedial damp proof course within a barn will never be 100% perfect, be it stone or brick, because very often conservation requirements limit the degree of pre-injection repair, with this compounding the fact that the method of replastering must be seen as an  integral part of the remedial damp proof course.

While I have mainly mentioned potential problems in barn conversions, anyone with long term experience in dealing with dampness in buildings would confirm that similar pitfalls can occur in refurbishment of many other types of buildings.

Only last week a plasterer who served his apprenticeship with us came back to help us, advising that he had recently spent a year applying dabbed plasterboard to walls within 300 former council houses being refurbished, only to spend a further three months going back to the same properties to remove sections where cold bridging and hygroscopic dampness at the dab locations had caused widespread latent spots.

Guess what? He was told to repair using plaster dabs again, so I do not need to wonder what the long term result of that will be!

The simple lesson is that wherever feasible in a very wet and or salt contaminated building, advise the safest option, i.e. isolation of  plaster from the substrates by some means, including vapour barriers where appropriate, and where such may involve a previously derelict and saturated building, be it a barn or not, you will provide secondary benefits of (i) allowing the substrates to dry out gradually without compromising internal finishes, (ii) isolating vulnerable joinery from dampness, in which post completion rot commonly occurs during the drying out period if not so isolated, and (ii) assisting in meeting required U Values. While this is not what manufacturers of direct applied plaster products want to hear, from the point of view of a specialist contractor who is liable for advice given, and who is always the first to be called back when post completion problems occur, we can design dampness and latent rot out of the equation with linings instead of taking on unknown, fingers crossed, risk.

I know that many specialist companies think this way, and notwithstanding that we all continue to apply ‘plaster’ direct in ordinary cases, with the actual resulting percentage of problems being acceptably low, GPI have noted an increase in insurance registrations where cavity drainage membranes are applied instead of typical remedial replastering in association with ordinary damp proofing. In a remedial situation up to now, my own company has followed this course of action mainly where our direct applied plaster has failed, this designing the problem and our future risk, out of the equation.