How to retain and repair traditional plasterwork

Following the high-profile collapse of a decorative plaster ceiling at the Apollo Theatre in Westminster, London in 2013, the Association of British Theatre Technicians (ABTT) published a guidance note in 2015, Advice to Theatre Owners and Managers regarding Suspended Fibrous Plaster Ceilings; Survey Certification Record Keeping etc. Earlier this year, Historic England added to this work with Historic Fibrous Plaster in the UK designed as interim guidance for conservation professionals and building managers responsible for UK properties with fibrous work.

Fibrous plasterwork is a form of decoration made from plaster of Paris reinforced with hessian and thin strips of timber. Ceilings of this type were cast in workshops in large panels and dried before being transported to site for installation. Use of fibrous plasterwork increased in the mid to late 19th century because it was lighter, quicker to produce and therefore cheaper than the traditional lime-based plasterwork applied on timber laths.

Although often associated with public buildings, theatres, music halls and cinemas, fibrous plasterwork is increasingly being discovered in private houses that were built, extended or altered in the late 19th and early 20th centuries. This therefore requires surveyors to be able to identify the type of plasterwork present in buildings so they can appraise the risk of defects and failures.

Fibrous plasterwork was often fixed to, or suspended from, the structure above using hessian ties soaked in plaster of Paris, a method known as wadding. Investigations and research carried out since the 2013 Apollo collapse have shown that wad ties can be susceptible to poor environmental conditions, penetrating dampness, damage from later interventions and additional loading. The Historic England guidance considers wadding to be one of the greatest risks to the integrity of fibrous plaster.

The ABTT guidance, which has largely been incorporated into Historic England's publication, requires those responsible for fibrous plaster ceilings to carry out regular inspections. This is especially important for ceilings supported by unreinforced hessian wads, which the ABTT considers to be a deleterious material likely to require additional reinforcement to minimise the risk of collapse (see photo 1). It also advises that, in areas that cannot be reasonably inspected, the use of unreinforced wads should be presumed.

The baseline survey should be used to inform any repairs required, advise future management of the asset, and decide when the next ceiling inspection should be carried out. This may require additional support ties, or replacement of existing ones; removing accumulated debris or reducing imposed loads such as walkways; consolidation; and in some cases, replacing parts of the ceiling.

Between these main inspections, ceilings should be regularly checked for signs of change, including cracking and staining, and any evidence of this should be acted on in a timely manner.

While the advice has been written for fibrous plasterwork, many of the same principles apply to inspecting traditional lime-based plasterwork, which was much more commonly used in British buildings until the early to mid 20th century, and which can be equally susceptible to many of the same issues.

Traditional lime-based decorative ceilings were usually applied to timber laths nailed to the underside of a floor or roof structure or a secondary structure attached to the principal structure above. Ceilings, cornices and mouldings were built out in coats of haired plaster– so-called coarse stuff – and finished with a finer finishing or setting coat. Decorative elements were either created in situ by hand, or cast from moulds and stuck into place afterwards.

Checks should be made to assess the condition of the plaster nibs or rivets that are formed on the upper face of the laths when the first coat of plaster is applied, and which provide a mechanical key or bond. These can be inadvertently damaged as a result of additional loading, later reservicing and alterations, insulation upgrades, or vibrations from site activities. The condition of timber laths, ties and connections between secondary and principal structures should be checked, as should the surrounding fabric for signs of water ingress.

A close inspection of the underside of the ceiling should be carried out, with defects – including cracks, detachment and surface disruption – accurately recorded. As with fibrous plasterwork, the method of construction and any defects observed should be accurately recorded and used to inform future repairs and monitoring.

In most cases, retention and repair of traditional lime-based plasterwork is possible and should be the first consideration, especially in significant buildings. Repairs of traditional plasterwork should be carried out following guidance from English Heritage, or, north of the border, Historic Environment Scotland. Wholesale replacement should be the last resort, and is generally avoidable.

Dr William Napier MRICS is a chartered building surveyor, an RICS-certified historic buildings professional and a fully trained decorative plasterer wnapier@adamsnapier.co.uk