A fibrous ceiling in Gosford House, East Lothian, Scotland © Dr William Napier
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 on suspended fibrous plaster ceilings in 2015.
Following this, in 2019, Historic England published Historic fibrous plaster in the UK, as interim guidance for conservation professionals and building managers responsible for UK properties with fibrous plasterwork.
Use of fibrous plasterwork in private houses
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. Therefore, surveyors must 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 Apollo Theatre London's ceiling collapsed in 2013 have shown that wad ties are 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.
Specialist organisations have published guidance advice
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.
Historic England also advises that, in areas that cannot be reasonably inspected, the use of unreinforced wads should be presumed.
Both the ABTT and the Historic England guidance advise the following.
- Structures supporting suspended fibrous ceilings should be inspected by a competent structural engineer, and plaster parts by a competent plaster inspector.
- Ceilings should be inspected from both above and below, and surrounding fabric should be inspected for signs of water ingress – past and present – as well as structural movement.
- A baseline survey should be carried out to give an accurate record of the ceiling's construction, including all supports.
Unreinforced wad ties – a common method of fixing decorative fibrous plasterwork – will require replacement to minimise the risk of failure, as recommended by new guidance © Dr William Napier
Baseline surveys inform repairs and management
The baseline survey should be used to inform any repairs required and advise future management of the asset. 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.
Baseline surveys should also inform when the next ceiling inspection should be carried out.
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.
Both the ABTT and Historic England provide survey templates.
Retention and repair of traditional plasterwork should be considered as the primary option where possible © Dr William Napier
Traditional lime-based plasterwork
While the professional guidance has been written for fibrous plasterwork, many of the same principles apply to inspecting traditional lime-based plasterwork. This was much more commonly used in British buildings until the early to mid-20th century, and 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 applied 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, and from Cadw in Wales. Wholesale replacement should be the last resort and is generally avoidable.
A version of this article was originally published on 15 November 2019.
'While the professional guidance has been written for fibrous plasterwork, many of the same principles apply to inspecting traditional lime-based plasterwork'
Dr William Napier MRICS is a director at Adams Napier Partnership, chartered building surveyor, an RICS-certified historic buildings professional and a fully trained decorative plasterer
Contact Dr Napier: Email
Related competencies include: Building pathology, Conservation and restoration
Top tips for working with plaster
- Identify the type of plasterwork present, for example traditional lime-based plasterwork or fibrous plasterwork.
- Establish the method of fixing and support used, and their condition.
- Assess condition of surrounding fabric and consider impact of later interventions such as services, alterations and imposed loadings.
- In significant buildings, consider a conservative approach that allows retention of as much historic plasterwork as possible.
- Encourage contractors and building professionals who do not have conservation experience to attend training courses.
- Implement regular inspections informed by a baseline plaster survey.