Structural Assessment of Historic Plaster Ceilings
Kevin Clark
The highly decorative fibrous plaster ceiling of Wyndham’s Theatre in London, which is regularly inspected and defect-free (Photo: Conisbee, 2008)
Much welcome guidance has been published in recent years in connection with the inspection and assessment of historic plaster ceilings, however there is limited equivalent published information concerning assessment of their supporting structures. These often overlooked elements perform an essential role in ensuring the continued integrity of historic plaster ceilings and this article will explore their common forms and the importance of a thorough assessment of their structural integrity.
Obtaining a detailed understanding of the configuration and condition of the structure to which ceiling plasterwork is attached is usually the primary objective, and although the plasterwork itself is typically formed with great skill and care using proven traditional materials and techniques, the structure to which it is fixed is almost always hidden from view, difficult or impossible to access and occasionally assembled in an ad hoc manner with little thought of how it might be inspected or maintained in years to come.
It is therefore essential that appropriate advice is obtained from a plaster conservation specialist acting in cooperation with a structural engineer, both of whom should have a comprehensive knowledge of the various construction materials, as well as of the challenges and risks that are encountered during the appraisal and repair of historic plaster ceilings.
These requirements are not new, of course, but they have received greater attention since 2013 when part of the ceiling in the auditorium of London’s Apollo Theatre collapsed during a packed evening performance. The subsequent investigation by the Health and Safety Executive and plaster specialists culminated in the release in 2015 of guidance by the Association of British Theatre Technicians (ABTT) for the owners and operators of properties containing similar ceilings1 which recommends a thorough survey by a competent plaster specialist and structural engineer detailing the composition, arrangement and condition of the plaster and the structure to ensure the provision of appropriate advice to building owners and operators.
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| Figure 1. Solid plaster fixed by lath across a heavy timber framework (Photo: Conisbee, 2019) |
Historic Ceiling Types
Comprehensive accounts of the historic development of plasterwork are available (see Recommended Reading) but in simple terms historic plaster ceilings are essentially of two types; solid plaster or fibrous plaster.
Solid plaster may be composed of either lime plaster (calcium hydroxide from calcined limestone mixed with sand and other additives), or gypsum plaster (calcium sulphate) and often reinforced with animal hair.
Fibrous plaster is composed of relatively thin panels of gypsum reinforced with a woven hessian fabric and timber battens. Wet lime plaster has been in widespread use since ancient times and can be applied in situ directly to masonry but also to thin timber strips of oak or pine (laths) interspersed with narrow gaps through which the plaster squeezes as it is trowelled on.
When cured, the plaster that has squeezed-through forms resilient nibs that grip the laths and hold the thin plaster membrane in place. Lime plaster was sometimes gauged with gypsum to give it a faster set, and plaster of Paris, a relatively pure form of gypsum, was also used for decorative plasterwork in much the same way as lime, particularly from the late 18th century.
In simple ceilings of the modern era consisting mostly of monoplanar flatwork, the laths are nailed directly to the underside of the floor or roof structure, but in ceilings incorporating more ambitious geometries and decorative schemes the laths are secured to a secondary structure suspended from the primary structure, often comprising a bespoke and complex framework of timber members (see Figure 1).
In later 19th and early 20th century ceilings the timber laths were replaced by expanded metal lath, a multipurpose steel mesh performing the same function and secured to the supporting framework in a similar way. By the mid-19th century solid plaster had been replaced by fibrous plaster for most large-scale ceilings due to the faster assembly and lower cost arising from the repetitive pre-fabrication methods used in its creation.
For its attachment fibrous plaster dispenses with laboriously fixed individual timber laths and relies instead on wads of gypsum-soaked hessian, often reinforced with steel wire ties, twisted around a supporting structure at discrete locations in a grid-like layout. As with traditional lime plaster, the fibrous plaster panels forming the ceiling may be secured directly to the primary structure or, more commonly, to a secondary timber or steel framed structure arranged to suit the ceiling’s geometrical and decorative scheme (see Figure 2).
Figure 2. A fibrous plaster ceiling attached to a timber framework by wire ties under wads of hessian soaked in plaster (Photo: Conisbee, 2017)
Common Problems and Challenges
Structures built before the middle part of the 19th century were not designed using empirically derived structural analysis techniques such as those routinely used today, but were instead constructed using contemporary rules of thumb and a general reliance on tried and tested forms that had previously proved successful.
This approach is perfectly adequate, albeit somewhat inefficient, when applied to traditional structures but is less successful when applied to bespoke lightweight framed structures like those built to support plaster ceilings, particularly when constructed under time pressure near the end of a project while funds were short and an unscrupulous builder would have been aware that any shoddy workmanship or materials would eventually be hidden from view by the enclosing plasterwork.
Consequently, structural problems can arise, and where problems lead to the dislocation and movement of poorly assembled members, plasterwork can suffer from the associated distress and damage. Poor connections between frame members are a common concern, particularly where timber frames are nailed to the supporting structures from below (see Figure 3). This connection would doubtless have been quick and effective when installing the ceiling structure, but their widespread use raises concern regarding the structural integrity of many ceilings.
The issue is assessing the pull out resistance of historic square cross section wrought iron nails from the parent timber in which they are embedded, and whether this might be sufficient to withstand tensile loads, especially those applied dynamically. One of our key challenges in conserving historic timber structure involves justifying the retention of areas affected by insect attack, particularly where it is advanced and severe.
Generally, only the edible sapwood is affected by common furniture beetle or powderpost beetle, but this is not always the case. Careful assessment must be made in areas of advanced live attack to establish the interior condition of timbers and critical connections in order to ensure their load carrying capability is not compromised. Micro-drilling can be undertaken in appropriate locations to quantify the extent of section loss, so that the adequacy of the structure can then be assessed by structural calculation.
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| Figure 3. A failing connection where a secondary frame had been fixed to a structural timber from below using wrought iron nails (Photo: Conisbee, 2019) |
Similar issues arise where the decay of timber and the corrosion of metalwork is due to water ingress. This is a frequently encountered problem where timbers are built into exterior walls or near gutters, and remains an ever present threat to the macro-integrity of the ceiling structure. Water ingress presents a particular risk to the integrity of the wads and any embedded wirework that secures fibrous plaster panels to the supporting structure.
The discrete arrangement of these fixing points means that fibrous plaster ceilings possess far less structural redundancy than lime plaster ceilings, which are effectively secured in a continuous manner, so special attention must be paid to their construction and condition. All of these considerations are amplified where the supporting structure is hidden behind the ceiling and access is limited or non-existent, preventing routine inspection and maintenance.
In these conditions the degradation of timber, metalwork and other elements can progress undetected and unhindered, sometimes over a period of many years, until structural failure occurs. Conversely, easy access to a roof space often leads to overloading from ill-considered storage, causing excessive deflection or even failure of the supporting structure. The structural integrity of the roof and its ability to safely support the attached plasterwork may also be compromised by ill-considered alteration of the ceiling support structure for pipes and cables.
Access may also be restricted by the extensive presence of asbestos, debris, dust and detritus, particularly in confined spaces. An alternative might be to make new openings, but in a listed building where minimal intervention is required, the widespread removal of floor finishes or plasterwork is unlikely to be permitted. Therefore a carefully considered strategy should be undertaken entailing a comprehensive process of enabling works preceding detailed structural and plaster inspections and further steps which generally comprise: 1) desk study, 2) digital survey, 3) access and cleaning 4) inspection and assessment, 6) repair and certification and 8) monitoring and management.
Desk Study
Original drawings or other information detailing the construction of historic plaster ceilings are extremely rare, and any surviving contemporary information that may be available in accounts or similar sources is typically of limited relevance from a structural perspective. Usually to gain sufficient understanding of the actual ceiling construction, including any alterations that have been made, it is necessary to undertake a detailed methodical inspection from above and below using a combination of digital survey techniques and hands-on inspection.
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| Figure 4 A 2D representation of a ceiling showing deviations from an agreed datum plane. Red, green and yellow represent areas close to the datum plane, while blue indicates significant localised downward movement of the ceiling, necessitating targeted detailed inspection and assessment (Photo: Conisbee, 2017) |
Digital Survey
In most cases photogrammetric reflected ceiling plans should be produced by combining point cloud laser scans with conventional digital photographs, augmented by a gridded overlay for cross-referencing specific areas during close up visual inspection.
Undertaking a laser scan enables collation of a 3D point-cloud database for future use including the formation of 3D models, 2D reflected ceiling plans and deformation mapping, which allows targeting of key areas in complicated structural arrangements with challenging access constraints.
It also provides the opportunity to incorporate movement or vibration monitoring if subsequently found to be necessary, such as to investigate the influence of nearby construction works on the ceiling’s geometry and condition.
To enable deformations to be graphically identified from this baseline, scan contoured distortion maps of the ceiling may be produced which illustrate deviations from an agreed datum plane (see Figure 4).
If deemed necessary, repeated periodic scans may be undertaken (usually annually) at a relatively low cost to allow clear and rapid identification of new or ongoing distortions which may require targeted hands-on investigation and further attention.
This approach, combined with remote digital monitoring, can identify local distortions which are not easily detectable by other means and positively addresses re-inspection requirements and problematic access into constrained voids that may exist above the ceiling.
Access and Cleaning
Close up access to the underside and topside of ceilings is advisable to permit hands on inspection of the plasterwork and its supporting structure, as it is important to physically examine both sides as much as practicably possible. This requires access to any voids that may be present above the ceiling. Close up inspection of the ceiling’s visible surface requires the use of a scaffold tower or in loftier spaces a MEWP – a mobile elevated working platform (see Figure 5).
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| Figure 5. A MEWP in use to inspect the condition of a fibrous plaster ceiling (Photo: Conisbee) |
The spatial and logistical challenges associated with manoeuvring and installing essential access equipment in and out of historic buildings should not be underestimated, and structural assessment of floors and stairs is often necessary to justify their ability to safely withstand the additional weight of temporary access equipment, which may be substantial.
Once access to the ceiling void is obtained, cleaning is frequently found to be necessary due to the accumulation of dust and debris on the ceiling surfaces over the decades, obscuring the structural connections and projecting plaster nibs securing the ceiling to the laths.
Asbestos may also be present within and around ceiling voids in residues from services encasement or firestopping so all spaces must be tested by a specialist asbestos consultant before access is gained by others. Close collaboration with the client, plaster conservator and asbestos contractor is necessary to assess and manage the health and safety implications of any contaminated environment and to ensure cleaning of the plaster nibs can be achieved without causing their damage or loss.
In some cases personal air monitoring equipment may be mandatory to maintain exposure to asbestos below acceptable threshold levels. It should not be assumed that any existing walkways or crawlboards are structurally sound, appropriately positioned or securely fixed, and a structural assessment should be undertaken before access is permitted.
In some cases the construction of new access walkways may be required to facilitate inspection and assessment of the ceiling and its supporting structure, and where this is not possible or where falls from height pose a significant risk, temporary rope access systems will be required, developed in collaboration with specialist access providers having extensive experience of working in and around historic buildings.
Where access to the topside of the ceiling is not possible, the use of more intrusive inspection techniques should be considered, such as digital borescopes, following negotiation and agreement with the client, conservation officer and other stakeholders (see Figure 7).
Inspection and Assessment
Once cleaned and safely accessible, the inspection of all relevant areas can begin. Each ceiling is unique so a site-specific strategy should be developed to suit particular considerations. Nevertheless the assessment of the ceiling support structure generally requires:
- Understanding the structural system (construction, load transfer, integrity, stability)
- Performing a condition assessment of structural members and investigating the causes of any defects
- Making a detailed investigation of structure directly supporting defective areas of plaster as identified in the underside inspections
- Carrying out a quantitative and qualitative investigation of structure supporting areas where considerable deviation/deflection of plaster has been recorded during underside inspections.
It is vital to appreciate that the assessment of any historic plaster ceiling should be approached holistically taking proper account of the geometry, composition and condition of both the plasterwork and its supporting structure. This process is fundamentally collaborative and requires close cooperation between the plaster conservator and the structural engineer if it is to result in meaningful and appropriate advice to the building owners and operator.
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| Figure 6. Rope access is necessary where there is a risk that the ceiling might not support conventional access methods (Photo: Conisbee) |
Structural analysis is typically undertaken through hand calculations to understand the existing stress state of the structural elements and to gauge the overall response to imposed loading. More often than not primary structural members and their connections are found to be of good quality and mostly of adequate capacity to perform as intended.
However, it is often discovered that modern serviceability limit states for deflection are not satisfied, so access restrictions have to be introduced to minimise further distress to the plasterwork from bowing, sagging and similar movement of the structure. In special cases load testing of unusual materials or potentially compromised structural configurations may be necessary to assess their adequacy.
This is necessary where the only evidence that the structure will be able to continue to satisfactorily support the dead and imposed loads to which they are subjected is wholly based on their historic performance. This is especially important when enhanced access resulting from a routine inspection and maintenance regime is to be introduced.
Structural repairs should be designed in accordance with accepted conservation engineering principles and practice, and commonly comprise relatively simple but reversible and distinct bolts, straps and other traditional repairs where needed to consolidate failing hangers or connections.
With a view to ensuring the retention of as much original fabric as possible all structural intervention should be minimal and limited to areas where repairs are essential. Implementing a considered regime of monitoring and management are preferable in lieu of more invasive interventions.
It is imperative to provide detailed reports and documentation of the construction, condition and safety of each ceiling and its supporting structure, plus further documentation outlining material condition, performance, management, access, cleaning requirements and asbestos status.
The Association of British Theatre Technicians’ guidance note 20 requires the production of certificates outlining the condition and status of both the plasterwork and structure, and this should be a collaborative document produced by the structural engineer and plaster specialist. This certification may take the form of comprehensive condition reports and one-page summary documents for use as quick reference guides outlining the current condition and safety of the ceiling, maintenance requirements and recommended actions.
Monitoring and Management
Access constraints can sometimes be so limiting that it is practically impossible to inspect the ceiling and its supporting structure from above, and whilst it is possible to draw a reasonable degree of confidence from underside inspections a programme of remote underside monitoring can be conducted to inform initial recommendations and clarify any ambiguities. This is particularly relevant where the geometry and construction of the ceiling is concerning or complex.
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| Figure 7. Using a boroscope to inspect the ceiling supports where access from above is impossible (Photo: Conisbee) |
Appropriate control protocols may also be established to protect the ceiling, including regulating access to safe access walkways and other areas above especially sensitive ceilings in order to reduce movement and vibration related damage.
This guidance should be included together with any ongoing monitoring information within the operation and maintenance manual under the aegis of the building owner and operator.
It can be appreciated that the strategic inspection and assessment of historic plaster ceilings and their corresponding supporting structures requires multiple skillsets and substantial experience drawing on several areas of expertise.
This process should implement the recommendations contained within the latest ABTT and other relevant guidance and demonstrate the many practical challenges encountered when attempting to satisfy certification requirements.
To safely access often restricted, confined, obscured and contaminated spaces requires considerable planning, design and execution, particularly where they are located above fragile plasterwork ceilings suspended over rooms in regular use. Some of these challenges and limitations are site-specific owing to the special and unique genesis and usage of each building, but many are generally and usefully applicable to a wide range of common building types from all periods.
Recommended Reading
(1) 1Guidance Note 20 – Suspended Fibrous Plaster Ceilings: Survey and Inspections, Association of British Theatre Technicians (2015)
Practical Building Conservation: Mortars, Renders and Plaster, Historic England (2012)
‘Decorative lime plaster – conservation and repair’, The Journal of the Building Limes Forum pp 24–39, Richard Ireland (2014)
