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518 Fibrous plaster ceiling features

Report ID: 518

Published: Newsletter No 39 - July 2015

Report Overview

This recent report of a serious concern about an old fibrous plaster ceiling has similarities to the Apollo Theatre ceiling collapse in London in December 2013.It is thought that complete collapse was averted by the arched shape of the trough section and that if the ceiling was flat, prior collapse would have occurred.

Report Content

This recent report of a serious concern about an old fibrous plaster ceiling has similarities to the Apollo Theatre ceiling collapse in London in December 2013. In retrieving a helium party balloon, says the reporter, a crack was discovered in a down-standing feature arch in a local authority hall built circa 1926. On closer inspection this was a non-structural plaster trough section with ornate scroll features. The crack 6mm wide was present over two thirds of the span and occurred on both sides. Inspection from the roof space revealed a system of small timber battens (30x18mm) tied together with hessian “wadding” type rope with “balls” of plaster around the joints – no mechanical fixings. The assembly was suspended from timber joists by similar wadding rope wrapped around the joist. Many of the wadding ropes were snapped at various places around the loop. Sometimes this was hidden below the joist, at the side or on top, particularly if there were signs of past crawl boards having been placed upon the rope or dragged over them. Where ties were intact, they could be snapped by finger pressure. Closer examination of the wadding rope showed a high volume of plaster in the cross section and upon crushing, the fibres were found to be interwoven confirming the origin of rolled cloth impregnated with plaster rather than preformed rope dipped in plaster. The hessian strings had fully degraded and could be pulled apart with light finger pressure. It is thought that complete collapse was averted by the arched shape of the trough section and that if the ceiling was flat, prior collapse would have occurred. The estimated dead load of the trough is 1.51kN/m (154kg/m) and it is 11m to the crown. Two other halls (with flat ceilings) were inspected and found to have down-stand architectural features with similar wadding ropes. Past reports have highlighted “Theatres” and there has been recent issue of the ABTT Guidance Note 20, by the Association of British Theatre Technicians (see below) but there must be many similar Public buildings including Council Chambers/meeting rooms, Town/Concert/Assembly Halls, School/University Buildings, and Stately Homes that are not specifically designated as a “Theatre” that may have circa 90 year old fibrous plaster ceilings. Clearly fully inspection of these ties is extremely difficult and in this case it was considered all ties were suspect (as all examined were) and alternative means of support were installed. The reporter hopes that his photos will help to pass on the message that these may be hazardous.

Comments

This is an issue of real and immediate concern and duplicates the findings following the Apollo Theatre Collapse when 88 patrons were injured as plasterwork fell during a performance. Advice issued after this by Westminster City Council is given in CROSS report 442.  When hessian ties are holding up heavy ceilings in places of public assembly there is a risk of failure as described above, when the ties deteriorate. There may also be loading changes if lights or other items have been added over the years and there may be dynamic effects which exacerbate the situation. In February 2014 Westminster City Council said: “As a result (of the Apollo Theatre incident), we recommend that in addition to what was stated in the interim guidance the wadding ties of all suspended ornate ceilings are thoroughly inspected from above as a matter of urgency by a competent historic plaster specialist and a structural engineer.” In response to the Apollo incident The Association of British Theatre Technicians published Guidance Note 20 –Suspended Fibrous Plaster Ceilings in May 2015. This has been compiled by specialists in the field and provides excellent guidance. More publicity is needed about the potential risks to other buildings and local and national government agencies and others are asked to draw attention to these.

 

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