Structural Engineer Magazine Article :”Fragile Roofs – Problems &
Potential Solutions” originally published in February 2004
by Dr Satish Desai
Chairman’s Response published as Viewpoint article November 2004
I write to you as Chairman of the Advisory Committee for Roofwork (ACR) to comment on the above article by Dr Satish Desai and the response made by Richard Eastwood (Sheffield) in the 6th July issue headed Fragile Roofs.
Following the work done by the HSE to determine the loads applied to structural roofing elements when a person trips and falls, the ACR group was formed in conjunction with the HSE, to determine a standard means of testing a roof assembly to be strong enough to be deemed non-fragile. This is a critical requirement for designers, specifiers and builders since the CDM Regulations impose a duty that, where ever possible, fragile roofs are designed out of the specification. For this to happen there has to be a definition for what is a non-fragile roof.
The ACR published their findings in 2000, called “Test for Fragility of Roofing Assemblies” – ACR(M)001:2000. This document has the full endorsement of the HSE and is now accepted by manufacturers and the building industry as the formal means to determine non-fragility for a roof assembly.
Correcting the comments by Dr. Desai, the follow-on publication by ACR was published in 2003 and titled “Recommended Practice for Work on Profiled Sheeted Roofs” -
ACR(CP)001:2003. This is an excellent “good practice” guide that is again fully endorsed by the HSE. Of specific note, Annex C at the end of the publication, provides an Aide Memoir for designers, specifiers and inspectors to go through with the roofing contractor to ensure that the full roofing specification has been complied with in every detail to ensure that the complete roof assembly is non-fragile.
One of the key statements in this Code of Practice is item 4 :- “Before commencing any work on any existing roof, it is absolutely essential that you find out whether your roof is fragile or non-fragile. This is a key decision which should only be made by persons competent to do so. If there are any doubts, assume that the roof is fragile”.
Accidents on roofs occur both during the construction phase and in the maintenance phase. During the construction phase, contractors need to be aware that whereas the roof assembly will (normally) be designed to be non-fragile, it will only be non-fragile when the complete roof assembly has been fully fixed. During assembly the roof must be considered fragile and systems designed accordingly.
During the maintenance phase, building owners need to be fully aware that the roof may have been fragile when built (with appropriate yellow / black signs in place) or has become fragile with time due to weathering and material decay. Only competent persons should be employed to assess the roof, and if the competent person cannot certify that the roof is non-fragile, then the roof must be deemed to be fragile and appropriate management systems adopted to ensure the required work is carried out safely.
The other area of concern with Dr. Desai’s article is the implication that BS 6399 Part 3 would need to be uplifted by 600% to satisfy non-fragility requirements. Firstly, the loads that are applied when a person falls have been established by practical experiment and a modest safety factor applied to the resultant figures. Secondly, it will not generally be the case that the supporting structure needs to be upgraded to address the human load, it will be the external envelope that will generally fail the impact and thus it is the design of the external envelope that requires immediate attention to ensure non-fragility. Manufacturers of external roof components will generally be able to give sound advice on material, design and fixing specifications that will readily achieve a non-fragile structure (at least at the time of installation). Even rooflights, that in the past have been considered as the number 1 danger area, can now be made to be as strong and non-fragile as the rest of the surrounding roof without losing any appreciable light transmission – Richard Eastwood please note.
The key point of the CDM Regulations is that fragility should be designed out at the drawing board stage. Eliminate the hazard should be the prime task of the designer.
Having said that amending the prime building structure will not generally be an issue to achieve non-fragility, it should be born in mind by structural engineers that the new Building Regs Part L require buildings to have a far greater insulation value requiring almost a doubling of the thickness of rockwool or glass mat insulation. The resulting increased imposed load on the roof structure to achieve Part L, together with a thickening of external sheet materials to achieve non-fragility and the additional weight when loading up the roof prior to fixing, is now becoming a major issue for purlin design. If purlins sag or twist when the roof is loaded out with an external envelope of composite panel, then the panels may not have a proper bearing with sufficient end laps and fixings correctly installed. If this were to happen, the roof assembly that has been designed to be non-fragile, may in practice be fragile arising from inadequate installation. This situation could become even more of an issue when further revisions to Part L are issued in 2006.
In respect of monitoring accidents over say a 3 year period from new, I would contest is not an issue. If the roof has been designed to be non-fragile at the outset, it should still be non-fragile 3 years later, subject to the specification designed to suit the environment. If failure does occur within this period, it is almost certain that it will be due to a material specification below that deemed to be non-fragile or that it had been installed incorrectly to the manufacturers recommendations. As indicated above, reference to Annex C of ACR (CP) 001:2003 will assist any competent roofer to determine if the roof is still non-fragile.
All should be aware that even non-fragile roofs could be dangerous places to work dependent on roof pitch and weather conditions such as ice, rain and high winds.
Arising out of the comments made by Richard Eastwood, I would make the following points.
Under the CDM Regulations, designers have a prime responsibility to design out hazards at source. To allow yourself the excuse that most accidents that occur at roof level arise from carelessness, negligence and the inherent hazards of roof work is an opt out for good design and safe working practice. We are all human and subject to being careless at times. For an office worker that may not be serious but for a roofer it could be fatal. Designers, building material manufacturers and contractors need to ensure that designs and systems of work are such that if carelessness creeps in, risk to life is not serious.
It is true to say that many roof accidents occur today due to people falling through fragile rooflights and asbestos cement sheets. On both counts, since the CDM Regs and the creation of a Non-Fragile Standard drawn up by ACR, both rooflights and (now) fibre cement roofing can be manufactured to be non-fragile and there is no excuse why the new ongoing roofing stock should not all be non-fragile. Mr Eastwood is wrong to conclude that the need for fibre cement is declining and will reduce to nothing – it is the preferred material for most agricultural live stock buildings and the fibre cement industry are to be applauded for improving their products to comply with the ACR Non-Fragility Test.
Mr Eastwood is correct when he says that current Building Regs require buildings to be better insulated but that does not imply that they are safer. During the construction phase, the individual elements still need to be assembled and as such the roof may be fragile whilst it is being assembled. And when fully assembled the additional weight on the roof may have caused purlins to sag such that fixings are not correctly sited as I have indicated in a preceding paragraph. Designers please make sure that when designing out one hazard you do not introduce another.
The ACR and the HSE are working together to increase the awareness of safe working design and practice when working on roofs and I recommend everyone with responsibilities in this area to obtain a copy of ACR(CP)001:2003 “Recommended Practice for Work on Profiled Sheeted Roofs”. Copies are available from any of the trade associations that are members of the ACR
Chris Pearce - Chairman