CROSS Safety Report
Storm damage to pressed metal roof covering
Overview
A modern dwelling with two separate V-shaped roofs sustained damage to the standing seam pressed metal roof covering during two separate weather events.
Key Learning Outcomes
For construction professionals including cladding contractors:
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Cladding design and installation should be given the same degree of attention as the primary structure during both design and construction to improve safety, reliability and longevity
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Connections can often be the weak link in structures and attention to detail is required, both in the design and construction phases
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Fixings should be installed in accordance with the manufacturer's specifications. If the details are not clear the manufacturer should be consulted
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Consider having representatives of the manufacturer attending site to train operatives on best practices. This can help raise industry standards
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Adequate quality assurance and competent supervision can help to ensure that cladding systems are installed in accordance with the design
For building owners/managers:
- The specification of any inspection and maintenance requirements for cladding fixings should also be considered and recorded in the operation and maintenance manual
- The fixings may require a combination of regular visual inspections with full inspections at appropriate intervals
- The fixings should be inspected by a competent person
For architects and civil and structural design engineers:
- Select the correct fixings and materials (such as plywood substrates) for the given loads and environment conditions to ensure the products safety and longevity
- Design with robustness in mind
- If possible, attend site to inspect the installation of cladding systems and their fixings
Full Report
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
A modern dwelling with two separate V-shaped roofs sustained damage to the standing seam pressed metal roof covering during two separate weather events. The roof structure was of reinforced concrete, which was overlaid by plywood sheeting, a breather membrane and lastly a pressed metal, standing seam, aluminium roofing system. Figure 1 shows the sheeting at the valley gutter and the roof edge being temporarily held down.
The failures occurred in wind conditions that included gusts of Force 10 – 12; however, the wind conditions were significantly less than (only 40 – 64% of) what the design of these roofs should have been capable of resisting. The implication is that there must be an underlying cause for the damage, other than the wind conditions during each event. The following deficiencies in the design and construction of the roof were identified as the root cause of these incidents.
Spacing of the clip fixings
There was a failure to provide a sufficient number of clip fixings in the areas of roof exposed to the greatest wind loading. The clip fixings were installed at a spacing of 600 millimetres, whereas twice this amount should have been provided to achieve the manufacturer’s required 300 millimetres spacing for the wind loading conditions that were to be expected at this site.
Position of the first clip fixing
The first clip fixing (at the base of the roof pitch) was installed at a spacing of approximately 210 millimetres. In contrast, the manufacturer has recommended that this dimension should be 100 millimetres. The effect of not complying with this 100 millimetre spacing requirement is to increase the amount of flexure at the end of the roof sheeting (above the valley gutter). This flexure creates an increased risk of rainwater passing back up along the underside of the roof sheeting and towards the plywood sub base.
Roof sheeting lengths
The installed lengths of some roofing sheets varied considerably. This created an inappropriate overhang (above the valley gutter) at the ends of these sheets. This also creates an increased risk of rainwater passing back up along the underside of the roof sheeting towards the plywood sub base.
Deterioration of the plywood
The plywood in the vicinity of the valley gutter was wet and badly degraded as shown in Figure 2. This occurred due to the deficiencies in the construction of the roof sheeting which allowed water to pass back up along the underside of the sheeting. Near the gutter the clip fixings were not significantly deformed which indicates that failure occurred within the plywood and not within the clip fixing.
The reporter feels that while there were storm force wind conditions on the dates that the roofs were damaged, these winds were not significant enough to damage a roof that had been properly designed and constructed.
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Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
The failure reported here occurred under severe but not exceptional wind conditions. There is sometimes not enough understanding of the disproportionately large wind loads that occur at the sharp edges of buildings and roofs, and at the edges of cladding panels, all of which need to be adequately fixed and restrained.
Designers should provide robust designs where key areas of installation are highlighted and effectively communicated to the cladding contractor. This will help ensure safe designs. CROSS recommends that cladding design and installation is given the same degree of attention as the primary structure during both the design and construction phases. This will improve safety, reliability, and longevity.
Fixing selection and installation
Clearly, appropriate fixings must be designed and specified. Fasteners must be fixed in accordance with the manufacturer’s requirements, including with the correct equipment to ensure that adequate installation is achieved. Fixings can vibrate loose, and it can be difficult to predict the air pressure fluctuations that can cause this.
The correct choice of fixing can ensure that they do not loosen through wind or vibration induced effects. If fixings are into a timber substrate, then the timber specification and detailing of the fixings are also important to ensure there are no durability issues such as the ones that look to have occurred in this report. The timbers and fixings should be carefully selected for the given environment.
It is essential that there is adequate supervision of subcontractor activities to ensure the satisfactory execution of works
Correct site fixing of sheeting must be assured. Marking out of site fixings must accord with the design to ensure that minimum edge distances and the like are achieved. Quality control procedures should demonstrate that the installation meets the cladding system specification which sets down materials, workmanship, and testing requirements.
It is essential that there is adequate supervision of subcontractor activities to ensure the satisfactory execution of works. Arrangements for the inspection and acceptance, including any testing requirements, of subcontractor packages should be carefully considered and agreed upon. The demonstration of the adequacy of concealed fixings requires thought.
Consideration should be given to the provision of independent supervision for all aspects of the construction process where safety critical fixings are concerned.
Further reading
References that readers may find helpful include: