CROSS Safety Report
Condition assessments of exposed lightweight canopy roofs
A reporter has identified several issues with lightweight canopy roofs using profiled steel sheeting or sandwich panels, and in many cases with the roof sheeting underslung from the structure. Concerns raised include excessive deflections, corrosion at critical locations, and lack of regular inspections and maintenance.
Key Learning Outcomes
For designers and specifiers:
- Identify a suitable maintenance regime in the specification, including safe access and cleaning requirements
- Specify fixing details and materials
- Seek certification from a qualified structural engineer prior to making any changes to the specified works
- Carry out replacement works with like-for-like materials and details, unless otherwise certified by a qualified structural engineer
For owners and asset managers:
- Ensure that specified maintenance inspections are adhered to at nominated intervals by suitably qualified personnel
- Ensure regular cleaning of the roof structure
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The reporter has undertaken a number of condition assessments of lightweight canopy roofs and has also been consulted when issues concerning structural performance have arisen with this type of roof.
In most cases, no urgent action in the interest of safety has been necessary, apart from two incidents of particular concern to the reporter.
Many of these canopies are constructed with roofing sheets underslung below structural beams and purlins, leaving the main structural elements exposed. The primary supports in some cases are individual cantilever columns; others may have a pair of columns.
The roofing material is usually profiled steel sheeting but on some occasions it consists of sandwich “polypanels” (aluminium facings with a polystyrene core).
In one case, the canopy fascia was supported by heavy-gauge profiled ceiling sheets which cantilevered 2m to support concealed gutters and fascias. The upper profiled roof spanned 2m, supported on double cold-formed girts with a timber-formed fascia supported on the lower heavy-gauge ceiling.
The reporter was called in after the fascia sagged following replacement of ageing ceiling structural members with lightweight 0.45mm Base Metal Thickness (BMT) steel-profiled roofing sheets. Fortunately, no one had accessed the cantilever roof!
The reporter also visited a site in a corrosive environment involving underslung polypanels supported by stainless steel fixings. Some sagging had been observed after heavy rainfall; and there was significant corrosion and loss of support at some polypanel fixings.
Corrosion at the bases of cantilever posts has been encountered on several occasions.
The reporter is concerned that such conditions may lead to structural issues. Factors that may have contributed to the observed conditions include design issues, and a lack of ready access for inspection, maintenance and cleaning.
concerned that such conditions could lead to partial structural collapse
Potential lessons are:
- need for regular inspections;
- corrosion should be addressed early;
- ceiling and roofing replacement should be with materials with at least similar strength and stiffness as the original materials; and
- areas of potential corrosion in highly-loaded locations should be readily accessible.
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Expert Panel Comments
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This report highlights the need to design using durable materials appropriate for the environment, and to specify regular inspection of the structure by a qualified structural engineer with an appropriate maintenance program (particularly if exposed to the weather), including requirements for safe access.
The reporter has identified a particular issue with lightweight canopies or awnings, where the sheeting, sometimes made of sandwich panels (also known as “laminate panels” or "polypanels”), is often affixed below the supporting framework with fixings invisible from below. Due to this type of arrangement, the sheeting or façade commonly suffers from:
It should further be noted that expanded polystyrene sandwich (EPS) panels are not permitted as a roofing material in certain construction (e.g., Types A and B) and care should be taken to avoid material non-compliance.
A further issue of undiscovered corrosion of the frame (in addition to the fixings) can also occur with exposed structures of this nature where the frame cannot be easily inspected.
Such issues highlight the following potential problems:
Finally, the reporter has identified a situation in which replacement works were undertaken with materials of a lesser strength than that of the original design, highlighting the fact that any replacement should always be with elements of equal or greater strength than the original, unless otherwise certified by a qualified structural engineer.