Cut-Edge Corrosion – Case Study
Expanding on our previous Insight (Industrial Roof Condition), we now focus on a specific defect known as cut-edge corrosion.
Regardless of your interest in the property (landlord or tenant), you will likely want to ensure that the roof remains in good condition and without defects. Cut-edge corrosion is often overlooked.
What is cut-edge corrosion?
Cut-edge corrosion occurs when the factory coating on profiled metal sheets peels back at the sheet edges (mid-laps, end-laps, or both). The coating material varies between manufacturers but is typically either a PVC Plastisol or polyester powder-coated finish.
This occurrence is partly due to factory processes. During manufacturing, the metal sheets are finished with a suitable coating before being rolled into coils. The coils are then cut to size, creating a weakness in the sheet edges. The edges are susceptible to weather and UV degradation.
Eventually, the factory coating begins to peel back and exposes the metal substrate underneath. Where the substrate is exposed, the metal begins to corrode and, if left untreated, may lead to perforation of the roof sheets.
How can cut-edge corrosion be identified?
Unfortunately, regular maintenance of roofs is often overlooked and not considered until defects are already apparent.
As obvious as it sounds, the early warning signs of this occurrence are the factory coating peeling at the edges. If discovered, it does not necessarily mean immediate repairs are required. A reasoned assessment of the age of the cut edges needs to be made before recommending certain repairs.
In the example below, the sheet edges have started peeling, but there is no evidence of significant corrosion at this stage. This is an important consideration when it comes to repairs, as budgets may need to be allocated to cover unforeseen repairs. If identified early, the extent of remedial works may be reduced.
Another clear example of identifying cut-edge corrosion is by evidence of existing cut-edge corrosion repairs. This is a sign that defects have previously been identified and repaired.
Tip / Consideration: If not implemented, we would recommend a cyclical maintenance regime which inspects not only the cut edges but gutters, rooflights and roof sheets. If identified early, the repair methods and costs will be more economical. The inspection can be undertaken by a commercial building surveyor or competent roofing professional.
Great, the cut edges have been repaired, why do I need to be concerned?
Depending on the age of the cut-edge repairs, they may no longer be protecting the sheet edges as they should. If such repairs have been applied poorly or not in accordance with the manufacturer’s recommendations, the risk of corrosion and eventual perforation remains.
If the system can be identified through existing O&M manuals, the manufacturer/contractor could be contacted to understand when they were installed.
If no such manuals are available, a visual examination of the existing sheet edges should be undertaken. If there is evidence of splits or breakdown of the product, corrosion may be continuing under the repairs.
The photo below shows such an example of existing cut-edge repairs being in disrepair. No evidence of the product or date of installation was available. Through a visual examination, it was determined that the repairs had reached the end of their serviceable life. At this stage, it was unclear what extent of corrosion was continuing under the surface of the repairs.
Tip / Consideration: When inspecting such repairs, it is not always possible to identify whether the sheet edges remain in fair condition. Only through physical/intrusive examination can the true condition be determined. In the case above, only after jet washing and removing the old repairs was the true condition of the sheet edges discovered.
Our previous cut-edge repairs are in poor condition, now what?
When the layperson views corrosion, they understandably think the worst. However, not all corrosion signifies the end of the base material life.
If the sheets remain sound and are not perforated, an assessment can be undertaken to check the extent of rust. The Swedish Standards are used as a reference point for assessing surface preparation of steel surfaces. Within these standards, there are four rust grades ranging from A to D. Each has a specific criterion for assessing the rust grade, which determines the type of surface preparation required.
Depending on the coating material being proposed, different surface classifications are required, typically to Swedish Standard ST2 or ST3. This requires the rust to be scraped with a metal scraper or other powered handheld tool, such as a disc sander, to remove loose scale and rust. If within the tolerances of the Swedish Standards, the new coating to the sheet edges may be permitted.
However, if the sheets have significant corrosion and perforations are evident throughout, more extensive repairs will be required. In this case study, perforations were so widespread to the sheet edges that new profiled metal sleeves were required.
First stage – New sealant applied to existing surface and new matching profiled metal sleeve installed to perforated sheet edges. The sleeves are fixed (top and bottom) with new rivet fixings.
Second stage – Base coat sealant to the top and bottom of the sleeve, ensuring it is fully encapsulated.
Third stage – New base coat application over roof sheets encapsulating the sleeve and coating. Alternatively, if the roof sheets are not being recoated, a top coat sealant would be applied to the top and bottom of the sleeve in a neat finish.
Following a top coat finish to the above, the manufacturer guarantees can be provided. Guarantees typically range from 15-25 years.
Conclusion
Ensure regular maintenance of your roof is undertaken to identify any defects early. The various stages of cut-edge corrosion will determine which repair is recommended. If identified early, the repairs will be more cost-effective compared to dealing with perforated roof sheets. Perforated roof sheets will likely require additional measures such as new profiled metal sleeves and additional sealant.
Each client’s requirement is different, and various circumstances need to be considered. Perhaps you are the freeholder looking to sell your asset. Do you undertake such repairs before placing your property on the market? Or do you allow the defect to be discovered on a building survey, which potential purchasers will consider in their bid?
You may be a tenant and have certain repair liabilities. Does the lease require you to repair such defects, or does a schedule of condition protect you?
Please get in touch with any queries or to discuss your situation, and we will gladly assist.
Insight provided by Lewis Livesey.