Identifying Hidden Defects on Metal Roofs
A real example of why visible roof condition does not always tell the full story, and why the right recommendation starts with investigation, not product selection.
Why Technical Knowledge and Proper Inspection Matter More Than Product Recommendation
When a metal roof is being considered for refurbishment, the first question shouldn’t be which coating system to use.
It should be: what condition is the roof actually in?
These photographs demonstrate why investigating concealed defects is essential before making any product recommendation.
What these photographs show
In the first image, a steel rule has been inserted into the mid-lap joint of a profiled steel roof.
The purpose of that simple test was to check whether a butyl seal was present within the lap.
In this case, the rule clearly passed the fixing line, indicating that no effective butyl strip was present at that point within the joint. That immediately raises the possibility of wind-driven water ingress through the lap.
At first glance, the roof surface itself does not appear especially alarming. There is visible weathering and some corrosion at the lap line, but nothing that fully suggests what is happening beneath.
That changed when the rule was removed.
In the second image, the material recovered from the lap joint can be seen resting on the roof surface. This is heavily corroded steel that came out from within the overlap.
That is the important point.
Externally, the roof detail looked fairly typical. Internally, the condition was far worse than the exposed surface suggested.
Why this matters
This is exactly why technical knowledge and inspection matter more than product recommendation.
If somebody were to look only at the visible outer surface, they might simply specify standard cut edge corrosion treatment and move on.
But that would miss the more important issue.
The real concern here is not just the exposed lap edge. It is the concealed corrosion occurring within the overlap itself.
That changes the discussion completely.
Instead of asking, what product do we sell for this? the proper question becomes:
What is actually happening here, and what treatment is appropriate in light of the roof’s true condition?
A steel rule inserted into the mid-lap passes the fixing line, indicating that no effective butyl strip is present at that point within the lap.
When the rule was removed, heavily corroded steel debris came out from within the lap, showing concealed deterioration far more severe than the visible outer surface suggested.
From this limited inspection, we cannot determine whether the missing seal and extent of underside corrosion is widespread or localised.
Consequently, it would be advisable, if practicable to do so, to undo the fixings and lift the sheets to see what is happening beneath.
Often this is not practical because lifting sheets can buckle the roof sheets, especially if a butyl strip is present to the majority of the joint.
When hidden defects like this are discovered, the right approach is to investigate further.
When time and budget is considered however, the easy option is to cover it up by indiscriminately sealing the mid-lap joint where the issue will be exacerbated by trapped moisture.
Lap sealing should only be considered after assessing the extent of corrosion and the potential for trapping moisture within the joint.
What may be happening within the lap
In this case, the lack of an effective butyl seal may allow wind-driven rain to enter the lap.
At the same time, the lap itself appears very tight. Where two steel sheets sit closely together, moisture and soiling can become trapped within the interface. Once present, that moisture may remain there for prolonged periods.
That creates conditions in which concealed underside corrosion can develop and continue, even when the outer face of the roof still appears broadly serviceable.
Importantly, this also shows the limitation of superficial drying methods.
Blowing the roof surface or exposed lap edge with leaf blowers may remove some accessible moisture, but it does not mean the hidden interface between the sheets has actually dried.
Fixings may also be part of the problem
The fixings immediately above the lap should not be ignored.
If a fixing is loose, deteriorated, or has a failed washer, water may be entering through the fixing position and tracking beneath the upper sheet into the lap area.
That means the lap itself may not be the only point of water entry.
In this case, the fact that the corrosion debris came out on top of the rule as it was withdrawn suggests that the corrosion products were sitting above the inserted blade, reinforcing the suspicion that deterioration is occurring higher within the joint.
That is why a proper remedial specification should consider more than just the lap edge itself.
It may be sensible to replace the fixings immediately above the lap where practical, if they are found to be defective.
Where replacement is not practical, sealing those fixings with an appropriate detail treatment such as Non-Sag Sealer may also form part of the remedial works, but note that sealing fixings cannot restore their mechanical integrity.
Again, the point is not to jump to a standard product recommendation.
The point is to understand the defect first.
Why this supports a “Knowledge Before Products” approach
At Liquasil, we believe that product recommendation should follow diagnosis, not replace it.
Any manufacturer can suggest a coating.
The more important question is whether the person making that suggestion has properly understood the condition of the roof, the likely mechanism of failure, and the limitations of the proposed treatment.
These photographs demonstrate the importance of this approach.
If we simply recommended a coating system without identifying the missing lap seal, the concealed underside corrosion, and the possible role of defective fixings, the recommendation would be incomplete.
Worse still, it could give a false impression that the visible outer condition tells the whole story.
It does not.
The right recommendation is not always the quickest one, and it is rarely the one made without proper inspection.
What proper inspection should achieve
Before any coating or remedial specification is prepared, the inspection should aim to establish:
- whether water is entering through the lap, the fixings, or both
- whether sealant or butyl is present within the lap
- the extent of visible and concealed corrosion
- whether localised sheet deterioration has progressed beyond what surface treatment alone can sensibly address
- whether the surrounding fixings remain sound and weather-tight
- whether lap sealing is likely to be beneficial, or whether it could create further moisture retention risks if applied without proper consideration
That is the difference between a product-led response and a technically led response.
So what is the likely recommendation here?
Based on a detail like this, the answer is unlikely to be a single off-the-shelf instruction.
A technically informed recommendation may include:
- cut edge corrosion treatment to the exposed lap edge
- careful consideration of lap sealing to reduce wind-driven water ingress
- replacement of defective fixings immediately above the lap where practical
- sealing of relevant fixings where replacement is not practical
- acknowledgement that sealing the lap may help slow further deterioration, but cannot reverse corrosion already occurring within inaccessible interfaces
- localised repair or sheet replacement if investigation shows more significant section loss
In other words, the recommendation should reflect the condition of the roof detail, not a standard product script.
The bigger lesson
This example is a good reminder that the most important part of a roof coating specification is not the coating.
It is the understanding behind it.
A roof may appear relatively acceptable from the outside and still conceal serious deterioration within laps, interfaces, or fixing lines.
That is why we continue to push a Knowledge Before Products approach.
Because on real roofs, the correct recommendation depends on what is actually there, not what we assume is there.
Need help assessing a metal roof refurbishment project?
If you are dealing with corrosion at end laps, cut edges, fixings or concealed roof defects, the first step is not choosing a product. The first step is understanding the roof.
If you want a technically informed view on the condition of a metal roof and the suitability of refurbishment works, get in touch with Liquasil.