Plastisol Deterioration on Industrial Metal Roofs
What is Plastisol & Why Does it Degrade?
Plastisol accounts for around 90% of all finishes on steel profiled roofs in the UK. It is typically recognised by its leathergrain appearance, though in recent years, other patterned finishes such as scintilla emboss have been available.
The plastisol finish is applied evenly to steel coils which are then pressed and cut into the profiled roof and cladding sheets we commonly see on industrial premsies.
Over time, the original finish deteriorates and will eventually require maintenance recoating, though the need for this will depend on a number of factors
Early Indicators of Plastisol Degradation
The early signs of deterioration are often laregly cosmetic, where the surface starts to chalk and become dull.
Over time, because of UV exposure, the surface of the plastisol loses its flexibility and becomes more brittle.
Micro-checking and cracking, resulting in surface roughening and increased dirt retention with biological growth will then develop.
At this stage, deterioration ,may still be laregly superficial, although the plastisol finish is beginning to lose its original protective performance.
If left untreated, the plastisol will continue to deteriorate, since moisture will be reatined where the surface of the plastisol is broken.
With weather cycling, the affected areas will become more widespread throughout the roof.
Progression of Plastisol Degradation
If left untreated, localised delamination will follow and following rainfall, dampness may be apparent where moisture is retained beneath the plastisol coating.
With thermal cycling, the edges of the affected plastisol will begin to lift further over time and small pockets of bare galvanised steel will be evident across the roof.
In the early stage depicted in this image, we can see areas of delamination mainly affecting the webs (uprights) of the profiled sheet.
Examination of the sheets will probably show more microchecking and it is often the case that conducting an adhesion test by cutting a V shape into the coating will demonstrate instability of the plastisol.
Progression of Plastisol Degradation
If left untreated, localised delamination will follow and following rainfall, dampness may be apparent where moisture is retained beneath the plastisol coating.
With thermal cycling, the edges of the affected plastisol will begin to lift further over time and small pockets of bare galvanised steel will be evident across the roof.
In the early stage depicted in this image, we can see areas of delamination mainly affecting the webs (uprights) of the profiled sheet.
Examination of the sheets will probably show more microchecking and it is often the case that conducting an adhesion test by cutting a V shape into the coating will demonstrate instability of the plastisol.
Widespread Delamination of Plastisol Coating
Eventually, the roof surface will suffer large-scale delamination across much of the area of the roof, exposing the protective galvanised substrate to the elements.
Over time, the loss of the galvanising protection will result in the bare steel substrate being exposed to the elements and the formation of spot corrosion is likely.
Delamination of the Plastisol often becomes apparent when pieces start falling from the roof and collecting in the areas outside of the building. You don’t always need a roof inspection to see this defect. It shows itself natuarally in windy conditions.
Cost Implication of Delaminating Plastisol
As deterioration progresses and adhesion to the galvanised substrate becomes unstable, metal roof refurbishment preparation requirements can increase significantly.
In many cases, the removal of delaminating plastisol becomes one of the most labour-intensive stages of the refurbishment process.
It is always best to consider the condition of the roof and any existing finishes before a project goes out to tender, since contractors often don’t visits sites before being awarded their contracts. To do so is not cost effective for them.
Therefore, it is best to carry our a thorough roof inspection, taking all things into account before tendering any roof refurbishment works.
Loss of Galvanisning Protection and Spot Corrosion
The substrate of most plastisol coated roofs tend to be galvanised steel.
Galvanising refers to the application of a sacrificial and microscopic layer of zinc that is applied to the steel during manufacure.
The zinc galvanising is the last layer of defence for the steel substrate and as this deteriorates, the steel becomes susceptible to spot corrosion.
Unlike cut edge corrosion, which originates at sheet edges, spot corrosion occurs across the weathering surface where protective coatings and galvanising have broken down.
The areas of spot corrosion can expand rapidly once established and this is usually the last opportunity for refurbishment, before replacement becomes necessary.
Loss of Galvanisning Protection and Spot Corrosion
The substrate of most plastisol coated roofs tend to be galvanised steel.
Galvanising refers to the application of a sacrificial and microscopic layer of zinc that is applied to the steel during manufacure.
The zinc galvanising is the last layer of defence for the steel substrate and as this deteriorates, the steel becomes susceptible to spot corrosion.
Unlike cut edge corrosion, which originates at sheet edges, spot corrosion occurs across the weathering surface where protective coatings and galvanising have broken down.
The areas of spot corrosion can expand rapidly once established and this is usually the last opportunity for refurbishment, before replacement becomes necessary.
Delaminating Plastisol
This video shows how unstable the plastisol coating has become over time.
It is possible to remove the plastisol with almost no pressure using a small blade during the roof inspection.
Although it looks bad, the presence of this defect does not mean that the roof has to be replaced.
In many cases, such roofs can be prepared and refurbished, but the cost of removing the existing finish must be factored into the refurbishment budget.
This can be around £5-6 per square metre more than simply cleaning the roof, because it is slower and more labour intensive.
Delaminating Plastisol
This video shows how unstable the plastisol coating has become over time.
It is possible to remove the plastisol with almost no pressure using a small blade during the roof inspection.
Although it looks bad, the presence of this defect does not mean that the roof has to be replaced.
In many cases, such roofs can be prepared and refurbished, but the cost of removing the existing finish must be factored into the refurbishment budget.
This can be around £5-6 per square metre more than simply cleaning the roof, because it is slower and more labour intensive.
Sometimes, it is possible to mitigate the additional costs by choosing a more economical metal roof refurbishment system.
It is important that the lance is held closely to the surface of the roof, ideally 2-4 inches, with all leading edges of the delaminating plastisol being attacked.
As well as the main troughs and crowns, the operator then has to prepare the webs (vertical surfaces) of the profiles, ensuring that all plasisol is removed as far as reasonably practicable,
Stable residue can often remain as long as the edges of the residue are not lifting or peeling.
This is where the additional labour costs occur, because the roof will probably need more than one pass.
Confusing cleaning with surface preparation is where costly budgeting mistakes can occur.
PVDF / PVF Coatings Delaminate Too
It isn’t only plastisol coated roofs that suffer from surface delamination.
Even thin coatings, such as PVDF / PVF2 can suffer from UV degradation and although not as common, we have seen several cases during our inspections.
PVDF coatings are recognised by their very smooth finish. There will not be any pattern present on the surface.
They are called thin coatings because the thickness of the coating is usually only 25-50 microns, whereas plastisol is usually around 200 microns.
Additional Considerations For Coating After Stripping Plastisol
After delaminating plastisol has been stripped, it will require an additional treatment, most likely a mordant T-wash.
The reason is that the newly exposed zinc layer can and probably will cause adhesion issues with a new coating.
The application of a T-wash solution will neutralise the zinc by way of chemical reaction which blackens the surface.
Once this reaction has taken place, the treated areas then need to be rinsed off with clean water.
The cost of the T-wash solution is very low, but once again, the cost of labour for application and rinsing immediately afterward will probably add £1-2 to the cleaning and preparation stage.
This can easily be missed by specifiers, quantity surveyors and contractors.
When Is a Plastisol Roof No Longer Economically Viable to Refurbish?
There will be circumstances when a steel roof becomes econmically unviable to refurbish.
For example, extensive underside corrosion presence, where structural integrity of the sheets has been lost, or when there is complete loss of the galvanising layer, resulting in widesprewad corrosion and perforation or weakening of the roof sheets.
In such cases, the choice probably becomes overcladding or replacement, not only because of the more involved surface preparation, but also for the saftey of the individuals involved in completing the works.