Rust Staining After Roof Coating
Technical Defect Analysis
What The Client Sees
Brown rust-coloured marks below sheet laps, after the roof has been coated.
Why it is often mis-diagnosed
It may be reported as cut edge corrosion, coating failure or failed remedial treatment.
What may actually be happening
Water has entered the sheet lap, contacted corrosion on the underside of the sheet and then flowed back out carrying rust particles.
How the coating can be checked
On a sound newly coated surface, the staining can often be wiped away with a damp cloth. The intact coating remains beneath it.
Why sealing the lap is not automatically the answer
Sealing may hide the symptom while trapping moisture within the joint and worsening concealed underside corrosion.
What should happen next?
The source and extent of the staining should be investigated before further remedial work is specified.
A coated roof showing rust staining from beneath the mid-lap joint. This is a different defect from cut edge corrosion.
To determine whether the defect is rust staining, the simplest test is to take a wet or damp cloth and wipe the surface. On most coatings, the staining will simply wipe off.
Wiping away the stain confirms the condition of the visible coating, but it does not remove the source of the corrosion products and the staining may return.
Should The Mid-Lap Joint Be Sealed During Coating?
Liquasil does not recommend blanket external sealing of mid-lap joints as a routine part of cut-edge corrosion treatment.
Existing laps should first be inspected for water ingress, failed internal seals and concealed underside corrosion.
Sealing may hide the symptom while restricting drying. If moisture continues to enter from elsewhere, concealed underside corrosion may worsen.