Before applying a new roof coating over an existing coating, contractors should verify that the old coating is firmly bonded and compatible with the proposed new system. A new coating cannot correct a poorly adhered underlying layer. If the existing coating is peeling, blistering, or separating from the roof substrate, the new coating may fail along with it. For this reason, adhesion testing is an important part of evaluating an existing commercial roof before recoating.
What Is an Adhesion Test?
An adhesion test determines how strongly the existing coating is bonded to the roof and, in some cases, how well the proposed new coating bonds to the prepared surface. Testing should be performed after the surface has been cleaned and prepared in substantially the same manner planned for the actual recoating project.
The test location should represent the roof’s actual conditions. Multiple areas may need to be tested when the roof contains different coating types, substrates, repair areas, weathered sections, ponding-water locations, or coatings installed during different phases. Manufacturers may specify the number and location of required tests for warranty purposes.
How Is the Test Performed?
One common field approach is a test patch or fabric pull test. The contractor first cleans and dries a representative area, removing loose coating, dirt, grease, chalk, and other contaminants. If the proposed system requires primer, the primer should be incorporated into the test procedure.
A small area of the proposed coating is then applied at the manufacturer’s specified thickness. A reinforcing fabric strip is embedded into the wet coating, leaving a portion exposed so it can be pulled after the coating has properly cured. After the required curing period, the strip is pulled steadily and perpendicular to the roof surface. The contractor records the force required and observes how the coating fails. KARNAK, for example, describes a field procedure using a fabric strip and digital scale and recommends testing representative roof surfaces before coating.
The most important observation is where the failure occurs. If the new coating separates cleanly from the existing coating, the problem may be poor intercoat adhesion or incompatibility. If the existing coating pulls away from the roof substrate, the underlying coating may already be unsound. If the coating remains strongly bonded and the fabric or coating itself fails cohesively, the result generally indicates a stronger bond.
What About ASTM Adhesion Tests?
Depending on the coating, substrate, and project requirements, contractors may use recognized adhesion-testing methods such as ASTM D3359, ASTM D6677, ASTM D903, or ASTM D4541. These methods are not interchangeable, and the appropriate procedure depends on the roofing system and manufacturer’s requirements. ASTM D3359, for example, uses a tape-based evaluation of coating adhesion, while pull-off methods provide quantitative adhesion measurements.
There is no single universal field adhesion test that applies to every roof-coating system. Some manufacturers establish their own modified procedures and acceptance criteria. Therefore, the coating manufacturer’s written instructions should control the test method and minimum acceptable result.
What Happens If the Existing Coating Fails?
A failed adhesion test does not necessarily mean the entire roof must be replaced. Depending on the severity and location of the failure, the contractor may remove poorly bonded coating, mechanically prepare the surface, apply a compatible primer, or select a different coating system. RCMA guidance notes that previously coated roofs may require primers depending on the existing coating’s type and condition. Silicone coatings, for example, can be particularly difficult to coat with incompatible materials.
Manufacturers such as GAF also recommend test patches on previously coated roofs to verify adhesion before proceeding with the full application.
Why Adhesion Testing Matters
Adhesion testing gives a commercial building owner evidence that the proposed coating system can bond to the prepared existing roof. It helps identify weak areas before thousands of square feet are recoated and can prevent premature peeling, delamination, and coating failure.
For a commercial roof in Florida, Shieldline Roofing recommends documenting test locations, surface preparation, coating products, primer used, curing conditions, test results, and photographs. A qualified roofing contractor should also verify substrate condition, moisture, drainage, existing leaks, and coating compatibility before approving the roof for recoating. A successful adhesion test is one important part of determining whether an existing roof is a good candidate for restoration.
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