Compare: An Adhesive Failure Vs. a Cohesive Failure in an Asphalt Shingle

Posted by in Shingles, Wind Damage | September 16, 2013

The wind resistance of asphalt shingles is strongly correlated to the sealing of the tabs.  That is, if the tabs are well sealed, the shingle have excellent resistance to damage from exposure to high winds.  An exception to this is when the shingles are poorly nailed, that is, not nailed according to the manufacturer’s instructions on the packaging for the shingle bundle, but that is another discussion for another time.

Shingle tabs are commonly self-sealing through the factory applied line or dashes of soft asphalt sealant.  The sealant is self-sealing because it is softened through the heat generated by the radiant sunshine.  The sealing of new shingle tabs are tested and rated by their respective manufacturers for winds measuring 110 to 130 MPH.

The failure of a sealant is commonly classified as an “adhesive failure” or a “cohesive failure.”  An adhesive failure is a failure between the sealant and the surface of the shingles, either the top of the underlying shingle or the underside of the overlying shingle.  That is, the sealant unsticks from the surface with which it has made contact.  This type of failure generally occurs when the shingles are expanding-contracting from thermal variation or shrinking-swelling from moisture variation.  The sealant is stretched and the bond is sheared by the lengthening or shortening of the shingle tab relative to the underlying shingle.  The stretching or shearing action may be progressive as the fracture extends through the sealant over time: if the fracture surface is not excessively contaminated with dirt or debris, re-heating of the sealant by the radiant heat of the sun may cause the sealant to re-seal.  An adhesive failure may also occur when the effectiveness of the sealant has been weakened by a relatively weak sealant material, a contaminated bonding surface, inadequate contact area, inability of the sealant material to soften in the heat, etc.

A cohesive failure is a failure within the sealant or within the shingle mat (See Figure CF-1).  That is, the seal of the shingle is broken because the bond between the sealant and the surface of the shingle is better than the material strength of the shingle mat or the sealant itself.  When most people see a cohesive failure they describe it as “chunks of the shingle were pulled out.”  This type of failure occurs when the shingle tabs are forcibly loosened from the underlying shingles.  This type of failure does not reseal when the shingles are re-heated.

 

CF-1 Cohesive failure of shingle tab sealant.

Shingle tabs may be forcibly loosened by uplift pressures from excessive winds, that is, at least 60 MPH for three-tab shingles and at least 70 MPH for the thicker shingles often called dimensional shingles, or by hand lifting.  Our measurement and calculation of in-place and aged shingle tab seal strength (based on methods developed through research for the shingle manufacturing industry) conservatively indicates that most reasonably well bonded shingle tabs (at least half the length bonded) will resist uplift pressures generated by winds of at least 80 MPH.  If a shingle tab has been unsealed by wind pressures, there will be other obvious wind related damages to the subject structure and/or in the surrounding area.  Additionally, through our measurements and calculations we have also found that even relatively poorly bonded shingle tabs have a significant resistance to uplift pressures generated by wind.

From this we conclude that decades of improvements by the shingle manufacturing industry have been successful at improving the resistance of asphalt shingles to wind related damages.

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