Mastering Roof Inspections: Asphalt Composition Shingles, Part 51

Blow-Off

Shingles may become detached and blow completely off of the roof. This type of failure is often caused by poor bonding of the adhesive strips, resulting in tab-uplift forces that break the bonds completely. Wind can then lift the tab and enter beneath the affected shingle, increasing the uplift forces on that entire shingle.

If fasteners have been improperly placed, the wind may be able to pull the shingle over the heads of their fasteners, displacing the shingle or blowing it entirely off the roof.

In the photo above, you can see a number of creased shingles, indicating that the shingles were not bonded adequately. Where the shingles are missing, you can see in the course below that the shingles were high-nailed. High-nailing shingles drastically reduces the wind resistance of the roof. In this case, wind damage can be expected to continue.

The recommendation for this condition is complete re-fastening of the entire roof, followed by hand-sealing the entire roof.  If all the shingles are poorly bonded, this may be possible.

Adhesive Strips

Understanding the importance of full bonding of the adhesive strip is key to inspecting asphalt composition shingle roofs in areas exposed to high winds.  By bonding shingles to each other, the adhesive strip limits the area of shingle that is exposed to wind forces.

A shingle roof has maximum resistance when the wind is blowing at a 90° angle to the length of the shingle. This is the case when the wind is blowing straight at the eave. Under these conditions, the wind will be blowing at the tab edges, which are usually sealed along their entire length.

Wind blowing at a rake will be blowing at the ends of the shingles.  In this situation, the portion of the adhesive strip facing the wind is typically a maximum of ¾-inch wide. This reduced area of sealant can leave shingles near the rake edge more vulnerable to damage.

Failure of adhesive strips can be caused by a number of different conditions:

• Shingles installed during cold months in cold regions of North America may not bond until mid-spring of the following year.
• Wind may blow dust or debris into the adhesive strip, contaminating it.
• Shingles that never experience direct sunlight due to shade from trees or topographical features, such as mountains, may never fully seal.
• In an effort to keep roofs cool and extend the shingles' service life, manufacturers have developed granule coatings that reflect sunlight more effectively than conventional coatings. In areas and situations in which the available sunlight only marginally warms the roof, highly reflective granule coatings may prevent adhesive strips from bonding adequately.
  
  

Transfer

Sometimes, only part of an adhesive strip may fail and a section of the adhesive strip may remain fully bonded.  Wind may be strong enough to break the overlying shingle tab loose from the underlying shingle to which it’s bonded. The shingle may delaminate at the fully bonded section.

When this happens, a divot may be pulled loose and remain attached to the opposing shingle. This is called transfer because part of one shingle is transferred to another.  Both shingles should be replaced.

Inflation

Wind may break the bond of the adhesive strips, but it may not be strong enough to crease or tear the shingle tabs. One of the ways this can happen is when wind is able to enter the space between the shingles and underlayment. This is most likely to happen when wind is blowing against a rake rather than against an eave. In this situation, the wind can inflate this space, raising the air pressure to the point at which the bonds of the adhesive strips fail.