SHINGLE  CURLING

By

Charles C. Roberts, Jr. Ph.D., P.E.

Claims Magazine January 2011

 

 

 

Figure 1 is a photo of a typical glass fiber substrate asphalt shingle, a common roofing shingle used throughout the United States. A fiberglass mat is impregnated with asphalt and then coated with a ceramic grit called granules. Claims of roof shingle wind damage often arise when discovering curling of the shingle tabs after a severe wind storm (Figures 2 and 3). Curling is typically a long term related deterioration of the shingle brought on by a variety of causes not related to wind damage.

 

 

Figure 1

 

The following are typical causes of shingle curling:

 

1. The purpose of the sealing strip is to hold the neighboring shingle tabs down to prevent curling or lift during high winds. If the shingles are installed improperly where the sealing strip does not line up with the next shingle according to the manufacturer’s recommendation, then the shingle tabs will not be secure and will curl upward or inward over time.

 

 

 

 

Figure 2

 

 

Figure 3

 

2. Improper nailing is a common cause of shingle curl. In Figure 1, the nails are placed according to the shingle manufacturer’s recommendations. In Figure 4, a nail has been driven into the sealing strip which limits the effectiveness of the seal on the sealing strip. Over time, as a result of thermal cycles (heating during the day and cooling at night) the nail can back out, causing the shingle tab to lift and part from the seal, causing curling as shown in Figures 4 and 5. Also, insufficient nailing can lead to curling.

 

 

Figure 4

 

 

Figure 5

 

3. A lack of attic space ventilation is another cause of shingle curl. Moisture evolves in the attic as a result of moisture generation in the home and from the outside environment. Normally, moisture laden air in a well ventilated attic will be swept away by air flow and not cause a problem. In poorly ventilated attics, water vapor will increase and water will eventually condense on the roof sheathing. As the sheathing becomes moisture laden, shingle curling can occur. A lack of attic space ventilation can also cause high attic temperatures which can reduce the effectiveness of the sealing strip adhesive, causing curling over time. Most building codes require at least one square foot of free air opening for every 150 square feet of attic area. In homes with an acceptable vapor retarder, 300 square feet of attic space is allowed per one square foot of open ventilation area. Of course, attic ventilation openings should be distributed so that approximately 50% of the open area is at the ridge and approximately 50% at the soffits.
4. Product defects are also a possible cause of shingle curling. Inadequate adhesion of bituminous coating and grit, poor quality substrate, deterioration rates higher than that put forth in warranties and inadequate sealing strip are all problems associated with defective manufacture.  The recent evolution of class action suits against CertainTeed regarding premature failure of organic substrate shingles is an example of premature degradation of shingles. Figures 6a & 6b are photographs of 25 year warranty, CertainTeed shingles, 9 years after installation. There is evidence of inward curling and loss of granules from cracking of the cardboard substrate, which easily deteriorates in normal weather related conditions.

 

 

Figure 6a

 

 

Figure 6b

 

 

In conclusion, fractured shingle tabs can be related to wind damage, however curling of shingles is a long term problem associated with lack of ventilation, improper installation or product associated defects.