A while back, I inspected a new construction home that had dimensional asphalt shingles installed. It had a dormer on one side that had a 3/12 slope. The odd thing was the shingles were not dimensional type but instead were strip type shingles.
I mentioned it in the report as odd but not necessarily wrong and I recommended they ask the builder why it was different. From the roof I could see that other houses in the development were the same way.
The builder’s response was that dimensional shingles should not be installed on slopes less than 3.5/12. Given the code allows installation down to 2/12, and even manufacturers allow it, I was skeptical.
However, the builder provided information from the Western States Roofing Contractors Association (WSRCA) that supported what they were saying.
Apparently dimensional asphalt shingles can be problematic when installed on slopes below 3.5/12. This is due to the way the slope of the face of the shingle decreases because of the extra shingle thickness. It has been demonstrated by testing by WSRCA that more leaks occur on roofs with dimensional shingles installed on slopes below 3.5/12.
For roof slopes between 2/12 and 4/12 there are “no-line” (strip) asphalt shingles with similar life expectancy that can be used in place of the dimensional shingles and have the “look” of dimensional shingles.
WSRCA has produced a technical bulletin about this problem and roofing contractors that are members appear to be heeding its warning.
This is an example of where both codes and manufacturers may need to play catch-up.
Real Estate Inspections in Seattle