According to code, asphalt shingles installed on a roof with slope 4:12 or greater require an underlayment of which material?

• A. One layer #15 non-perforated felt
• B. Two layers felt
• C. No underlayment
• D. One layer of tar paper

Answer: (A)

The requirement for an underlayment when installing asphalt shingles on roofs with a slope of 4:12 or greater is to use one layer of #15 non-perforated felt. Non-perforated felt acts as a moisture barrier, protecting the roof deck from water infiltration. Underlayment is essential, particularly in areas prone to wind-driven rain or where there is a potential for ice damming, as it helps to prevent leaks that could occur due to the roof's exposure to harsh weather conditions.

Using #15 felt specifically provides a good balance of durability and cost-effectiveness. It is thick enough to resist tearing and degradation during installation and provides additional protection against moisture that may reach the shingles. Although other forms of underlayment exist, such as two layers of felt or other materials, the specific need for just one layer of #15 felt aligns with standard building codes and best practices for roofing in these conditions. This choice ensures adequate protection without unnecessary complexity or expense.