What is the specification for the number of underlayment layers needed for wood shakes on a roof?

The correct specification for the underlayment layers needed for wood shakes on a roof is that the underlayment should be not less than Number 30 felt. This requirement is based on the need for a durable and moisture-resistant barrier beneath the wood shakes, which are a natural roofing material that can be subject to water infiltration if not properly installed.

Using Number 30 felt provides a reliable level of protection against leaks and helps manage moisture that can accumulate under the shakes, especially in regions with heavy rainfall or snow. This layer serves as an important secondary defense, as wood shakes can absorb water and lose their effectiveness over time if proper underlayment is not used.

In contrast to the other options, which suggest varying numbers of layers, specifying the use of Number 30 felt establishes a standard for quality and performance that aligns with building codes and best practices in roofing. Furthermore, simply stating a number of underlayment layers without specifying the type or quality of the underlayment does not ensure adequate protection or compliance with industry standards.