Felt underlayment is mandatory under the following conditions of application:

• All glass-based shingles installed on slopes less than 8/12. It is optional for slopes greater than 8/12.
• Underlayment is required under Building Products of Canada's Eclipse laminated shingles on slopes less than 6/12. It is optional for slopes greater than 6/12.
• In order to achieve fire-resistant ratings (Class A or C), an underlayment must be used regardless of the type of shingle being installed.
• A minimum of 2 layers of felt is required for roofs with slopes less than 4/12. In such cases, the application must meet the requirements for low slope roofs.

Advantages of using underlayment include:

• Reducing the risk of water infiltration due to wind driven rain, or if shingles are torn or cracked;
• Act as a buffer area between the shingles and the deck.

Considering the relatively low cost of felt underlayment, Building Products of Canada recommends it be used under the vast majority of roof applications.