When starting a 3 tab shingle roof, how should the starter course be installed?

Installing the starter course of a 3-tab shingle roof upside down is crucial for ensuring proper water drainage and preventing leaks. The starter course is designed to create a seal along the eaves before the main shingles are installed. When placed upside down, the exposed strip at the bottom allows for effective water runoff, keeping excess moisture from getting trapped underneath the shingles. This setup helps to direct water off the roof rather than allowing it to seep in where the shingles overlap.

The other options do not offer the same protective benefits. If the starter course were installed right side up, it would not effectively address water runoff and could lead to water being trapped under the first course of shingles, increasing the risk of leaks. Similarly, installing it squarely or with a left side up orientation does not follow the intended design and may compromise the integrity of the roof system. Thus, upside down installation is the correct practice for ensuring the longevity and efficiency of the roofing system.