measure your roof

https://research-wiki.win/index.php/What_Is_a_Roofing_Square_and_Why_Every_Contractor_Needs_One

Before you start any roofing project, you need accurate measurements. Grab a tape measure, ladder, and notepad. Measure the length and width of each roof plane, accounting for overhangs and pitch. For steep roofs, work safely from the ground using satellite images or a drone if possible. Adding 10% for waste ensures you have enough material.