Most people understand that the human body consists of more than just the outer appearance (e.g., skin, hair, eyes, ears, nose, mouth, etc.). Although you cannot see another person’s internal organs or other systems, you understand they exist. The same analogy applies to roof layers. The outside layer of the roofing system is the visible surface material (typically asphalt shingles, wood shakes, or clay tiles). Under that layer, several sublayers exist, each serving a crucial function. These layers must work harmoniously to ensure a roofing system performs optimally.