← U.S. Residential Roofing Basics
What is underneath the shingles?
By the end: Explain the distinct roles of decking, underlayment, leak barriers, shingles, and flashing.
Work from structure to weather surface. Rafters or trusses carry loads. Decking or sheathing creates the base above them. Underlayment sits beneath the roof covering as an additional protective layer. Localized self-adhered leak barriers may protect vulnerable areas where the design and local requirements call for them. They are not interchangeable with the deck.
For an asphalt-shingle assembly, starter products, field shingles, and hip or ridge caps do different jobs. Flashings handle boundaries and penetrations. Ventilation components may also be part of the assembly. Materials must work together according to the selected system, the building design, and applicable requirements. A roof made of individually good products can still fail at their connections.
Think of the layers as a team, not a ranking. If the deck is deteriorated, adding a premium surface does not restore its load-carrying capacity. If a transition is wrong, a second layer elsewhere may not stop water entering there. Ask an estimator to identify every layer they will assess, replace, retain, or exclude.
Diagram description
Four separated layers show shingles above underlayment and localized leak protection, above decking, supported by framing. Flashing and vents are coordinated at interfaces.
Watch for how the manufacturer groups protective layers and ventilation. Pause to match each visible part to the diagram. This is a branded system example, not a universal specification.
Quick check
Which component provides the base beneath underlayment?
End of module 1