Asphalt shingles are the visible layer, but underlayment and flashing are the hidden components that make a roof watertight. Cutting corners on these materials is one of the most common causes of premature roof failure. Here’s what you need to know.
Underlayment: The Waterproof Barrier. Underlayment is the material installed between the roof deck and the shingles. It provides a secondary layer of protection against water that gets past the shingles. Two main types:
- Asphalt‑saturated felt (tar paper): The traditional option, available in 15‑pound and 30‑pound weights. Affordable and effective.
- Synthetic underlayment: Newer, made from woven polypropylene. Stronger, lighter, more resistant to tearing and UV exposure, and increasingly preferred by professionals.
Flashing: Sealing the Transitions. Flashing is the metal strips that seal the roof’s vulnerable points—where shingles meet chimneys, skylights, vents, walls, and valleys. Common materials include aluminum, galvanized steel, and copper. Properly installed flashing prevents leaks at these critical junctions. For how these are installed, see the installation process.
Why These Components Matter: Shingles alone cannot waterproof a roof. The underlayment and flashing work together to channel water away from the deck and vulnerable transitions. For cost implications, see asphalt shingle installation cost. For how long a properly protected roof should last, see asphalt shingle lifespan. And for maintenance of these components, see asphalt shingle maintenance.
Avoiding Mistakes: Incorrect flashing installation is a leading cause of leaks. Learn more in common installation mistakes.
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