Know Your Roof
Roof Underlayment and Roof Deck Terms
Under the shingles are two layers most homeowners never see: the underlayment and the roof deck it sits on. These guides and terms explain what each layer does, the common types and why decking problems usually only show up once the old roof is off.
Looking for a single word? See the roofing terms glossary, A to Z.
Underlayment & Roof Deck guides
Underlayment
Underlayment is the water-resistant layer between the deck and the shingles. It's the backup if water gets past the shingles. Most roofs use synthetic or felt underlayment across the deck, with self-adhered membrane at the eaves, valleys and around penetrations, where leaks are most likely to start.
Key terms in this topic
- Board Decking
- Individual boards with small gaps, found on older homes.
- Decking Replacement
- Swapping out damaged deck boards during a roof replacement. The amount is only known once the old roof is off.
- Dry-In
- Getting underlayment on the deck so the roof is weather-tight until shingles go on.
- Felt Underlayment
- Asphalt-saturated paper or fiberglass. The traditional choice, also called tar paper.
- Ice and Water Barrier
- A self-adhering waterproof membrane along the roof edge. A lot of roofers put it at the eaves because ice dams push water up under the shingles there.
- Leak Barrier
- Another name for self-adhered membrane used at eaves, valleys and around penetrations.
- OSB Decking
- A roof deck made from compressed wood strands. Common, and it swells if it stays wet.
- Plywood Decking
- Layered wood sheets used as a roof deck.
- Roof Deck
- The plywood or OSB boards nailed over the rafters that the roof is built on. Also called sheathing or decking.
- Roofing Nail
- A galvanized nail with a wide head, made to hold shingles without tearing through.
- Rotted Decking
- Soft or spongy deck boards from long-term moisture. They have to be replaced before new roofing goes on.
- Self-Adhered Underlayment
- A peel-and-stick waterproof membrane. Ice and water barrier is one type.
- Synthetic Underlayment
- A polymer sheet that resists tearing and doesn't soak up water.
Need a hand with this?
Inspections are free, and we respond within 2 hours, 24/7.
Roof replacement
Full roof replacements by an Owens Corning Preferred Contractor.
Free roof inspections
We check the roof, the edge and the flashing, and tell you what we find.
Ready to protect your home?
Get your FREE inspection today. Calls are answered 24/7, and we serve the greater Cincinnati area!