What roof underlayment means
Underlayment is the sheet laid over the roof deck before the shingles or metal. It is a second line of defense against water that gets past the roofing, and it keeps the deck dry during the job.
Also called felt, synthetic underlayment, tar paper.
Where it matters on a roofing job
Felt and synthetic underlayment price very differently and handle walking and weather differently. Name the product on the estimate. On low pitch roofs the code asks for a double layer, which doubles that line.
What the model code says
- On asphalt shingle roofs from 2 in 12 up to 4 in 12, the underlayment must be doubled. (IRC 2021, Chapter 9)
Check which code edition your building department enforces for roof underlayment before you quote.
What it costs
The roof replacement cost guide has the typical job at $9,350 and the usual range at $5,800 to $13,000. The guide shows its sources and what moves the price. The roofing materials calculator prices a job line by line.
Work it out
- Free calculator: Roofing materials calculator
- Cost guide: Roof replacement cost
- Licensing: Roofing license rules by state
- Tools, templates and guides: Free roofing resources
Sources: 2021 International Residential Code, Chapter 9, Roof Assemblies (ICC). Checked October 9, 2026.