What is the difference between roof underlayment and tar paper?
Short answer. Tar paper is one type of roof underlayment: asphalt-saturated felt, sold as #15 or #30. Today the word underlayment usually refers to synthetic sheets, which are lighter, stronger, more tear resistant and more tolerant of sun and rain during installation, while tar paper costs less but wrinkles and tears more easily.
What tar paper actually is
Despite the name, modern tar paper is a felt mat, made from organic fibers or fiberglass, saturated with asphalt rather than coal tar. It has been the traditional roof underlayment for generations and is made to ASTM standards in #15 and #30 weights.
How synthetic underlayment differs
- Material: woven or spun polypropylene or polyethylene, often coated, with a textured walking surface.
- Strength: much higher tear resistance, so it survives wind and foot traffic before shingles go on.
- Weight and coverage: a roll of #15 felt typically covers about 4 squares and #30 about 2, while many synthetic rolls cover 10 squares at a fraction of the weight. Fewer rolls means fewer seams and less lifting.
- Water and sun: felt absorbs moisture and wrinkles, then dries brittle in the sun. Synthetics stay flat and have longer UV exposure ratings.
- Vapor: felt passes some vapor; many synthetics pass less, though breathable versions exist.
- Cost: felt is cheaper per square, but the difference is small relative to a whole roof.
Where tar paper still shows up
Budget reroofs, some tile installations, small repairs and historic work still use felt. It meets code when installed correctly.
What most Georgia roofs get today
Synthetic over the field, with self-adhered leak barrier in valleys and around penetrations. Shingle manufacturers also tie their enhanced warranties to their own underlayment products.
See synthetic underlayment vs felt and ice and water shield in Georgia.