When shingles are torn off, the underlayment beneath becomes the primary barrier between your decking and the elements. What type gets installed matters.
The Two Types
Felt underlayment (tar paper): Traditional roofing felt made from organic or fiberglass-reinforced paper saturated with asphalt. Has been used in roofing for decades.
Synthetic underlayment: Made from woven or spun polypropylene or polyethylene. No paper, no organic content. Dramatically different performance.
Why Synthetic Has Become Standard in Atlanta
Moisture resistance: Felt is water-resistant, not waterproof. In Georgia’s climate, where a roof might sit exposed for hours during installation, synthetic provides significantly better temporary protection against a rain event.
Tear resistance: Felt tears easily under foot traffic during installation. Synthetic is substantially stronger — fewer tears means fewer potential leak points.
UV stability: Felt degrades rapidly under UV exposure. Synthetic maintains performance for extended periods. Critical when installation is delayed by weather.
Mold resistance: No organic content means no food source for mold. In Georgia’s humidity, this matters.
The Trade-off
Synthetic costs $0.10-$0.20 per sqft more than 15-lb felt. On a 2,000 sqft roof, that’s $200-$400 added to the project. It’s essentially always worth it.
What to Ask Your Contractor
Ask specifically what underlayment they’re using and make sure it’s in the written estimate. Quality products include GAF FeltBuster, CertainTeed Diamond Deck, and Grace Ice & Water Shield for valleys and eaves.
A note on ice and water shield: Self-adhering ice and water barrier in valleys and at eaves is a worthwhile upgrade in Atlanta. It’s required by code in colder climates and provides an added waterproofing layer at your roof’s most vulnerable zones.