How thick is roof underlayment?
Short answer. Synthetic roof underlayment ranges from roughly 7 to 30 mils thick depending on the product, while self-adhered ice and water membranes start at 40 mils, about 1 millimeter, under the ASTM D1970 standard. Felt is graded by weight as #15 or #30, and the #30 grade is the heavier, thicker one.
Thickness by type
- Synthetic: woven or spun polymer sheets. Data sheets list nominal thicknesses from under 10 mils for spun products to around 30 mils for heavier coated woven ones.
- Self-adhered membranes (ice and water shield): rubberized asphalt with a sticky back. ASTM D1970 sets a 40 mil minimum, and many products are thicker.
- Felt (tar paper): asphalt saturated felt sold as #15 and #30. The numbers once referred to weight per square; today #30 is still the heavier, more durable grade.
Why thicker is not automatically better
A thin synthetic can be far stronger than thick felt because its woven fibers resist tearing. What matters more than thickness:
- Tear and tensile strength, so it survives foot traffic and wind before shingles go on.
- Walkability, since a slick surface is dangerous on steep Georgia roofs.
- UV exposure rating, how long it can sit uncovered.
- Heat rating, especially under metal roofs.
- Code listing and the shingle manufacturer's approval for warranty purposes.
Where each one goes
On a typical Georgia shingle roof, synthetic covers the open field of the deck, and a self-adhered membrane goes in valleys, around chimneys and skylights, and at other leak-prone spots. Some homeowners choose full peel and stick coverage for extra protection.
This page is about roof underlayment, not flooring. Compare roof products in synthetic underlayment vs felt and ice and water shield in Georgia.