What is the best roof underlayment for shingles?
Short answer. For a typical Georgia shingle roof, the usual combination is a synthetic underlayment over the open deck plus a self-adhered leak barrier in valleys, around chimneys, skylights and other penetrations, and on low-slope areas. Using the shingle manufacturer's own underlayment and leak barrier keeps the whole system eligible for its enhanced warranties.
The options
- Synthetic underlayment: woven or spun polymer sheets. Strong, light, tear resistant, and able to sit exposed longer than felt. The default on most new roofs.
- Felt (#15 or #30): asphalt-saturated felt. Cheaper, but it tears more easily, wrinkles when wet, and breaks down faster in sun.
- Self-adhered leak barrier: rubberized asphalt that sticks to the deck and seals around nails. Used where water concentrates or moves slowly.
Where each goes on a typical Georgia roof
- Valleys: leak barrier, because two slopes dump water into one channel.
- Around chimneys, skylights, walls and pipe penetrations: leak barrier under the flashing.
- Low-slope sections from 2 in 12 to 4 in 12: double underlayment or leak barrier.
- Eaves: leak barrier where ice dams are possible, mostly in the North Georgia mountains.
- Everything else: synthetic underlayment.
When full peel and stick makes sense
Covering the entire deck with leak barrier creates a sealed deck that keeps water out even if shingles blow off. It costs more, makes the next tear-off harder, and needs a well-ventilated attic since it blocks vapor. It suits wind-exposed homes and owners pursuing a FORTIFIED designation.
Match the system
Shingle manufacturers offer enhanced warranties when their own underlayment, leak barrier, starter, caps and ventilation are used together. Mixing brands can still meet code but may limit warranty coverage.
See synthetic underlayment vs felt and ice and water shield in Georgia.