What is underlayment?
A protective moisture barrier material installed directly on the roof deck beneath shingles or panels to block water penetration and provide secondary weatherproofing.
Underlayment is a waterproof or water-resistant layer applied to the roof deck before shingles, metal panels, or tile are installed. It serves as a secondary defense against leaks by catching water that works past the primary roofing material, particularly in valleys, at eaves, and around roof penetrations like vents and chimneys.
Two main types are used in Georgia residential and commercial roofing. Felt underlayment, typically 15-pound or 30-pound saturated paper, has been the traditional choice for decades and remains affordable. Synthetic underlayment, made from woven polypropylene or polyester, offers greater durability, better traction for workers during installation, and superior resistance to tearing and UV exposure compared to felt. Synthetic products also perform better in high-heat conditions common to Georgia summers.
Building codes and shingle manufacturers often specify underlayment requirements based on roof slope, wind zone, and climate. Steeper pitches and lower slopes (common in many Georgia homes) typically mandate specific underlayment grades. Proper installation without wrinkles and with adequate overlap is critical to function, as is correct fastening to prevent blow-off during storms. When replacing a roof, contractors inspect existing underlayment for damage and deterioration before deciding whether to recover or remove it, since compromised underlayment can lead to interior water damage even if the visible shingles appear intact.