What is roof underlayment?

Roof underlayment sits between your roof deck (the plywood or OSB) and the shingles themselves, acting as a backup water barrier when primary shingle protection fails. It catches water that gets underneath shingles during heavy rain, ice dam conditions, or wind-driven moisture, directing it toward gutters and downspouts instead of into the attic.

Two main types are common in the Hartford Metro region. Felt underlayment, the traditional choice, is a saturated organic or synthetic material that costs less but requires careful installation to avoid wrinkles and tears. Synthetic underlayment, made from polypropylene or polyester, resists tearing, dries faster, and performs better in damp conditions. Most modern roof replacements use synthetic because it handles Connecticut's freeze-thaw cycles and high humidity more reliably than felt.

Building codes typically require underlayment on new roofs and most replacements. The material anchors down with nails or fasteners before shingles go down, and proper overlap between strips prevents water from pooling or seeping at seams. Without underlayment, ice backup or storm-driven rain can damage decking and framing far faster than shingles alone would allow.

When getting quotes for roof work from local contractors, ask whether they include synthetic or felt underlayment and how they handle overlap and fastening. Choosing the right underlayment protects your home's interior during the lifespan of your roof, especially during Hartford's heavy spring and fall rain seasons.