Definition
Underlayment is the protective barrier between your roof deck and shingles. It's your backup if water gets past the shingles—synthetic underlayment offers superior protection compared to old felt.
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Underlayment is critical protection against water infiltration if shingles are damaged or lifted by wind. Traditional felt underlayment is inexpensive but deteriorates over time. Synthetic underlayment (polypropylene, polyethylene) provides superior protection and durability, lasting 30+ years. High-quality underlayment prevents ice dam leaks in cold climates where water backs up under shingles. Inspectors assess underlayment presence and condition when visible from the attic or during roof replacement.
Pro Tip
If you live in a cold climate, insist on synthetic underlayment during roof replacement. It prevents ice dam leaks.
Example
A roof inspection in Minnesota revealed the home lacked synthetic underlayment, relying on old, deteriorated felt. During spring snowmelt, ice dams formed at the gutters, backing water under shingles and into the attic. Installation of synthetic underlayment during the next roof replacement would prevent future ice dam leaks.