Synthetic Underlayment vs Felt: Why It Matters

Underlayment is the layer between the roof deck and the shingles. Nobody sees it; nobody asks about it in the showroom. But the wrong underlayment choice can shorten roof life by 5+ years and create leaks you can't track down to a source. Here's why we use synthetic underlayment on every NJ install.

Traditional felt: what it is and where it fails. Asphalt-saturated organic felt has been the residential roof underlayment standard since the 1920s. It's cheap, it sheds water, and it works fine — for about 20 years. The problems: felt absorbs water and starts to rot if it gets wet (during install or through any small leak), it tears easily in installer foot traffic, it wrinkles and lays flat unevenly under shingles, and the asphalt binder dries out and becomes brittle over time.

Synthetic underlayment: what changed. Modern synthetic underlayment is woven polypropylene or polyethylene — essentially a tear-resistant plastic sheet engineered for roof use. It's significantly lighter than felt, doesn't absorb water, lies flat without wrinkles, can be left exposed for weeks during install if needed, and lasts roughly 4× longer than felt before degrading.

Real-world difference. On a 30-year asphalt roof, the shingles are what your homeowner's-insurance estimator sees. But the underlayment is what actually keeps water out when a shingle is missing, lifted, or fails. Old felt under a 25-year-old roof has often dried out completely — meaning the shingle is doing 100% of the waterproofing. One blown-off shingle in a storm becomes an active leak. Synthetic underlayment provides true backup waterproofing that holds up for the life of the roof.

Cost difference. Synthetic underlayment costs slightly more at the material level than felt, but the install is faster (lighter rolls, faster cuts, no wrinkles to flatten). Total cost difference on a typical install is small — and the lifecycle return is huge. Every Tri-State roof we install in NJ uses synthetic underlayment as the baseline spec.

Ice-and-water shield is different. Don't confuse general underlayment with ice-and-water shield. Ice-and-water shield (a fully self-adhered modified bitumen sheet) is required by NJ code at the eaves and valleys — the places where ice dams form and water pools. Synthetic underlayment covers the rest of the deck. Both are part of a complete spec.