Roof Underlayment — Felt vs Synthetic and Why the Difference Matters
Updated June 2026 · 7 min read
Homeowners focus on shingles — the visible layer. But the underlayment is the part that actually keeps water out of your home when a shingle fails, blows off in a storm, or lifts at an edge. It's the secondary moisture barrier between your shingles and the structural deck.
The difference between good and cheap underlayment isn't visible once the shingles are on. That's why it's a common place for contractors to cut costs without the homeowner knowing. Here's what to specify and why.
What Underlayment Actually Does
Underlayment has three jobs: it provides a second line of defense against water infiltration if a shingle is compromised; it protects the deck during installation if rain falls before shingles are placed; and it provides a smooth, consistent surface that helps shingles lay flat and nail down properly. It's not a luxury layer — it's required by building code on any new roof installation.
15-lb Felt
Lowest30-lb Felt
Low-moderateSynthetic Underlayment
ModerateIce-and-Water Barrier
HigherWhat to Specify on Your Estimate
Your estimate should specify by name, not category. Ask for this wording in the scope:
Field underlayment: Synthetic underlayment — [brand/product] — over entire roof deck
Eaves and valleys: Self-adhesive ice-and-water barrier — [brand] — [X] feet from drip edge at eaves; applied to all valleys
If an estimate just says "underlayment" with no specification, ask. Most contractors who skip the specification default to the cheapest option.
Frequently Asked Questions
Is synthetic or felt underlayment better?
Synthetic underlayment is better in almost every measurable way: it's lighter (easier to handle on the roof), stronger (won't tear in high wind during installation), more UV-resistant if the shingles go on slowly, more water-resistant if rain hits before shingles are installed, and more slip-resistant for the crew. It costs slightly more — typically $200–$400 more on a full reroof — which is a rounding error relative to the total project cost.
What is ice-and-water barrier and is it different from underlayment?
Ice-and-water barrier (also called ice-and-water shield) is a self-adhesive, rubberized asphalt membrane installed at the eaves and in valleys. It's different from standard underlayment — it's thicker, fully adhered to the deck, and seals around nail holes rather than just shedding water. Most building codes require 2–6 feet of ice-and-water barrier at the eave edge in cold climates, in addition to standard underlayment over the rest of the roof.
Can I reroof over existing underlayment?
No — you can install a second layer of shingles over existing shingles without removing underlayment, but if you're doing a full tearoff (recommended), new underlayment goes on the bare deck. The point of full tearoff is to get down to the deck so it can be inspected and so new underlayment provides proper secondary protection. Reusing old felt under new shingles defeats much of the purpose.
Get a Quote That Specifies Every Component
Licensed local contractors, written scope including underlayment spec — free estimate.