Felt vs Synthetic Underlayment
The hidden layer that protects your roof deck. Felt has a 100-year track record, but synthetic is quickly becoming the new standard. Here is why.
$10 – $25
per square (100 sq ft)
Tear Resistance
Low
UV Exposure
30–60 days
Roll Coverage
2–4 squares
Breathability
Moderate
Best for: Tight budgets, same-day shingle install, vapor-permeable assemblies
$25 – $60
per square (100 sq ft)
Tear Resistance
High
UV Exposure
Up to 6 months
Roll Coverage
10 squares
Slip Resistance
Non-slip
Best for: Multi-day installs, steep roofs, metal roofing, best long-term protection
Head-to-Head Comparison
| Feature | Felt Underlayment | Synthetic Underlayment |
|---|---|---|
| Material | Asphalt-saturated organic/fiberglass | Woven/spunbond polypropylene |
| Cost Per Square | $10 – $25 | $25 – $60 |
| Weight Per Square | 15 – 30 lbs | 4 – 8 lbs |
| Tear Resistance | Low (tears easily) | High (very durable) |
| UV Exposure Limit | 30 – 60 days | Up to 6 months |
| Slip Resistance | Slippery when wet | Non-slip surface |
| Coverage Per Roll | 2 – 4 squares | 10 squares |
| Breathability | Moderate | Varies by product |
| Moisture Absorption | Absorbs water, wrinkles | Does not absorb water |
Cost Comparison (20 Squares / 2,000 Sq Ft)
| Component | Felt (30 lb) | Synthetic |
|---|---|---|
| Materials | $200 – $500 | $500 – $1,200 |
| Labor Impact | Slower (heavier, tears) | Faster (lighter, durable) |
| Net Added Cost | Synthetic adds $300–$700 to a typical roof (about 2–5% of total project cost) | |
Calculate how much underlayment you need with our underlayment calculation guide.
The Real Cost in Context
On a $12,000 roofing project, upgrading from felt to synthetic adds roughly $300-$700 (2-5% of total cost). For that small premium you get dramatically better tear resistance, safer working conditions for your crew, and protection that lasts if the project hits a rain delay. Most roofing pros consider synthetic a no-brainer upgrade.
Pros & Cons
Felt Underlayment
Pros
- +Roughly half the price of synthetic
- +100+ year proven track record
- +Good breathability reduces trapped condensation
- +Accepted by all building codes
Cons
- -Tears easily from foot traffic and staples
- -Absorbs water, wrinkles, and buckles
- -Must be covered within 30-60 days
- -Slippery when wet (safety hazard)
- -Heavy rolls cover less area
- -Wrinkles can telegraph through shingles
Synthetic Underlayment
Pros
- +Extremely durable, resists tearing
- +Waterproof, stays flat even after rain
- +Non-slip surface reduces fall risk
- +Can be left exposed 3-6 months
- +Lightweight rolls cover 10 squares each
- +Faster installation, fewer fasteners needed
Cons
- -2-3x more expensive per square foot
- -Some products are vapor-impermeable
- -Slippery underside if not properly fastened
Choose Felt When...
- ✓Budget is extremely tight and every dollar counts
- ✓Roof will be shingled the same day (no weather exposure)
- ✓Codes or warranties specifically require felt
- ✓You need vapor permeability for the roof assembly
Choose Synthetic When...
- ✓Project may span multiple days with weather risk
- ✓Roof pitch is steep (over 6/12) for worker safety
- ✓Installing metal roofing (wrinkles from felt amplified)
- ✓Manufacturer warranty requires or recommends it
- ✓You want the best protection (cost difference is minimal)
Frequently Asked Questions
Should I use 15 lb or 30 lb felt?
Does underlayment type affect my shingle warranty?
Do I need ice and water shield in addition to underlayment?
How much underlayment do I need?
Related Tools
Underlayment Calculation Guide
Figure out exactly how much underlayment your project needs.
Roof Area Calculator
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Roof Replacement Cost Calculator
Get a full cost estimate including underlayment.
Asphalt vs Metal Roof
Compare the two most popular roofing materials.