Synthetic Underlayment vs. Felt
Why this matters
Underlayment is the layer between the roof deck + the roofing material. Traditional asphalt felt is being replaced by synthetic underlayment in modern installations. The choice affects performance + cost. Customers benefit from understanding the option.
Traditional felt
| Type | Properties |
|---|---|
| #15 felt | Standard residential; thin |
| #30 felt | Premium; thicker; longer-lasting |
Felt:
- Asphalt-saturated paper
- Standard for decades
- Cost-effective
- Limited service life when exposed
Synthetic underlayment
| Type | Properties |
|---|---|
| Polypropylene-based | Modern; lightweight; durable |
| Polyester-based | Premium; very durable |
| Hybrid | Multiple layer construction |
Synthetic:
- Long-lasting
- UV-resistant
- Lighter weight
- Tear-resistant
- More expensive
Comparison
| Aspect | Felt #30 | Synthetic |
|---|---|---|
| Weight | Heavy | Light |
| Walkability | Good | Excellent |
| Wind resistance | Limited | High |
| UV resistance | Limited | Excellent |
| Service life (exposed) | Limited; days to weeks | Months to years |
| Cost | Lower | Higher (1.5-2.5x) |
| Customer's roof life | Standard | Enhanced |
Modern installations favor synthetic.
When to use synthetic
| Customer situation | Recommendation |
|---|---|
| Premium installation | Synthetic |
| Long-term ownership | Synthetic |
| Cold climate (ice + water shield + synthetic) | Synthetic |
| Hot climate (UV + heat) | Synthetic |
| Standard residential | Either acceptable; synthetic preferred |
| Budget-constrained customer | Felt acceptable |
Installation differences
For synthetic:
- Larger sheets (typically)
- Faster installation
- Less waste
- Customer's investment
Standard procedure with adaptations.
Customer education
For underlayment:
- "Modern roofing uses synthetic for longer life"
- "Cost premium pays back through service life"
- "Better UV + tear resistance"
- "Customer's investment in quality"
A premium choice.
Cost framework
For 100 squares of underlayment:
| Material | Cost (relative) |
|---|---|
| Felt #15 | Baseline |
| Felt #30 | 1.3-1.5x |
| Synthetic | 1.5-2.5x |
| Premium synthetic | 2.5-4x |
Customer's choice.
Common pitfalls
- Skipping underlayment - code violation
- Wrong underlayment for the application - performance issues
- Improper installation - leak source
- Mixing brands - sometimes warranty issues
Common installation issues
For underlayment:
- Improper overlap
- Wrong fastener
- Wrinkles / bubbles
- Exposure exceeds limits
Each requires care.
Code requirements
For underlayment:
- Per IRC R905
- Specific overlap minimums
- Customer's specific slope
- Customer's specific climate
Verify.
Service life when covered
Both:
- Once shingles cover, service life is multi-decade
- Felt + synthetic both protected
The exposed period matters most.
Specialty: integrated synthetic
For some products:
- Synthetic with integrated ice + water shield strips
- Customer's specific choice
- Specialty installation
- Premium
Manufacturer combinations
For warranty:
- Some shingle manufacturers require specific underlayment
- Customer's specific warranty
- Verify before installation
Important for customer warranty.
Documentation
For each installation:
- Underlayment specified
- Manufacturer
- Customer file note
When the customer's reroof requires upgrade
For comprehensive reroof:
- Recommend synthetic
- Customer's investment in long-term
Customer's specific situation
For each customer:
- Felt acceptable for some
- Synthetic for others
- Customer's preferences guide
Common substrate considerations
For underlayment:
- Wood plywood
- OSB
- Older deck materials
- Verify before specification
Specialty: ice + water shield underlayment
Different from standard:
- Self-adhering
- Premium protection
- Specific zones
Not the same as standard underlayment.
When to use double-layer
For specific situations:
- Low-slope roofs (per code)
- High-wind areas
- Customer's specific protection
Specific applications.
Customer's investment
For underlayment choice:
- Small cost difference
- Significant performance difference
- Customer's long-term protection
A worthwhile decision.
Synthetic + shingle warranty
For some manufacturers:
- Synthetic underlayment required for full warranty
- Verify the shingle warranty terms
Important.
Final notes
Synthetic underlayment is:
- Modern standard
- Customer's premium choice
- Long-term value
- Manufacturer-required sometimes
A common modern upgrade.
References
- IRC R905 (Roofing)
- ASTM standards for underlayment
- Manufacturer documentation (Owens Corning, GAF, etc.)
- NRCA installation manuals
- Manuall internal: Asphalt Shingle Replacement SOP, Ice and Water Shield, Roof Slope and Measurements