Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
Underlayment is the layer of material installed between your roof decking and the shingles themselves. It’s the second line of defense – when wind-driven rain or ice dam meltwater finds its way past the shingles, the underlayment is what stops the water from reaching the deck. For decades, the standard was 15-lb felt (“tarpaper”). Today, synthetic underlayment is the right answer on virtually every Illinois install. Here’s why, with the technical specs.
What underlayment does
Underlayment serves four functions in a properly installed roof system:
- Backup water barrier – water that gets past the shingles (lifted in wind, meltwater under ice dam, condensation drip) hits the underlayment before reaching the deck
- Deck protection during install – when shingles are off mid-installation and an afternoon storm rolls in, underlayment keeps the deck dry for the rest of the project
- Smoothing layer – underlayment evens out small imperfections in the deck so the shingles lie flat
- Vapor management – modern synthetics allow some vapor to pass while blocking liquid water
Ice and water shield (a self-adhered rubberized membrane) is a different product – installed in valleys, at eaves, and around penetrations. Underlayment covers the rest of the deck. Both are required by IL code in different zones.
15-lb felt – the old standard
Asphalt-saturated felt (“15-lb felt” or “tarpaper”) was the default underlayment for residential roofs through the 1990s. It’s still widely available, still inexpensive, and still occasionally installed by contractors looking to shave dollars off a quote.
Felt specs:
- Weight: 15-lb felt is actually about 8 lbs per 100 sq ft (the “15-lb” name is historical and inaccurate). 30-lb felt is heavier – maybe 16 lbs per square – and more durable.
- Roll size: typically 36 inches wide, 432 sq ft per roll
- Tear resistance: low. Felt tears easily with foot traffic or fastener pull-through.
- UV exposure tolerance: 7-14 days. Felt degrades quickly when exposed to sun before shingles go on.
- Wet performance: felt absorbs water, wrinkles when wet, can tear when wet
- Cost: $20-$30 per 432 sq ft roll (about $0.05-$0.07 per sq ft)
The real problem with felt isn’t the cost – it’s the failure mode. Felt installed on a Wednesday and exposed to weather through Friday before the shingles arrive on Monday loses 30-50% of its tear resistance. By the time the roof system needs it (5-15 years in, when something happens above), it may have already degraded into something that doesn’t perform.
Synthetic underlayment – the current standard
Synthetic underlayment is woven polypropylene or polyester treated with stabilizers. It’s the standard install on every Trill Roofing replacement and on most professional installs since about 2010.
Synthetic specs (varies by product):
- Weight: 4-6 lbs per square – substantially lighter than felt
- Roll size: wider (48 inches typical) and longer (1,000+ sq ft per roll), so fewer overlaps and fewer fasteners
- Tear resistance: 5-10x felt – synthetic resists tear-through at staples and walked-on areas
- UV exposure tolerance: 60-180 days depending on product. Synthetic can sit exposed through extended weather delays without degradation.
- Wet performance: doesn’t absorb water, doesn’t wrinkle, maintains specs when wet
- Slip resistance: textured surface for safer foot traffic during install
- Cost: $80-$200 per 1,000 sq ft roll (about $0.08-$0.20 per sq ft)
Common products we install: GAF Tiger Paw (paired with their shingle warranties), CertainTeed DiamondDeck, Owens Corning Deck Defense, Atlas Summit.
Side-by-side specs
| Spec | 15-lb Felt | Synthetic |
|---|---|---|
| Weight per square | 8 lbs | 4-6 lbs |
| Tear resistance | Low | 5-10x felt |
| UV tolerance | 1-2 weeks | 2-6 months |
| Wet performance | Absorbs, wrinkles | Hydrophobic |
| Roll width | 36″ | 48″ |
| Cost per sq ft | $0.05-0.07 | $0.08-0.20 |
| Warranty backing | Often excluded from premium warranties | Required for premium tier warranties |
The cost difference for a 2,000 sq ft roof: $60-$260 between felt and synthetic. On a $12,000-$15,000 roof replacement that’s less than 2% of the total cost – and it’s a 30-year decision.
Warranty implications
This is where it actually matters in dollars. Most major shingle manufacturers (GAF, CertainTeed, Owens Corning) tier their warranties:
- Standard warranty – minimum specs, applies regardless of underlayment
- Enhanced warranty – requires manufacturer-certified contractor + manufacturer-spec underlayment (their synthetic, not generic felt)
- Top-tier warranty – requires full system installed by an elite-certified contractor with synthetic underlayment AND ice barrier in code-required zones AND ridge venting
For example, GAF’s Golden Pledge warranty (which we can register as GAF Certified Contractors) requires GAF Tiger Paw synthetic underlayment. Installing felt instead drops the warranty to a much weaker standard tier – saving maybe $100 in materials and losing ~$5,000 of labor warranty value over the roof’s life.
Ask any roofing quote you receive: what underlayment is specified? If it’s felt, ask why – and what warranty tier applies.
Where ice and water shield fits
Ice and water shield is a self-adhered rubberized asphalt membrane – different product, different role. IL building code (IRC R905.1.2) requires ice barrier:
- From the lowest roof edge to a point 24″ inside the warm-wall line
- In all valleys
- Around all penetrations (chimney, plumbing vents, skylights)
Synthetic underlayment goes everywhere else. So a typical roof system has ice barrier in vulnerable zones and synthetic across the rest of the deck. Both layers; complementary purposes.
What to ask on a quote
If you’re collecting roofing quotes:
- What underlayment is specified by brand and product name?
- If synthetic – what’s the manufacturer’s UV tolerance rating?
- What ice and water shield product and coverage?
- Does the underlayment qualify for the manufacturer’s premium warranty tier?
Vague answers (“the standard stuff” or “whatever the manufacturer recommends”) are signs the contractor hasn’t actually specified the system. Get product names in writing.
Schedule a Trill Roofing free inspection at /free-inspection/ for an itemized quote with all materials specified.
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