Felt vs synthetic underlayment is the first choice most homeowners never hear about, because the layer disappears under shingles on day one. It still decides how your roof handles wind-driven rain, ice at the eaves and the weeks between tear-off and the last bundle.
Think of this page as a felt vs synthetic underlayment swatch book. Each sample gets a care label, a fiber content line and a by-the-spec look at where it belongs. Then you can tilt a roof yourself and see what the Ohio code asks for at each pitch.
The 30-Second Version
Felt vs synthetic underlayment in one breath: felt is the classic asphalt sheet. Synthetic is a lighter woven or spunbond polymer sheet that resists wrinkling. Self-adhered leak barrier is a sticky membrane for eaves, valleys and flashings. Ohio code wants two layers on 2:12 to 4:12 slopes, one layer at 4:12 and steeper, and an ice barrier where ice dams are a known problem.
Measure Twice: Seven Marks on the Tape
Every Good Coat Has a Lining
Shingles shed most of the water. The underlayment is the lining that catches what sneaks past: wind-driven rain pushed under a tab, melt that backs up behind an ice dam, a drip at a nail line. It also keeps the deck dry while the crew works.
Our roofing system components guide lists it as component 2 of 7: leak barrier, underlayment, starter strip, shingles, flashing (including drip edge), hip and ridge cap and attic ventilation, all resting on the deck. This page goes deep on that second layer and on the felt vs synthetic underlayment decision you’ll see on an estimate.
Felt vs Synthetic Underlayment, Plus Peel-and-Stick
In any felt vs synthetic underlayment comparison, every swatch is allowed by code when it carries the right label. The Ohio roof assemblies rule says underlayment covered by ASTM D226, D1970, D4869 and D6757 shall bear a label, so the roll itself tells you what it is.
Asphalt-Saturated Felt
- Fiber content
- Organic or glass mat saturated with asphalt, sold as #15 and #30.
- Standard
- ASTM D226 (Type I and Type II) or ASTM D4869.
- Care
- Absorbs moisture if left wet, which is where wrinkles start. Cover it promptly.
- Wears well on
- Budget reroofs and crews who know its quirks.
Synthetic (Polymeric)
- Fiber content
- Woven or spunbond polypropylene with a slip-resistant face.
- Standard
- ASTM D8257, or felt standards plus an ICC-ES evaluation report.
- Care
- Install with the cap nails or staples the maker lists; respect its UV window.
- Wears well on
- Most full replacements with modern laminated shingles.
Self-Adhered Leak Barrier
- Fiber content
- Polymer-modified bitumen sheet with a peel-off release film.
- Standard
- ASTM D1970.
- Care
- Needs a clean, dry deck to bond. Not a full-roof substitute on most homes.
- Wears well on
- Eaves, valleys and around chimneys and walls.
Why two paths for synthetics? The Florida Roofing and Sheet Metal Contractors Association explains that early synthetics were qualified under felt standards that are “primarily composition driven,” so only a few tests applied. ASTM D8257 was written for polymeric sheets, while ICC-ES acceptance criteria AC188 supports evaluation reports.
Pitch & Pick: Tilt the Roof, Choose the Fabric
Pick a slope to see how many layers the Ohio code asks for and where the leak barrier goes. Then pick a swatch to compare felt vs synthetic underlayment and peel-and-stick side by side.
The lowest slope where asphalt shingles are allowed. Code calls for double underlayment: a 19-inch starter strip at the eave, then 36-inch sheets lapped 19 inches. Leak barrier at the eaves where ice dams are designated.
Still in the double-underlayment range (2:12 up to 4:12). Water drains slowly here, so the doubled lap pattern matters more than the brand on the roll.
At 4:12 and steeper, one layer applied shingle fashion with 2-inch head laps meets the asphalt-shingle table. Eave protection still applies where required.
A common Miami Valley pitch. One layer of underlayment, ice barrier from the eave to 24 inches inside the exterior wall, and peel-and-stick in valleys.
Steep enough that footing becomes the story. Code needs one layer; a slip-resistant synthetic face helps the crew work safely.
- StandardASTM D226 Type I (#15) or Type II (#30), or D4869
- Roll widthTypically 36 inches
- MoistureCan absorb water and wrinkle if left exposed
- Where it goesField of the roof, under shingles
- StandardASTM D8257 or ICC-ES report; FeltBuster also meets D226 Type II and D4869 physicals
- Roll width48 inches on GAF Deck-Armor, about 33% more per course than 36-inch felt
- MoistureDoes not absorb water, which helps prevent wrinkles
- UV windowUp to 90 days (FeltBuster) or 180 days (Tiger Paw, Deck-Armor)
- StandardASTM D1970 self-adhering polymer-modified bitumen
- BondSticks to the deck so water can’t run underneath
- Code roleSatisfies the ice barrier at eaves
- Where it goesEaves, valleys, walls and penetrations
What Ohio Code Says, Slope by Slope
The Residential Code of Ohio handles roofs in Ohio Admin. Code 4101:8-9-01. For asphalt shingles, three lines do most of the work, and they apply whichever side of the felt vs synthetic underlayment question you land on.
| Roof slope | Layers | How it goes down |
|---|---|---|
| Under 2:12 | No shingles | Asphalt shingles are only used on slopes of 2:12 or greater; low slopes need a different roof. |
| 2:12 up to 4:12 | Two | 19-inch strip at the eave, then 36-inch sheets overlapping 19 inches. |
| 4:12 and steeper | One | Applied shingle fashion, lapped 2 inches. |
| Eaves (ice areas) | Ice barrier | From the lowest edge to at least 24 inches inside the exterior wall line. |
The ice barrier applies where the local climate table designates a history of ice forming along the eaves. It can be two layers cemented together or a self-adhering polymer-modified bitumen sheet. Around here we treat it as standard, and our ice damming guide shows why.
Laps and Fasteners Are the Stitching
A seam is only as good as its stitch. Underlayment laps are the same: water runs downhill, so every upper sheet has to lie over the one below it. Laps matter more than the felt vs synthetic underlayment label. These are the code numbers worth remembering.
Fasteners follow the code table and the manufacturer. GAF’s FeltBuster data sheet, for example, calls for plastic cap nails or staples. Leak barrier goes on first at the eave, the drip edge and roof flashing tie in, and the field underlayment laps over the top.
Water always looks for the lap that runs uphill. A good crew never gives it one.
Why the Layer Matters for Your Warranty
Shingle makers build warranties around whole systems, not single bundles. GAF says FeltBuster is eligible for up to a Lifetime limited warranty when used with GAF Lifetime Shingles such as Timberline HDZ, Timberline UHDZ and Camelot II. Our GAF roof warranty guide walks through how the tiers stack.
GAF’s premium synthetics add more room for weather delays. Tiger Paw and Deck-Armor list 180-day UV resistance and at least 600% greater tear strength than standard #30 felt. Deck-Armor is also breathable, so moisture inside the roof system has a way out.
So the felt vs synthetic underlayment choice is partly a paperwork choice. Ask which underlayment the warranty on your estimate assumes, and get it in writing.
Felt vs Synthetic Underlayment: Where Jobs Unravel
Most felt vs synthetic underlayment trouble has nothing to do with the roll. It comes from timing and technique.
- Wrinkles that telegraph. Felt that soaks up rain can ripple, and ripples show through new shingles. Synthetics that don’t absorb water help, but any sheet should lie flat before shingles go on.
- Exposure past the UV window. A 90-day or 180-day rating covers sunlight only. GAF notes it does not mean resistance to water, snow or wind, and FeltBuster is not meant as a temporary roof.
- Missed or reversed laps. An upper sheet tucked under a lower one sends water straight to the deck.
- No ice barrier at the eaves. Melt that backs up behind an ice dam finds the first unsealed nail.
- Skipped valleys and walls. These spots carry the most water and want peel-and-stick under the flashing.
The Takeaway Worth Sharing
In the felt vs synthetic underlayment debate, the lap pattern beats the label. Pick the fabric your warranty expects, seal the eaves and keep it covered.
Send this felt vs synthetic underlayment guide to the neighbor comparing roof estimates.
Want the Right Layer Under Your Shingles? Let’s Look at Your Roof
Thrush & Son®: family-owned exterior pros since 1960.
Weighing felt vs synthetic underlayment on an estimate? If you’re in the Miami Valley and want to see how we handle the layers under GAF Timberline HDZ, read the Thrush & Son® roof underlayment guide, then book an inspection.
➡️ Schedule in Dayton➡️ View All ServicesCall (937) 833-5136 · 115 Jefferson St, Brookville, OH 45309
- GAF President’s Club Contractor
- Best of Dayton 2024 Winner · 2025 and 2026 Finalist
- BBB Accredited A+
Further Reading
Official sources: Ohio Admin. Code 4101:8-9-01 (Cornell LII), the Ohio Laws and Administrative Rules listing and the FRSA on ASTM D8257.
Felt vs Synthetic Underlayment FAQ
Which wins, felt vs synthetic underlayment?
Both meet code when labeled to the right standard. Synthetic is lighter, resists wrinkling and offers a longer exposure window, so it suits most full replacements. Felt still works when installed and covered promptly.
How many layers of underlayment does Ohio require?
For asphalt shingles, two layers on slopes from 2:12 up to 4:12, and one layer at 4:12 and steeper. Shingles are not used below 2:12.
Where does the ice barrier go?
Where the local climate table designates ice forming at the eaves, it runs from the lowest roof edge to at least 24 inches inside the exterior wall line.
How long can synthetic underlayment stay uncovered?
It depends on the product. GAF rates FeltBuster for up to 90 days of UV exposure and Tiger Paw and Deck-Armor for 180 days. Those ratings cover sunlight, not water, snow or wind.
Is peel-and-stick the same as underlayment?
No. Self-adhered leak barrier made to ASTM D1970 bonds to the deck and protects eaves, valleys and flashings. The rest of the roof gets mechanically fastened underlayment.
Written by Corey Thrush, Vice President and third generation of Thrush & Son®.
Leave a Reply