Synthetic underlayment is the better choice under a new roof in Western Pennsylvania: it is stronger, lighter, more water resistant, and safer to walk on than felt, it does not wrinkle or tear in the wind while the roof is going on, and it adds only about $300 to $700 to the cost of a typical roof replacement. Felt still works and is still allowed by code, but on a Pittsburgh roof that will see 30 winters of freeze thaw, synthetic paired with ice and water shield at the eaves and valleys is what we install and what the manufacturer warranties expect. Here is what each one does and why it matters more here than in a mild climate.
Last updated September 2026.
What does roof underlayment actually do?
Underlayment is the layer between the wood decking and the shingles. Shingles are the primary barrier, but they are not perfect. Wind drives rain up under the edges, ice backs water up behind the shingle laps, and a missing or cracked shingle leaves a gap. Underlayment is the second line of defense that keeps that water off the plywood until it can drain out at the eave.
It also protects the deck during the job. A roof that is stripped in the morning and not fully shingled by evening is protected overnight by the underlayment alone. And it gives the shingles a smooth, uniform surface to seal against, which matters for wind resistance.
What is felt underlayment?
Felt, sometimes called tar paper, is a mat of paper or fiberglass soaked in asphalt. It comes in two weights. Fifteen pound felt is the thin version, about the weight of heavy paper, and 30 pound felt is roughly twice as thick. Felt has been under roofs for a century and it does the basic job.
Its weaknesses show up on the roof. Felt absorbs water and wrinkles, which can telegraph through the shingles as bumps. It tears easily in wind, and a Western PA fall with lake effect gusts is hard on an exposed felt deck. It gets slick when wet and brittle when cold, and it degrades in sunlight within days. A roll of 15 pound felt covers only 400 square feet, so it takes a lot of laps, and every lap is a place water can get in.
What is synthetic underlayment?
Synthetic underlayment is a woven or spun polypropylene or polyethylene sheet, often with a nonslip coating on top. A single roll covers about 1,000 square feet and weighs less than a roll of felt. It does not absorb water, so it does not wrinkle. It resists tearing around the fasteners, which keeps it on the roof in wind. It can be left exposed for weeks or months without breaking down. And the walking surface is far better, which matters to a crew on a 10/12 pitch in Greensburg.
Owens Corning makes several synthetic underlayments, and their product pages explain the differences in weight, exposure rating, and walkability. You can compare them on the Owens Corning underlayment page.
Synthetic underlayment vs felt comparison
| Factor | 15 lb felt | 30 lb felt | Synthetic |
|---|---|---|---|
| Coverage per roll | About 400 sq ft | About 200 sq ft | About 1,000 sq ft |
| Weight per roll | About 30 lb | About 30 lb | About 25 lb |
| Water absorption | Absorbs, wrinkles | Absorbs, wrinkles less | Does not absorb |
| Tear resistance | Low | Moderate | High |
| Wind during install | Tears easily | Tears | Holds |
| Safe exposure time | A few days | A few days | Weeks to months |
| Walkability when wet | Slick | Slick | Nonslip coating |
| Cold weather handling | Brittle | Brittle | Stays flexible |
| Lifespan under shingles | 15 to 20 years | 20 to 25 years | 30 to 50 years |
| Cost per square (100 sq ft) | $5 to $10 | $8 to $15 | $12 to $25 |
| Added cost, typical 2,000 sq ft roof | Baseline | About $100 to $200 more | About $300 to $700 more |
| Meets Owens Corning premium warranty tiers | Not for top tiers | Not for top tiers | Yes, with matching components |
Where does ice and water shield go?
Ice and water shield is a different product from either felt or synthetic. It is a thick rubberized asphalt membrane with a peel off backing that sticks directly to the deck and seals around every nail that goes through it. It is the only underlayment that actually stops water that is backing up rather than running down.
The Pennsylvania Uniform Construction Code, which follows the International Residential Code, requires an ice barrier in areas with a history of ice forming along the eaves. All of Western Pennsylvania qualifies. The code calls for the membrane to extend from the edge of the roof to at least 24 inches inside the exterior wall line, measured horizontally. On a house in Irwin with a 12 inch overhang and a low pitch, that usually means two full courses of 36 inch membrane along every eave.
Code is the minimum. In our Westmoreland County jobs, we also run ice and water shield the full length of every valley, around every chimney, skylight, and pipe boot, and along any low slope section, because those are where every leak we get called to fix actually starts. On roofs with a pitch under 4/12, we often cover the whole deck with it.
Why does underlayment matter more in Pittsburgh?
Because of ice dams. Snow melts from heat escaping the attic, runs down to the cold overhang, and refreezes. The ice builds a dam, and the meltwater behind it creeps up under the shingles. Shingles shed water running downhill, not water sitting behind them, so the underlayment is the only thing between that water and your ceiling. Felt that has gone soft is not much of a barrier. Synthetic sheds it, and ice and water shield seals it out entirely. Add freeze thaw cycling, lake effect wind, and heavy fall leaf drop holding water at the eaves, and the case is easy. Underlayment is the cheapest part of the roof and the one you cannot upgrade later without tearing the shingles off.
What does Owens Corning require for their warranty?
This is where the choice stops being optional. Owens Corning’s premium warranty tiers, the ones that cover materials and workmanship for decades and that we can offer as a Platinum Preferred Contractor, require an Owens Corning roofing system installed with matching components. That means Owens Corning shingles, ice and water shield, underlayment, starter strip, hip and ridge, and ventilation. Generic felt under Owens Corning shingles will still be a warranted roof at the basic level, but it will not qualify for the top tier coverage that makes the Platinum program worth having. We explain how those tiers work in our guide to what Owens Corning Platinum Preferred means.
How much more does synthetic underlayment cost?
On a typical 2,000 square foot roof in the greater Pittsburgh area, synthetic underlayment adds about $300 to $700 over 15 pound felt. Ice and water shield runs about $50 to $90 per square installed and is required at the eaves and valleys either way, so it is not really a choice between products. It is a choice about how much of the roof gets it. Full ice and water coverage on a low pitch roof can add $1,000 to $2,000.
What moves the price: roof size, the number of valleys and penetrations, the pitch, the overhang depth, and whether the deck needs repair once the old roof comes off. In the context of a full replacement that runs $12,000 to $25,000 for most homes, the underlayment upgrade is a small line. You can see the full breakdown in our roof replacement cost guide.
Does better underlayment make a roof last longer?
Indirectly, yes. Underlayment does not extend the life of the shingles, but it prevents the leaks and deck rot that end a roof early. A roof with felt that failed in year 18 needs a full tear off and possibly new decking. A roof with synthetic and proper ice barrier can often reach the full life of the shingles, which for Owens Corning architectural shingles in our climate is 25 to 30 years. Our article on how long a roof lasts goes deeper on what shortens roof life in Western PA.
Frequently Asked Questions
Is synthetic underlayment better than felt?
Yes, for nearly every roof. It is stronger, does not absorb water or wrinkle, holds up in wind during installation, is safer to walk on, and lasts as long as the shingles. Felt is cheaper and still meets code, but on a Pittsburgh roof the $300 to $700 difference is money well spent.
Do I need ice and water shield in Pennsylvania?
Yes. Pennsylvania building code requires an ice barrier at the eaves in areas with a history of ice damming, which includes all of Western Pennsylvania. It has to extend at least 24 inches inside the exterior wall line. Most good roofers, including us, also run it in valleys and around chimneys and skylights.
Can I use felt under Owens Corning shingles?
You can, and the basic shingle warranty still applies. But the extended and platinum level warranties that cover workmanship and full system replacement require Owens Corning underlayment and components installed by a certified contractor. If you want the best warranty, felt is not an option.
How much does synthetic underlayment cost compared to felt?
Synthetic runs about $12 to $25 per 100 square feet against $5 to $10 for 15 pound felt. On a typical 2,000 square foot roof, that is roughly $300 to $700 more. Ice and water shield is separate and costs about $50 to $90 per square installed where it is used.
If you are planning a roof this fall, ask any contractor you talk to exactly what goes under the shingles and how far the ice and water shield runs. We will show you on your own roof during a free inspection. Fall roofing schedules across Westmoreland and Allegheny County fill by October, so call Bella Construction at (724) 515 5163 or visit our roofing services page to get on the calendar before the first hard freeze. Financing is available.