Homeowner Guides

Synthetic vs. Felt Underlayment: What Goes Under Maryland Shingles

Under every shingle roof there is a layer most homeowners never think about until it appears as a line item on an estimate: the underlayment. It is the sheet material rolled across the bare decking before the shingles go on, and for most of modern roofing history it was asphalt-saturated felt — the heavy black paper your parents' roof probably had. Today, most estimates offer synthetic underlayment instead: woven or spun plastic sheeting engineered for the job. Is synthetic actually better, or is it an upsell? Is felt a false economy, or a proven material being retired for no reason? The honest answer is that both can work, they fail in different ways, and for most Maryland homes synthetic has real advantages that have nothing to do with marketing. Our glossary entry on underlayment has the short version.

Getting estimates for a roof replacement? This is one of the line items worth understanding before you compare prices. If you want a walk- through on your own roof, call (301) 458-1003 for a free roof inspection.

What Underlayment Actually Does (and Does Not Do)

First, a myth to retire: underlayment is not your roof's waterproofing. Shingles are the primary water-shedding surface, and the waterproof membrane at the vulnerable points is ice and water shield — covered in our ice and water shield guide. Underlayment's jobs are the supporting ones, and they matter:

• A secondary barrier. If wind drives rain under a shingle edge, or a shingle is lost in a storm, the underlayment is the layer that keeps that water off the bare deck until repairs are made.
• Deck protection during installation. Between tear-off and the last shingle, your deck is covered only by underlayment. In Maryland, where a pop-up afternoon thunderstorm can arrive mid-install, that temporary roof has to actually work.
• A clean, even surface for shingles to lie flat on, so deck seams and imperfections do not telegraph through the finished roof.
• Separation between the wood deck and the asphalt shingles, so resins and moisture do not interact directly over the roof's life.

Felt Underlayment: The Traditional Option

Felt is a mat of organic or fiberglass material saturated with asphalt. It comes in weight grades — commonly referred to as #15 and #30, with #30 the heavier, more durable option — and it has protected roofs for generations. Its honest strengths: it is inexpensive, widely available, and it has a long track record under shingles that stayed put. Its weaknesses show up in modern Maryland conditions. Felt absorbs water; if it gets wet during installation it can wrinkle and ripple, and those wrinkles can show through the shingles and hold them off the deck. It tears comparatively easily under foot traffic and wind once rolled out, so a stripped roof covered only in felt is vulnerable if weather turns before the shingles are on. And it degrades with exposure — felt left uncovered in sun and humidity breaks down far faster than its synthetic counterpart, which matters on any job that runs more than a day.

Synthetic Underlayment: The Modern Default

Synthetic underlayments are engineered polymer sheets — typically woven or non-woven polypropylene or polyethylene — made specifically to replace felt. Their practical advantages map closely onto felt's weaknesses. They are far more tear-resistant, so they survive installation foot traffic and stay put in wind. They do not absorb water, so they lie flat when they go down and stay flat. They are lighter per square of coverage, which speeds installation, and they tolerate exposure for much longer than felt if weather or scheduling delays the shingle work — a real consideration in Maryland summers, when humidity and afternoon storms interrupt jobs. Most synthetics also carry a textured, walkable surface that is safer for crews on steep pitches than slick, damp felt. The trade-offs are straightforward: synthetic costs more per roll than felt, and quality varies by product — a bargain synthetic is not automatically better than heavy #30 felt, which is why the specific product matters, not just the word "synthetic" on an estimate.

Head to Head, for a Maryland Roof

• Rain during installation: Synthetic sheds it and stays intact; felt wets, wrinkles, and can tear. Edge: synthetic — and Maryland's sudden summer storms make this a common event, not an edge case.
• Humidity and exposure time: Synthetic tolerates days of exposure far better. Edge: synthetic, especially on larger homes in Columbia or Silver Spring where a replacement may span more than one day.
• As a backup barrier under lost shingles: Both work when intact; synthetic's tear resistance means it is more likely to still be intact after the storm that took the shingle. Edge: synthetic.
• Cost: Felt is cheaper as a material. But underlayment is a small share of a full replacement, and the difference rarely changes which estimate you should choose — other line items matter far more. Our roof replacement cost guide shows where the real cost drivers are.
• Track record: Felt has decades more history. Synthetics have been in wide use long enough now that their performance is established rather than experimental. Edge: even, for quality products installed correctly.

What Neither Product Replaces

Be clear-eyed about one sales pattern: underlayment — of either kind — is sometimes pitched as if upgrading it upgrades the whole roof's waterproofing. It does not. The places water actually backs up on a Maryland roof are the eaves, valleys, chimneys, skylights, and low-slope transitions, and those are protected by self-sealing ice and water shield and proper flashing, not by the field underlayment. A roof with premium synthetic underlayment but no membrane in the valleys is worse protected than a roof with basic felt and membrane in all the right places. Spend attention first on membrane placement and flashing scope; then take the better underlayment. Our roof valleys guide and chimney flashing guide cover those higher-stakes layers.

What to Ask in an Estimate

• Synthetic or felt — and if synthetic, which specific product? ("Synthetic" alone is a category, not a specification.)
• If felt: #15 or #30 weight? The heavier grade is the only felt comparison worth having on a modern architectural shingle roof.
• Does the shingle manufacturer's installation specification call for a particular underlayment type or weight on this pitch? Manufacturer specs govern, and your roofer should follow them.
• Where does ice and water shield go, separately from the field underlayment? (If the answer blurs the two together, ask again.)

Our guide to reading a roofing estimate puts these questions in the context of the full document.

The Bottom Line

For most Maryland homes today, a quality synthetic underlayment is the better default: it survives installation weather, lies flat, resists tearing, and serves as a more reliable backup barrier — for a modest difference in the total cost of a roof. Heavy felt remains a workable, proven option when it is installed dry, covered promptly, and paired with membrane in the right places. What matters most is not the label but the system: deck, membrane at the vulnerable points, underlayment, flashing, and shingles working together. Nucci Brothers Roofing has installed that system on Maryland homes since 1986, MHIC #24398. To talk through the layers of your next roof, request a free inspection or call (301) 458-1003.

Proudly Serving These Maryland Communities

Nucci Brothers Roofing provides roof inspections, repairs, and replacement for homeowners in Annapolis, Baltimore, Columbia, Glen Burnie, Silver Spring, and Towson — plus free roof inspections across Maryland. Request a free inspection today.