Ever wonder why some roofing jobs sail through inspections while others get red-tagged faster than you can say code violation? Or why can certain contractors bid 15% higher and still win projects while others scrape by on razor-thin margins?
It all comes down to one critical decision that most people completely overlook. The membrane barrier you install beneath the asphalt shingles or tiles can mean the difference between a bulletproof installation and a callback nightmare that might haunt you for years.
With local codes and specs getting stricter every year, you can no longer default to whatever you used last time. Is it Class A fire-rated? Does it meet ASTM D1970 for ice barrier? Can it handle a month of UV before you dry-in the rest? To make sure you meet all the performance standards while remaining efficient, you need the right protective base.
Self-Adhered vs. Synthetic vs. Nailable Felt Roof Underlayment Types
Self-Adhered Membranes (Polystick® Series)
If your project calls for maximum moisture protection, fire resistance, or you are working in areas with ice dam risk or low-slope sections (below 4/12), self-sealing underlayments are a high-performance go-to. The rolls bond directly to the roof deck using the proprietary dual-compound ADESO® technology.
Unlike traditional options, a self-adhesive rubberized asphalt underlayment fully seals to the substrate, so even the areas around fasteners are watertight. With Polystick® MX, Polystick® P, Polystick® TU P, Polystick® TU PLUS, and other products in this range, you get complete protection against wind-driven rain, snow melt, and freeze/thaw cycles. You will also appreciate how quick and clean the installation process is. Thanks to the SEALLap® and FASTLap® technology for overlap zones, seams are easier to align, seal, and finish without relying on numerous labor hours.
Features:
- Fully bonded to the deck
- No hot torch, nails, or adhesives required
- Built-in overlap seal zones for faster dry-in
- High durability; can stay exposed for 90 to 180 days
- Can safely handle up to 250°F and 265°F temperature
Mechanically Attached Synthetic Underlayment Sheets (Polyanchor® SU)
If you are looking for a tough, walkable, and lightweight base layer that installs fast and handles the foot traffic of a busy jobsite, Polyglass’s Polyanchor SU is what you want. It’s a lightweight, nail-down synthetic roof underlayment sheet — not a felt — perfect for when you are working on asphalt or synthetic shingle roofs, or natural cedar shakes. It’s also ideal for reroofs in suburban subdivisions, small commercial builds, or budget-conscious new construction.
Polyanchor SU is built with a non-woven fiber top surface and a polymeric-coated underside to grip the roof deck better and make it safer for crews to walk on angled roofs or in wet conditions. And because it comes in large 10-square rolls, you can dry-in quickly without constantly stopping to overlap more sheets. Although it’s not self-sealing around nails like the previous option, it can be paired with Polystick® IR-Xe at the eaves, valleys, and other exposed areas for full ice-and-water protection.
Features:
- Higher tear strength and wind resistance than felt
- Ideal for open exposures or high-elevation projects
- Quick coverage with 10-square lightweight rolls
- 90-day UV exposure rating
- More stable footing during installation
Traditional Felt Rolls (Nailable Asphalt-Saturated Sheets)
Many roofing contractors still use old-school felt (tar paper) mainly because it’s the cheapest material (at least upfront). But price alone should never be a factor. As easy as felt rolls are to install and meet the minimum code requirements, they come with quite a few trade-offs:
- Felt tears easily in windy conditions or when stretched over uneven surfaces.
- It offers minimal resistance to moisture over time, making it vulnerable to leaks if the final roofing layer is not installed quickly.
- It must be fastened manually using roofing nails or cap nails (sometimes staples).
- It gets slippery and unstable fast on steep pitches and wet days.
- It starts to break down within 12-20 years.
- It offers no sealing around fasteners.
Felt underlayments still have a place on low-budget residential projects, temporary builds, or when used beneath secondary layers in systems with multiple moisture barriers. But for long-term value and protection, it is a bare-minimum option; not something you’d choose for demanding environments or high-spec jobs.
8 Questions to Ask When Choosing the Right Roofing Base Sheet for Steep-Slope Projects
How critical is water intrusion protection?
If the area sees a lot of snow, ice dams, or torrential downpours, a fully bonded self-adhered sheet like Polystick® is your best bet. It will create a watertight layer around nails and fasteners, so there is less of a chance of leaks in the valleys, low slopes, and roof-to-wall intersections.
If you don’t need this type of coverage but still need targeted waterproofing, consider going hybrid: use Polystick IR-Xe in critical zones like eaves and transitions, and then cover the field with Polyanchor® SU to speed up dry-in while controlling material costs.
How long does the deck need to be exposed before the final roof system goes on?
Fast-paced commercial sites may get covered quickly, but in many projects, there are delays. In those cases, your base layer becomes your first line of defense. Self-adhered membranes from the Polystick line come with 90–180-day UV exposure ratings. This is stellar if your crews need a wider install window without redoing work. Polyanchor SU also gives you up to 90 days of UV protection. You want long-exposure rated materials if the final roof install might be delayed or if projects pause between phases.
What kind of foot traffic will the roof see during installation?
Is this a quiet reroofing on a house, or are you expecting heavy footfall, multiple trades on the roof, and material staging areas? Synthetic wraps like Polyanchor SU are reinforced to withstand more abuse. The non-woven surface also improves grip. By contrast, standard felt can become slick, soft, or rip under pressure.
What kind of slope are you working with?
Most steep-slope systems are above 4/12, but not all are created equal. On pitches lower than 4/12 (about 2/12 to 3/12), the risk of water pooling or flowing under laps is much higher. For those zones, a fully adhered sheet that self-seals and doesn’t rely on gravity to shed water (like Polystick) is essential. You can go with Polyanchor SU or even felt (if budget is the main concern) for sharper slopes above 4/12, provided you are not in a heavy-weather zone.
How fast do you want the installation to be?
Self-adhered sheets generally install faster than you’d expect; Polystick products in particular come with SEALLap® and FASTLap® to provide quick overlaps without needing torches or adhesives. And because they bond directly, there is less risk of loose edges or wind uplift during staging.
That said, Polyanchor SU is also known for fast, consistent installation across large square footage. You can nail down wide runs quickly if the weather is holding. Felt, while cheap, requires more care with overlaps and is more prone to wrinkles, increasing the risk of do-overs.
What kind of local building codes do you have to adhere to?
Before you even compare price tags or product specs, you need to know what your jurisdiction allows or mandates. In wildfire-prone regions like California or the Southwest, you will need a Class A fire-rated assembly to pass inspection. That not only means the top layer has to resist flames; the entire system, from deck to finish, needs to meet code.
Most Polystick membranes meet ASTM E108 or carry UL Class A ratings when used in approved assemblies. We recommend checking your local AHJ (Authority Having Jurisdiction) before finalizing specs. Look for systems rated for both wind uplift and fire resistance, not just standalone membrane data.
What roofing materials are you using on top?
Not all top layers play nice with all base sheets. Clay tiles, concrete, metal panels, wood shakes, and composite or asphalt shingle roofing: each creates different types of pressure, movement, and moisture challenges. Your base layer has to be chemically and thermally compatible with what’s going over it.
Go with a high-temp rated membrane that won’t break down under prolonged heat exposure for metal roofing; Polystick products are good for these projects. For concrete or clay tiles, you want a fully bonded surface (like Polystick TU Plus) that keeps moisture out in low-slope tile sections. Polyanchor SU is a great match for synthetic shingles that are lightweight, tough, and easy to install under manufactured systems.
Do you have mixed slopes or slope breaks?
Some buildings combine 3/12 or 4/12 slopes with steeper angles or tie into flat sections altogether, so not all steep-sloped roofs are steep from end to end. Lower-pitch sections (under 4/12) are more prone to ice damming and wind-driven rain, which makes a watertight, fully adhered membrane a must. For higher slopes over open spans, a synthetic wrap like Polyanchor SU will save time and weight.
You are almost always better off with a dual-system setup for mixed-pitch projects: self-adhered underlayment for low-pitch eaves, then nail-down synthetic up the rest of the slope.
Explore the Complete Range of Polyglass Products
If you have ever had to tear off wrinkled felt after an unexpected storm or replace blown-off synthetic under a partially finished tile underlayment job, you already know the importance of performance over lab specs. What you install beneath the finished roof becomes your first defense against moisture intrusion, weather delays, callbacks, and in some cases, insurance claims.
Polyglass offers a wide range of the best roof underlayment solutions engineered to meet all of your on-the-ground needs. If you have any questions or need help zeroing in on the best type of underlayment for steep or low-slope roofs, get in touch with us.