Roofing Felt & Underlayment

Underlayment is the layer that goes down on the roof deck before shingles or metal panels. Most folks think the shingles are the whole roof. They’re not. Underlayment is a water-resistant or waterproof barrier material installed directly onto the roof deck before any roofing material goes on top, serving as the last line of defense between a building’s interior and the elements — protecting the structure from wind-driven rain, ice dams, and moisture infiltration that can penetrate past shingles, tiles, or metal panels.

Here’s the part that matters for buyers: there are three types — felt, synthetic, and peel-and-stick — and matching the right one to your roof slope and Montana’s freeze-thaw climate is where people get it right or get it wrong. Choosing the wrong underlayment for the application is one of the most common and costly mistakes in roofing.

This guide covers what underlayment does, the three types, how to pick by slope, and where the code demands waterproof membrane in ice-dam country.

Why Every Roof Needs Underlayment

This isn’t an upsell. For more than 25 years organizations such as the National Roofing Contractors Association have included roofing felt in their roofing and waterproofing recommendations, and it is also a requirement under many building codes. Every building code in North America requires roof underlayment for good reason. Even the best roofing materials can fail under extreme weather, and fastener penetrations create potential leak points across every square foot of roof surface. Underlayment catches what the primary roofing material misses, prevents deck rot, and buys critical time if outer materials are damaged during a storm.

It does more than catch water. A layer of felt applied to a wood deck provides a vapor retarder between the wood deck and the shingles. Without the felt layer, the roof’s decking — dried by the heated temperatures found in most attics — will draw moisture from the shingles. The result will be shingles that are prematurely aged, becoming cracked and brittle as well as stuck to the wood surface when you attempt to remove them.

It also blocks staining. When you use certain types of wood to construct your deck, there is sometimes a risk the wood will leach small amounts of resin over time as it ages. Having a layer of roof felt will block the resin, preventing it from staining or compromising your roof shingles.

And it factors into fire ratings — which matters across the WUI zones that cover much of western Montana. When shingles are evaluated for their fire ratings, it is done on a small test deck with the roofing felt underlayment in place — without this underlayment, the shingles alone may not meet the Class A fire requirements.

The Three Types: Felt, Synthetic, and Peel-and-Stick

There are three primary roofing underlayment types used today: asphalt-saturated felt, synthetic underlayment, and self-adhering (peel and stick) membrane. Each serves a distinct purpose, and most roofing projects use at least one — sometimes a combination of two.

Felt is the old standby. Asphalt-saturated felt is the traditional roof underlayment that has been used for over a century. Made from a base mat of natural fibers saturated with asphalt, it provides a basic water-resistant barrier at a low cost. It comes in two weights. Felt underlayment is water-resistant, but not waterproof. It’s available in two thicknesses: 15-pound and 30-pound. Fifteen-pound felt has a perm rating of about 5, although this number can rise in high-humidity conditions. Thirty-pound felt is more resistant to damage during installation and will protect the roof longer if it should somehow become exposed to weather. Thirty-pound felt is much thicker and stiffer. Western Building Center carries both 15# felt in a 4-square roll and 30# felt in a 2-square roll.

Felt’s weakness shows in wind and cold. Saturated felts are susceptible to wind damage, often ripping off sections of the felt and leaving the roof deck unprotected. Felts also do not offer much resistance to UV rays, so they need to be covered as soon as possible.

Synthetic is what most pros reach for now. Synthetic roofing underlayment has become the industry standard for professional contractors over the past two decades. Made primarily from polypropylene or polyethylene polymers, it delivers superior performance across nearly every measurable category. The numbers back it up — superior tear strength up to 5-10x stronger than 30-lb felt, lightweight rolls that cover more area, extended UV exposure ratings, moisture resistance that won’t wrinkle or rot, and a non-slip surface that improves crew safety.

For Montana, the cold-weather behavior is the real argument. While asphalt felt wrinkles when wet and cracks and splits in cold weather, synthetic underlayment stays pliable and resilient and can be exposed to cold weather for generally 4-12 months without issue. One thing to know — most synthetics are vapor barriers. Most synthetic underlayments have permeance ratings under 1 perm, making them effective vapor barriers. Because these underlayments don’t allow roof sheathing to dry upward, manufacturers recommend they be used only over ventilated spaces — vented cathedral ceilings or vented attics — that allow downward drying. WBC’s synthetic options include Bigfoot 30 synthetic underlayment in a 10-square roll and a UDL30 synthetic in a 4-foot by 250-foot, 10-square roll.

Peel-and-stick is the heavy hitter for the spots that leak. Self-adhering underlayment is a premium waterproof membrane made from rubberized asphalt bonded to a polyethylene or polyester backing. Unlike felt and synthetic underlayment, which are water-resistant, peel and stick creates a fully waterproof, self-sealing barrier that bonds directly to the roof deck. The rubberized asphalt adhesive layer seals around nail and fastener penetrations, creating a watertight bond at every attachment point. WBC stocks several ice-and-water options including Malarkey ice/water in a 2-square roll, Shark HT ice & water (48” x 125’, 5-square), and Titan PSU-30 HT ice/water in a 2-square roll.

You don’t sheet the whole roof in it. Most contractors use peel and stick in combination with synthetic underlayment across the rest of the field — peel and stick in the critical areas, synthetic everywhere else. This approach balances waterproof protection where needed with cost efficiency across the full roof area.

Spec	Felt (#15/#30)	Synthetic	Peel-and-Stick
Weight per square	7–16 lbs	3–6 lbs	30–65 lbs
Tear strength (MD)	5–15 lbs	25–80 lbs	15–30 lbs
Permeability	5–60 perms	1–16 perms	0.05–0.5 perms
Lifespan	15–20 yrs	25–50 yrs	25–40 yrs
Waterproof?	Water-resistant	Water-resistant	Fully waterproof

Choosing by Slope — and Where Ice-and-Water Is Mandatory

Slope decides how much you overlap and whether you run a double layer. In low-slope roofs, which include 2:12 up to 4:12, felt courses should overlap a minimum of 19 inches. This provides a double layer of underlayment across the entire roof. In steep-slope roofs (4:12 and steeper), the upper courses of felt underlayment should overlap lower courses by at least 2 inches.

There’s a hard floor on pitch for any nailed product. All nailed-in-place roof underlayments should be installed at 2:12 or greater pitch per building codes.

Now the part that matters most in Montana cold country. The IRC Section R905 requires self-adhering ice and water shield membrane in specific locations. In areas where the average January temperature is 25 degrees F or less, an ice barrier must extend from the eave edge to at least 24 inches past the interior wall line; valleys, which concentrate water flow and are high-risk leak areas; around penetrations like plumbing vents, chimneys, and skylights; and at low-slope transitions where steep-slope roofing meets lower pitches.

That January-temperature trigger puts much of the state squarely in ice-dam territory. With the freeze-thaw cycling and snowmelt this climate dishes out, peel-and-stick at the eaves, valleys, and penetrations isn’t optional — it’s the first line of defense.

Installing Felt the Right Way

If you’re running felt, the fundamentals are simple but unforgiving. Beginning at a bottom corner of the roof, install felt courses horizontally. From behind the roll, nail down the top of the strip with roofing nails spaced 6 to 8 inches apart. Nail the middle and bottom of the felt similarly.

Mind the edge metal. Felt underlayment should overlap the edge metal at the eaves and be overlapped by edge metal on the rakes. At the top, when you reach the top course, lap about 6 inches of felt over the ridge top for temporary waterproofing.

One rule that saves backs and faces: never walk on felt that has not been nailed down.

For wind exposure — and the Rocky Mountain Front sees plenty — change up the fasteners. Felt is usually fastened with staples, but in high-wind areas, plastic windstrips may be used along the edges to prevent tearing. Felt may also be attached in high-wind areas using plastic caps. Plastic caps offer better wind resistance than staples and help prevent leakage through the holes made by the fasteners.

Don’t leave felt out in the weather. It has limited UV exposure tolerance, typically 30-60 days maximum, so get the finished roofing on promptly. And before a cold-weather job, treat the rolls right: keep rolls dry and completely protected from the weather. If work is to be done in cold weather, rolls of roofing should be placed in a warm place for 24 hours prior to application.

Frequently Asked Questions

Is underlayment really required, or can I skip it? It’s required. All roofs have underlayment. At a minimum, a roof with asphalt shingles should be completely covered with one layer of underlayment. Beyond code, it’s also what lets shingles meet their tested fire rating and what protects your deck during the build.

What’s the actual difference between #15 and #30 felt? Thickness and toughness. 30-lb felt is roughly twice as thick and significantly more durable, offers better protection against foot traffic during installation and improved water resistance, and is required by many shingle manufacturers for warranty compliance on lower-slope applications. The #15 is thinner and lighter, but it tears more readily during installation.

Why pick synthetic over felt in Montana? Cold and wind. Compared to asphalt felt, synthetic roofing underlayment installs quicker and is far more durable in high winds or when left exposed for long time periods. Felt cracks and splits when it’s cold; synthetic stays pliable. For a climate that swings hard between freeze and thaw, that’s a real durability edge.

Where exactly should I run peel-and-stick? The high-risk zones. Peel-and-stick provides superior waterproofing and reduces the risk of leaks in vulnerable areas like valleys, eaves, and around roof penetrations. In ice-dam country, code also requires it at eaves extending 24 inches past the interior wall line.

Is felt waterproof? No. Roofing felt can shed water, which helps it provide additional water protection. However, roofing felt does not offer the lowered moisture permeability characteristics of synthetic underlayment. Only peel-and-stick is truly waterproof and self-sealing around fasteners.

Can I use synthetic under a metal roof? It works for metal, but watch the venting. Because asphalt shingles and standing seam metal roofs are considered vapor barriers, creating an air gap above breathable roof underlayments should be considered. If you want a vapor-permeable layer that lets the deck dry upward, stay with asphalt felt — it’s the original ‘smart’ vapor retarder, having a perm rating of about 5 perms when dry but a much higher rating of 60 perms when wet.