The Types of Roofing Underlayment — and How Well They Perform

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Like choosing the right clothing for a hike based on the weather and trail conditions, it's just as important to consider several roofing underlayment options for your new home construction or a roof replacement project. Factors such as the climate, the roof slope (or pitch), and even how long it might be until the shingles go up are all reasons that one type of underlayment may perform better than another given the budget and the project conditions. It's the underlayment's job to help prevent water from seeping in should the roofing material fail. While classic felt underlayment works well in many conditions, it may not always be the best given the advancements in underlayment technology in the last couple decades.

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Felt Underlayment Basics

Felt underlayment, also called felt paper, tar paper, or asphalt-saturated felt, is the old-school standard when it comes to roofing underlayment. It also used to be the most common underlayment until synthetic options came along. While felt underlayment resists water, it isn't waterproof. The base of the felt paper is saturated in asphalt, bitumen, and often polyester; together, these create the material's water resistance. These additives also help the material resist hail and debris should any of the shingles come off in harsh weather, but ultimately, the roofing material still needs to be replaced, or the underlayment will fail too.

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Felt underlayment has either a cellulose or a fiberglass substrate, with the cellulose version sometimes called organic. In this case, "organic" means plant-based; it has nothing to do with the organic labeling used for some food products. The organic version is the more common of the two.

One of the biggest drawbacks of any felt underlayment is the limited amount of time it can sit uncovered and exposed to the elements. It's designed to go up the same day the shingles or other roofing material will be installed. Sitting out any longer than that, it breaks down and could fail prematurely. Moisture makes it ripple and wrinkle, cold weather causes cracks, and ultraviolet rays from the sun degrade it quickly, especially during hot weather. For any roofing job that can't be completed in one day, synthetic underlayment is a better option.

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15-Pound vs. 30-Pound Felt

Felt roofing underlayment comes in 15-pound and 30-pound thicknesses, with the 30-pound version being both thicker and heavier. The added thickness of the 30-pound underlayment means it holds up better to damage over time as well as to potential damage during the installation process. Since it's thicker, it costs a bit more than the 15-pound felt.

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For a roof that has a slope that isn't steep, 15-pound felt is often sufficient and is definitely the more economical choice. For a roof with a steep slope, 30-pound felt underlayment is the better option since it's less likely to become damaged during the roofing installation process. Walking upon the thinner felt could cause it to tear at such an angle, as it's harder to get good footing on a steep roof. Keep in mind that the roofers need to walk on the underlayment as they're installing the roofing, so the underlayment used on the project must make it through unscathed in order to properly do its job.

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Rubberized Asphalt Roofing Underlayment

Rubberized asphalt roofing underlayment has rubber polymers and asphalt mixed in, making the material waterproof. This also makes it a great choice for areas that receive heavy snowfall, as snow or ice that gets under the shingles and melts will not penetrate the underlayment. This type of underlayment looks and feels a bit rubbery compared to other underlayments. Peel-and-stick versions seal themselves around nails and staples used for the roofing material. Since this underlayment sticks to the roof deck without using any fasteners, a clean roof deck is critical, as any debris from old shingles could cause it to not adhere properly.

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This type of underlayment is completely waterproof and doesn't degrade easily from UV rays or the weather. It's a good option for a roof that's nearly flat, as it can stop water that puddles on the roof and gets through the roofing to the underlayment. In typical roofing scenarios, rubberized underlayment is not usually used for an entire roof. It's considerably thicker than other underlayments, which means less coverage per roll and therefore more rolls that need to be hauled up to the roof. If used under metal roofing, it's important to choose a rubberized asphalt underlayment with an adhesive rated for high heat since a lot of heat develops under the metal panels.

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Synthetic Roofing Underlayment

Synthetic underlayment, sometimes called nonbitumen synthetic underlayment, has many advantages over any felt underlayment, so much so that it has surpassed felt underlayment to become the most common type used on residential roofing projects. Synthetic underlayment is lighter and thinner, which means potentially more material per roll compared to any felt underlayment. It resists tears and damage better than felt, so it's less likely to be damaged during installation. Many types also have a nonslip coating, which makes it much easier to walk on without damage during the roofing installation.

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Synthetic underlayment is usually made of a plastic, such as polyethylene or polypropylene, with polypropylene being slightly more brittle until it softens in warm temperatures. Either plastic material is both strong and flexible as well as waterproof and resistant to fungal growth since water can't saturate the plastic. Synthetic is also the better choice for any job taking longer than one day, as it won't degrade in the elements. The average synthetic underlayment withstands upward of six months of exposure before it starts to break down, which is significantly better than any felt-based underlayment. This is a valuable trait even well after roofing installation, as any storm damage may leave the underlayment exposed for some time before roof repairs take place.

Attic ventilation is particularly important when using synthetic underlayment, as the roof needs to be able to "breathe" to allow vapor or moisture trapped in the home to get out. It's harder for vapor to pass through synthetic underlayment because it's less permeable than felt; in fact, some synthetics are considered vapor-impermeable, meaning they don't "breathe." A vapor-permeable option is available, although it costs more than the impermeable variety or up to 10 times as much as inexpensive felt paper underlayment.

Ice and Water Shield

Ice and water shield, also known as self-adhering underlayment or ice and water protector, is most useful in areas that receive heavy snowfall and on areas with somewhat low slopes where ice or snow might otherwise build up. The peel-and-stick, sticky-backed underlayment helps create a waterproof barrier between the underlayment and the roof deck beneath it. Installing it requires temperatures above 40 degrees Fahrenheit because in weather colder than that, it becomes stiff and not quite as sticky. Likewise, a sweltering summer day isn't ideal for self-adhering underlayment installation because it's so pliable that it's hard to set in place without creases.

Though not always used over an entire roof deck, ice and water shield is often installed around skylights, chimneys, vents, and any areas where snow, water, or ice may accumulate. In some snowy regions, installation of ice and water shield along roof eaves is required by building code. Rubberized asphalt underlayment is also sometimes used for the same purpose, as it's sold in narrow, peel-and-stick rolls used for areas of the roof where ice may build up.

Roofing Underlayment Costs

The price of roofing underlayment varies based on the type, brand, quality, and market availability. Generally speaking, 15-pound felt underlayment is the least expensive, costing about 5 cents per square foot, while a comparable 30-pound felt underlayment from the same product line costs double that. Rubberized asphalt roofing underlayment, such as CertainTeed's WinterGuard sand underlayment, runs around 70 cents per square foot.

The most variation in price comes within the vast array of synthetic underlayments, with vapor-impermeable underlayments running 11 to 15 cents per square foot and vapor-permeable synthetics running from 20 cents to a whopping 90 cents per square foot. For this reason, and if the conditions allow, many choose a felt-based underlayment if vapor permeability is important for the location since felt is inherently permeable.

Underlayment Installation Considerations

Since one underlayment type is completely different from the next in thickness and other physical characteristics, so are the methods used to install them to a roof deck. Most synthetic underlayment requires using cap nails instead of staples to secure the material. Cap nails have a plastic, washerlike cap just beneath the head. This cap helps minimize tearing of the underlayment, and it helps to prevent leaks by keeping moisture from seeping around the nail head. Staples would not work well on some synthetic underlayments since they're thin and could tear out around the staple — for instance, if a gust of wind got beneath the underlayment before the roofing installation was complete.

Any stick-on underlayment or ice and snow barrier requires an absolutely clean surface and often very specific weather conditions on installation day. Be sure to read the specifications on the actual product, as requirements vary based on the material and the product line. Although many are textured on top, some synthetic products and peel-and-stick underlayments are smooth and may be slippery to walk on, especially on a damp day. Also make sure the synthetic underlayment is compatible with the roofing material if it's not asphalt shingles, as some synthetics may not be meant for use with certain roofing materials, such as tile or wood. If you are using synthetic underlayment, there's still a good chance local code requires ice and water shield on specific areas if you live in a climate with cold winters.

When installing felt paper, it's important not to walk over the felt until it's been nailed into place; otherwise, it's likely to tear. Make sure one row overlaps the preceding row by about 2 inches; otherwise, it's easy for water to seep through. Though staples can be used to hold down the felt underlayment, nails are better in some cases. Cap nails are the best option if it is being installed on a windy day.

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