When most people think of a roof’s ability to prevent leaks, they usually think of the shingles. But, the truth is that shingles only serve as the first line of defense against moisture. Wind can blow water under shingles, and shingles can eventually dry out, allowing water to get past them. The unsung hero keeping your roof dry is roof underlayment.
True to its name, roof underlayment is installed under shingles and prevents any water or moisture that manages to get past them from reaching the roof sheathing and eventually the interior of the home.
Roof sheathing comes in various materials ranging from tried and true asphalt felt paper underlayment to high-performing synthetic to water-proof rubber. In this article, we’ll examine the roof underlayment types and options for your house, shed, or garage, so you can make the right choice.
- What Is Roof Underlayment?
- Is Roof Underlayment Necessary?
- Roof Underlayment Types and Options
- Roofing Underlayment Comparison Chart
- What Is the Best Type of Roof Underlayment?
- Popular Underlayment Products
- Best Roofing Underlayment for Different Types of Roofs
- Synthetic Roof Underlayment vs Felt
- Problems With Synthetic Roof Underlayment
What Is Roof Underlayment?
Roof underlayment is the material that goes between the shingles and the roof sheathing. Underlayment is responsible for providing an additional barrier against rain, wind, and snow. There are three types of roof underlayment: felt paper, rubberized asphalt, synthetic felt paper, and peel and stick.
Is Roof Underlayment Necessary?
While roofing underlayment isn’t always required by code in some areas, it’s almost always a good idea to use it. Roofing underlayment acts as an extra moisture barrier that protects the roof sheathing and framing and everything under the roof from water damage.
Given the catastrophic damage that water can cause to a home, it only makes sense to use this extra layer of defense. If shingles are the first line of defense, protecting against the sun’s harmful UV rays and blocking precipitation and wind, then roof underlayment is the second line of defense.
Shingles, on their own, typically aren’t enough. This is because shingles aren’t sealed in all places and can be lifted by strong winds, allowing rain to find its way underneath them to your roof’s vulnerable sheathing. Shingles also eventually age, become brittle and lose their effectiveness.
Roof Underlayment Types and Options
Roofing underlayment comes in different types. Selecting the right one is key to protecting your roof from the elements. In this section, we’ll discuss each of the four types of roof underlayment and how they compare.
1. Felt Paper (Asphalt-Saturated Felt)Asphalt saturated felt, more commonly referred to as felt paper, was the underlayment of choice up until about 15 years ago, when synthetic options became more popular and more affordable. It’s still commonly used, however.
This type of underlayment consists of a mix of plant fibers, polyester, bitumen, and asphalt. These ingredients make it waterproof while still remaining flexible enough to use.
Felt paper comes in two different thicknesses, 15 weight and 30 weight. Heavier 30-weight felt paper lasts longer and is better able to hold up against heavy snowfall, rain, or hail. It’s easy to install using tools most homeowners have in the workshop and is cheaper than synthetic options. That said, it isn’t as durable. It can crack in extreme cold, is heavier, making it hard to install, and will eventually succumb to intense heat.
- Easy for a DIYer to install
- A tried and true product
- Relatively short lifespan
- Can crack and peel easily
- Tears easily
2. Rubberized AsphaltRubberized asphalt is one of the most expensive types of roofing underlayment because it contains a high percentage of asphalt and rubber that makes it completely waterproof. This type of underlayment typically has a peel and stick backing that makes it one of the easier types of underlayment to install.
It’s especially useful when installed in valleys and around vents and other protrusions, which are often the first parts of a roof to fail.
Rubberized asphalt is also much more heat- and cold-resistant than felt underlayment. It’s easier to install, as the underlayment will hold up to direct exposure to weather for up to 6 months without taking damage. This makes it a popular choice for large commercial buildings.
Rubberized asphalt is also heavy and difficult to use. That, coupled with its high cost, means it is rarely employed in whole roof applications and is mainly used for vulnerable parts of the roof.
- 100 percent waterproof
- Very durable
- Can hold up to direct exposure to weather for months
- Very expensive
- Heavy and difficult to install
3. Synthetic Felt Paper (Non-Bitumen Synthetics)Synthetic underlayment is the most commonly used roofing underlayment today. It consists of bitumen saturated in asphalt with fiberglass mixed in. That fiberglass additive gives this roof underlayment better tear resistance than felt paper. It’s also lighter than felt paper, putting less strain on the roof. It won’t absorb moisture and resists mold growth.
Some synthetic underlayments even have scrim reinforcement that gives them better slip resistance qualities.
The downside of synthetic felt paper is that it’s more expensive and harder to install than asphalt-saturated felt. Most synthetic underlayment is peel and stick, which in some sense makes it easier to install. No nails or nail guns are required, which reduces labor costs.
That said, it can be tricky for someone who has never installed synthetic peel and stick underlayment before.
- Does not tear easily
- More water-resistant than felt paper
- Peel and stick installation reduces labor costs
- Hard to install for a DIYer
- More expensive than felt paper underlayment
Roofing Underlayment Comparison Chart
|Felt paper||Synthetic Felt Paper||Rubberized Asphalt|
|Average Lifespan||12-20 years||15-40 years||40 years|
|Fire Resistance||Combustible||Fire resistant||Fire resistant|
|Watertight||Perm rating of 5-7||Per rating or 1||Water-proof|
|Installation||Relatively easy project most DIYers can handle.||Fast but requires precise measurements and planning.||Time-consuming and requires professional expertise|
|Cost||About 8 cents a square foot||About 15 cents per square foot||About 45 cents per square foot|
|Environmentally Friendly||Petroleum-saturated felt paper is harmful to the environment||Some synthetic products are designed to be eco-friendly||Many rubberized asphalt underlayments use recycled tires.|
|Best for||Homes and sheds||Homes, especially those that face harsh weather conditions||Vulnerable parts of the roof, including edges of the eaves, valleys, and around vents.|
What Is the Best Type of Roof Underlayment?
When it comes to durability, the different types of roof underlayment vary. Felt roof underlayment typically lasts between 12 and 20 years. This means that after about a decade of use, that underlayment becomes more susceptible to roof leaks.
Synthetic underlayment is typically rated to last longer–many come with 15-year warranties. Premium synthetic roofing paper, such as Protective Platinum, have a 40-year warranty.
Underlayments tear strength also plays into life expectancy, especially when used under harsher roofing materials such as a slate. Synthetic roofing materials are much much stronger than felt, which will tear much more easily during installation. This also means synthetic underlayment is less likely to tear in high wind events.
When it comes to performance, synthetic and rubber asphalt win out. Synthetic asphalt is fortified with fiberglass and is very water-resistant. This means it prevents water from reaching the roof sheathing better than felt paper and is much less likely to tear during installation or in high winds.
Felt paper, in comparison, can tear easily and, though also water-resistant, is not as water-resistant as synthetic roofing paper.
Rubberized asphalt is the best of the three options when it comes to performance. While the other two types are merely water-resistant, rubberized asphalt is 100 percent waterproof, creating a tight seal over your roof sheathing. And, since it’s made of rubber, it’s nearly impossible to tear it.
In colder climates that experience a lot of snow each winter, there’s always the potential for ice dams to form on a roof. An ice dam occurs when snow collects on the roof, melts, then refreezes, forming a thick layer of ice around the eaves.
As the snow melts, the water gets trapped behind the dams. This can cause havoc on a roof. Trapped water will instantly make its way under the shingles to the roofing underlayment. If the underlayment is felt paper, it will soak through it in a matter of minutes, reaching the roof sheathing.
While roof design can help eliminate the potential for ice dams to grow, they are still possible. If you live in a region that sees significant snowfall and have a roof susceptible to ice dams, it’s a good idea to spend the extra money on a rubberized asphalt roof.
Rubberized asphalt is the only type of waterproof underlayment, ensuring that water will not reach the vulnerable sheathing even if an ice dam forms.
With all of these marks against the felt paper, why does anyone use it? The answer to that is cost. Felt paper is simply much much cheaper than synthetic options. A 220 square-foot roll of 30-pound roofing paper runs about $15, whereas a 1,000 square-foot roll of synthetic underlayment costs about $150.
This translates to about 7 cents per square foot for felt paper and 15 cents per square foot for synthetic underlayment. If we translate that into a full roof of 1,700 square feet, that would be $255 for synthetic roof underlayment versus about $70 for felt roof underlayment.
Rubber underlayment, in comparison, is the most expensive at a whopping 45 cents per square foot, which explains why it’s mainly reserved for vulnerable areas of the roof.
Keep in mind that installation costs for synthetic and rubber are more expensive than it is for felt. Installation for synthetic underlayment requires the installer to carefully line up each piece before removing the protective layer covering the adhesive backing.
Installation varies depending on the type of underlayment. Felt underlayments only require simple tools, a hammer, and tacks, to install on a roof. It’s also easy to install. Simply unroll and overlap the edges.
Synthetic underlayment may be easier for a prop to install than felt paper. Still, it requires specialized tools and expertise to prevent it from pulling away from the roof sheathing and allowing a place for water to infiltrate. This can be challenging for DIYers who don’t have experience installing this type of roof paper, making felt paper an attractive option.
Popular Underlayment Products
There are numerous underlayment products on the market. Below are some of the best and most affordable options.
This synthetic underlayment roll proves that one doesn’t have to invest a lot more money to upgrade from felt paper. This synthetic offers the tear-resistance and water-resistance of synthetic paper at an affordable price.
In addition to resisting tearing, it also won’t absorb moisture or become brittle under the intensity of the sun. With its medium gray color, it stays cooler than black asphalt. It is also easier to work with than heavy felt paper.
This synthetic also lays flatter than felt, eliminating odd inconsistencies in the roof look. And, since it’s 48 inches wide, installation goes faster with this paper than standard 36-inch wide rolls.
With a tacky exterior that gives it great non-slip quality, this aptly named roof underlayment is a great option for use on steeper roofs. The bottom and top are both textured to create increased friction for better grip.
In addition to its non-slip surface, it’s also more water-resistant and tear-resistant than standard roof felt. It’s also easier to work at about a fifth of the weight of 30-weight felt paper. It also conveniently includes an installation grid printed on it to aid in the installation of the shingles.
Grip-Rite comes in large 10-square rolls, which helps reduce waste, bringing it a little close to the cost of cheaper felt paper.
This classic felt paper from warrior roofing may not match the quality of its synthetic rivals, but it’s much more affordable and will still provide excellent protection for most roofs.
Its asphalt-saturated material resists water well, while its rough surface makes it slip-resistant. Warrior Roofing asphalt paper comes in 432-square-foot 15 weight and 216-square-foot 30 weight rolls.
Owens Corning WeatherLock
This polypropylene roof underlayment may not be cheap but offers the best possible defense against rain. This underlayment adheres to the roof and resists cracking during installation with excellent tear resistance.
And, unlike other roofing underlayment materials, this product offers a truly weatherproof seal. Granules integrated into the surface of the underlayment give it a slip-resistant surface that makes it easier to install on steeper pitches will preventing slipping. A split adhesive back sheet makes installation easier.
Best Roofing Underlayment for Different Types of Roofs
Sometimes the type of application is crucial in determining what type of roof underlayment is best for the job. In this section, we’ll look at some of the most common types of roofs and what underlayment works well with them.
While any of the above underlayments will work with asphalt shingles, synthetic underlayment is the best choice. It offers better protection from the elements than standard felt roof paper while still being affordable. Rubber, in comparison, is overkill given its cost.
For a higher level of protection, go with a mix of synthetic rubber used on vulnerable areas, such as around roof protrusions, around the edges of the roof deck, and in valleys where two separate roof planes meet.
That said, if you’re on a budget, there’s nothing wrong with tried and true felt roofing paper. When installed properly, it still provides adequate protection for most roofs.
Since metal roofs don’t function the same as traditional shingles, many people don’t think underlayment is necessary. Metal roofing comes in panels that prevent water from passing through. Like regular shingles, the metal roof is only the first line of defense.
Metal roofing is designed to last for a long time. In fact, some metal roofs will last for 70 years. With that in mind, you need an underlayment that will keep up with it. With its comparatively short lifespan, felt paper typically isn’t going to cut it.
Synthetic underlayment, with its ability to resist tearing and moving along with its tolerance of high temperatures and ability to keep out moisture, makes it the ideal underlayment for metal roofing.
As with metal roofing, tile roofs demand underlayment with more durability than what asphalt-saturated felt offers. Tile is heavy and can easily rip through asphalt felt. It also lasts a long time, so the underlayment underneath it should too.
With its superior strength features, synthetic underlayment is the best option for a tile roof. The non-slip coatings that are on most synthetics also prevent the underlayment from slipping.
When putting a brand new roof on a shed, some people skip the underlayment. Don’t. Just because a shed is a smaller structure doesn’t mean it doesn’t require the same protection as a full-size home.
That said, typically felt paper is adequate for protecting a shed’s roof. Go with a single roll of 30 weight felt sheathing or use two layers of 15 weight felt sheathing for a shed.
Low-slope roofs present a challenge for roof paper, mainly because rainwater is more likely to settle on a low-slope roof than it will a steep roof. With that in mind, the roofing underlayment needs to have greater water resistance.
Using asphalt-saturated felt, roofers will often double up the underlayment for added water resistance on a low slope roof. A better option is to go with a synthetic roofing paper that can better resist water if water does breach the shingles.
Synthetic Roof Underlayment vs Felt
As we discussed above, synthetic roof underlayment is superior to felt roof underlayment in numerous ways. It’s much less likely to tear, has greater water resistance, and lasts longer. Synthetic roof underlayment is also easier to install. Since it’s lighter, synthetic underlayment comes in wider and longer rolls, which means fewer rolls to haul up to the roof.
Those advantages don’t come cheap. Synthetic roof underlayment is also about twice as expensive as felt paper. While that cost presents a significant difference, some of that can be made up on the back end with the longer lifespan of a synthetic roof paper. So, while synthetic roof underlayment may be the best choice, it may not be within the budget.
Problems With Synthetic Roof Underlayment
While there are few cons with synthetic roof underlayment aside from cost, there are some things to consider. While peel and stick roof underlayment creates an excellent connection with roof sheathing, that connection can be too strong.
If you ever need to remove and replace it, getting it unstuck can be a real chore because the adhesive is so strong. If you do need to replace it, it’s often a good idea to replace the sheathing with it.
Another concern is that some underlayment will not close in around the shingle fasteners nailed through it. This can nullify the superior water-resistant attributes it has over cheaper felt roof underlayment. Look for a synthetic underlayment that does seal around the nail.
Finally, since synthetic roof underlayment allows very little water or water vapor to pass through, it can trap hot air in the attic, baking the roof. Felt paper, in comparison, allows hot air and water vapor to pass through.
With this in mind, it’s important to have a good ventilation system in place in the attic to allow hot air to escape.
While selecting the right shingles is an important part of installing a roof that will protect your home for many years to come, don’t underestimate the importance of roofing underlayment. This thin layer of material is crucial to preventing water from reaching the roof sheathing.
With that in mind, think carefully when selecting the type of underlayment and consider spending a little more on a synthetic product, especially if you live in a region that experiences extreme weather.