What is Roofing Underlayment, and Why is it Needed?
Your roof protects your home from the elements. Pretty basic stuff, right? Less obvious are the parts that go into the whole roof system. You may never have heard of roofing underlayment, but it’s doing a lot of work for you every day.
What is Roofing Underlayment?
Roofing underlayment is a water-resistant or waterproof barrier material installed directly onto your roof deck. It’s applied under all other roofing materials as an added layer of protection from severe weather such as rain, wind, and snow.
Why Do You Need Roofing Underlayment?
Roofing underlayment may not always be necessary, but many areas require felt or synthetic underlayment. It’s important because it acts as a moisture barrier to protect your roof from rain and other inclement weather.
Roofing underlayment provides your roof with an extra layer of protection that shingles alone can’t match. Having a shingle underlayment allows your shingles to serve their own purpose: they’re the first line of defense. Shingles reflect the sun’s rays, withstand wind and block precipitation.
But shingles overlap and aren’t sealed at all corners. They can be lifted by strong winds. This makes shingles vulnerable to tear-off or infiltration by wind-driven rain. Shingles can become brittle from age. The heat of an unventilated attic can cook your shingles, too.
Types of Roofing Underlayment
There are three main types of roof underlayment used under roof materials:
- Asphalt-saturated felt
- Non-bitumen synthetic underlayment (also known as “synthetic underlayment”).
- Rubberized asphalt underlayment
Asphalt-saturated felt and synthetic underlayment are water-resistant. Rubberized asphalt underlayment is waterproof.
Asphalt-saturated felt was the go-to roofing underlayment until about 15 years ago, though it is still a great shingle underlayment. That’s when synthetic products started gaining in popularity. This type of underlayment is commonly referred to as “felt paper” or “tar paper,” and it is made from varying blends of cellulose (natural plant fibers), polyester, bitumen or asphalt.
Typically, this underlayment has a basemat, or a flexible base layer, saturated with asphalt for water resistance. It is applied all across the entire roof deck, and depending on the local weather, a water waterproof underlayment may be recommended by your roofing contractor.
The advantage of this type of underlayment is that it comes in different thicknesses with varying resistances to damage and weather exposure. Thirty-pound asphalt-saturated felt, for example, can actually protect the roof longer if exposed to hail, wind, rain, and snow. It’s less expensive than many other materials, provides a good seepage barrier, and common tools can easily be used to install it.
The disadvantage is primarily its weight: it can be heavy, especially when it’s layered enough to be properly effective in harsher climates.
It can be more difficult to install since it is heavier than other options, it can wrinkle and crack when exposed to cold, and since asphalt-saturated roofing felt underlayment contains volatile compounds, these can dissipate when exposed to heat, causing the underlayment to absorb more moisture and break apart.
Synthetic underlayment (technically non-bitumen synthetic underlayment) is the preferred underlayment of most roofing professionals today.
A synthetic underlayment basemat is saturated in asphalt, and the addition of fiberglass gives it superior tear resistance and stability. High-quality synthetic underlayment like CertainTeed’s DiamondDeck™ has the added benefit of scrim reinforcement for added slip resistance, even when wet.
Synthetic underlayment is designed for application across the entire roof deck. It is sometimes used with waterproof products.
A disadvantage to synthetic underlayment is it tends to be more expensive than asphalt-saturated felt and require more tools and skill to install properly. But the bigger advantage to this type of underlayment is that it is a light, effective water barrier, so it doesn’t add a lot of weight to your roof.
It’s also more tear-resistant when exposed to high winds and does not absorb moisture, so it resists mold and fungal growth. It will maintain its integrity even when exposed to the cold and don’t wear down as quickly as others.
The image below shows the wind damage caused by an extreme weather event in Hollywood, Maryland on July 19, 2011. The 20-inch diameter pine tree in this photo was snapped in half by winds reported to be in excess of 80 miles per hour.
In the background, you can see a church undergoing a roof replacement. CertainTeed’s DiamondDeck™ water-resistant underlayment was the only roofing material on this roof when the storm hit.
This photograph of the church roof (below) was taken the same day as the one above. It shows that DiamondDeck™ alone protected this roof during an unexpected extreme weather event. These photographs show that no parts of the underlayment were torn or lifted. There were no leaks. That’s strong underlayment!
Rubberized asphalt is more expensive than other types of roofing underlayment. Why? It contains higher percentages of asphalt and rubber polymers, making it waterproof.
Rubberized asphalt underlayment often has a sticky back with a protective membrane that is removed prior to installation. This special backing creates the waterproof seal between the underlayment and a clean roof deck. In severe winter regions, waterproof roofing underlayment at the eaves of a roof is helpful.
The underlayment protects the edge of the roof deck from water damage. Rubberized asphalt roofing underlayment should be added at valleys and around roof protrusions as these spots often get leaky first.
Rubberized asphalt underlayment does a great job of protecting your roof against water damage and are much more heat and cold resistant than asphalt-saturated felt.
They last much longer when installed, and if your roofer needs to wait before installing your primary roof covering, rubberized asphalt underlayment can be exposed to weather elements for up to 180 days without severe negative effects.
A big disadvantage to rubberized asphalt is the cost. You will pay for the benefits you get from using this material, but it will be a higher upfront expense.
Water-Resistant Roofing Underlayment
Water-resistant underlayment is enough for the surfaces of most roofs. It provides the protection that your roof needs during and after installation. Even without the added protection of shingles, a superior product can withstand some seriously extreme weather.
There are regions where only waterproof roofing underlayment should be used. Waterproof underlayment is recommended for roofs in regions prone to high winds or ice damning.
Waterproof roofing underlayment should be used at:
- Protrusions (chimneys, vent pipes, skylights, etc.)
- Surfaces of low-slope roofs
How to Choose Roofing Underlayment that’s Right For Your Roof
The most important part of deciding how to choose a roofing underlayment that’s right for your home is finding what works best with your roof type.
Metal roofs, for example, will do well with a synthetic underlayment that has a higher heat-resistance (since the metal can trap more heat), while a traditional asphalt shingle roof will probably do best with a roofing felt underlayment.
It’s also important to consider the environment where you live and what the typical climate is. If you live in a hotter climate, then you want to choose an underlayment that can handle the heat, whereas if you live in a climate that is frequently snowy, you’ll need something that’s heavy-duty.
Lastly, you’ll want to consider price when it comes to choosing your underlayment. Different roofing underlayment works well with different budgets. Decide on your budget, and then you can more easily see what underlayment options are available to you.
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