What are roof underlayments?

When you look at rooftops from the street, you see a mosaic of shingles, tiles or metal ridges, but most of us rarely think about what lives in the space between this external layer and the attic below. Shingles are only the outermost layer of a roof; beneath them hides an underlayment, a layer of material that provides waterproofing for the roof deck , the architectural structure of the roof. 

Most roof underlayments fall into one of three categories: asphalt-saturated felt, rubberized asphalt or synthetic. As you might assume, synthetic underlayments represent the newer, more contemporary branch of underlayments. On the other hand, asphalt-saturated felt, or 'tar paper,' has been used for decades. Each has its place, and unless you're a professional roofer , knowing which to use isn't immediately clear to the layperson. Here's what you need to know about underlayments and the various types available.

What does roof underlayment do?

Underlayments have a few functions. Most importantly, they serve as an extra barrier to keep moisture from permeating your roof's deck. While shingles, metal or tiles all protect the roof from water incursion, none are a perfect shield against moisture buildup and unpredictable ice dams . Underlayment is a crucial component in making your roof weather-resistant .

This moisture barrier serves a dual function, as it also protects your roof from damage caused by high winds that may tear shingles away. Think of it as a second line of defense against storms, as well as day-to-day moisture build-up. With this goal in mind, strategically selected underlayment can be a crucial component of a storm-resistant "fortified roof."

Your roof is one of the most important insulators in your home, and your underlayment also serves as an extra layer of insulation. It helps retain warmth in the winter and shields your home from the intense heat of a shingle or metal roof.

Some underlayments have properties beyond acting as moisture barriers and insulators, and can also protect against fire and noise. For example, products like GAF VersaShield can protect your roof deck from flames. If you live in an area prone to wildfires, this added flame resistance could be extremely useful. 

Common roof underlayment types

There are three main types of underlayments: asphalt-saturated felt, modified bitumen ice and water shield and synthetic underlayment. 

Asphalt-saturated felt

Asphalt-saturated felt, also known as "tar paper," may resemble traditional paper, but it's actually an organic felt or fiberglass mat that has been thoroughly saturated with asphalt. The material is often found in two weights, #15 and #30, the latter being the heavier grade. The material serves as a barrier to moisture incursion while also being permeable enough to allow the roof deck to breathe. 

One of tar paper's downsides is that it easily rips, which can pose a danger to roofers. Put too much weight on a section of tar paper, and it may rip away, causing the roofer to lose their footing and potentially fall. It can also absorb moisture, which can cause it to retain water against the wood instead of protecting it.

According to Owens Corning , felt underlayment can also be incompatible with some roofing warranties. Check these details with your roofer before making a decision.

That said, there's one huge benefit to felt, and that's its price. This underlayment is very affordable compared to some others. 

Modified bitumen or 'ice and water shield'

Self-adhering modified bitumen underlayments — often called "ice and water shield" — are usually applied to roofs along the eaves and along the basin of any valleys to afford extra protection, even if the rest of the roof is finished with tar paper. However, this substance can also be used across the entire roof. Made of modified bitumen or rubberized asphalt, it provides more rigorous protection than standard asphalt-saturated felt. 

IKO, which offers a blog for roofing contractors, says that if ice and water shield is used to cover the entire roof, it can create a vapor barrier, which can lead to moisture build-up. Consequently, it's especially important to ensure that the home's attic is adequately ventilated if a self-adhering ice and water shield is used in this manner. The benefits are substantial, however, as ice and water shield can be an effective secondary barrier against moisture penetration if shingles are ripped away in a storm.

Modified bitumen ice and water shield is usually self-adhering. This means that roofers can peel off the material and stick it onto the roof deck. Importantly, the underlayment not only adheres to the roof but also self-adheres to nails, thereby reducing the likelihood of water leaking through these perforations.

This material can also have higher temperature limits than asphalt-saturated felt, making it a good pick for metal roofs, which can become quite hot due to sun exposure.

Synthetic polymer underlayments

Synthetic polymer underlayments are the veritable new kid on the block. Made of combinations of plastics and polymers, these materials are extremely effective at sealing the roof deck from water incursion and highly durable. They are often composed of polypropylene or polyester with coatings for waterproofing. 

While synthetic underlayments can be significantly lighter than asphalt-saturated felt, they are also less susceptible to tearing. This is a significant advantage for roofers walking on the roof surface, who may tear underlayments made of asphalt-saturated felt. However, some synthetics can be slippery, presenting a different danger.

Some synthetic polymer underlayments are very resistant to high temperatures. This makes them a good choice for parts of the country with very hot climates and high levels of sun exposure.

When set up correctly, synthetics allow a roof to 'breathe,' with properly coordinated vents. Some synthetics also have a degree of air permeability. That said, when improperly installed, this material's strict barrier can cause problems.

What type of roof underlayment is best?

Generally, it's best to consult your roofer about underlayment options. Your roof's venting system, decking and roof material will all contribute to determining the proper underlayment. 

That said, there's often a choice in the matter. If you have the budget and want to invest in an underlayment with added fire resistance or improved insulation, consult with your roofer to determine if the preference is compatible with your roof's needs. 

Additionally, if you're planning to invest in a cool roof or another non-standard roofing system, discuss how alternative underlayments could play a part in your decision.

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