This is a question that people ask us all the time! What type of underlayment should you use on the valleys of a roof? You’ve got a choice between traditional felt underlayment, synthetic underlayment, and peel and stick synthetic underlayment. All of these options will work—so long as they’re installed according to manufacturer instructions—but where valleys are concerned, there is definitely a clear winner! Read below to find out more.

Traditional Roofing Felt

Asphalt paper roofing felt has been the industry standard for decades. Most roofers are familiar with it and have used it to cover entire roofs, valleys included.

The problem with felt is that it’s just not as sturdy or watertight as other options available today. It doesn’t seal to the roof deck the way a peel and stick underlayment can, and it’s prone to cracking or loss of volatiles over the years, which are issues that can lead to water seeping beneath the underlayment itself. Along roof valleys, this is problematic because that is where water tends to flow when it rains—and snow and ice can dam in valleys, too. This is definitely an area of the roof that can use a little extra protection.

Synthetic Underlayment

You could consider this a step up in terms of protection. Most synthetic underlayments are water repellent, and they don’t lose volatile organic compounds over time, which means they last a lot longer than roofing felt. These products will do a better job protecting roof valleys—and the entire roof itself—but where valleys are concerned, there is still one disadvantage. Synthetic underlayment needs fasteners to hold it down, and anywhere there is a fastener, there is potential that water could someday seep past the underlayment to the roof deck itself. This is a larger risk in roof valleys than over the rest of the roof because of the sheer volume of water that flows here, and because of the potential for ice dams that can cause pooling.

Peel and Stick Underlayment

Where roof valleys are concerned, this option is the clear winner. Like non-peel and stick synthetics, there is no risk of cracking or loss of VOCs, which leads to the long-term degradation of the material. But the real beauty of this type of underlayment is that it seals to the roof deck—no fasteners required.

Our Plystik Plus features Ice Dam Protection, and that’s because peel and stick underlayment offers a truly waterproof barrier. As water flows along valleys, there is no chance that it will seep to the deck around fasteners since fasteners are not required, and because it is a watertight barrier, it’s a product that is often installed anywhere that ice dams may present a problem—most commonly along valleys and along the edges of the roof.

Hybrid Installation is an Option

With some roofing jobs, cost is a concern. Peel and stick underlayment is usually the most expensive option, though it is the longest lasting, which gives you the most bang for your buck over the lifetime of the roof. Roofing felt is the least expensive, with non-peel and stick synthetic falling somewhere in between. If cost is a concern, some roofers choose to use two different underlayment products on a roof. For example, peel and stick in the areas that need the most protection, like in the valleys and along the edges, and then less costly synthetic over the rest of the roof.

When it comes to preventing leaks along roof valleys, there are lots of options. But peel and stick underlayment is the only variety that provides a truly watertight seal, which makes it the clear winner for this application.un