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There’s a reason why building codes, roofers, and roofing associations alike recommend—and even often require—underlayment. But sometimes questions arise. Is underlayment truly necessary? How does it benefit the roof, particularly since underlayment is designed to be sealed away by shingles, steel, or some other roofing material?

Underlayment is a necessary part of any roofing system, one that comes with a lot of benefits. Let’s take a look at the reasons why to answer some of these questions.

1. An Extra Line of Defense

This is one of the biggest reasons why underlayment is necessary. Shingles, steel or whatever the primary roofing material is—these things are meant to be the roof’s first line of defense against water. On most roofs, the deck is made of OSB or plywood, which can be sensitive to water damage, plus any moisture leaking past the deck can cause problems with mold in attics or damage to rafters and trusses.

The problem is, severe weather often damages roofs—and it does so in a variety of different ways. High winds from thunderstorms or hurricanes can lift shingles or tear them off, which lets water past that first line of defense. The same goes for snow and ice, which can damage roofing materials and cause leaks. So the underlayment is there as an extra layer of protection to prevent water reaching the roof deck should the primary roofing material be damaged.

2. Underlayment Repels Water, Too

This is especially important on shingle roofs. In some conditions, shingles can trap and hold water. In instances where rainwater isn’t draining properly, much like with preventing water infiltration in the event of a storm, the underlayment is there to prevent water from reaching the deck. When this happens, the water should run off the underlayment, which keeps the roof deck below safe.

3. Underlayment Prevents Stained Shingles

Depending on the decking material, certain types of wood can leech resins over time. If there is no underlayment between the deck and shingles, then months or years after the roof’s installation, homeowners will start to notice stains as the shingles absorb these compounds. Underlayment does double duty, not only protecting the deck from water damage, but the shingles from resin stains.

4. Providing a Smooth Installation Surface

While builders do their best to install roof decks as flat and square as possible, in practicality, there are always some minor problem spots! Almost no roof deck is perfectly smooth. Underlayment over top of the decking helps level things out so that roofers have a smoother layer on which to install shingles. That’s important because without underlayment, the outline of sheets of decking is sometimes visible beneath shingles—a phenomenon referred to by roofers as “telegraphing.”

5. Protection During Installation

Another important reason to install underlayment is that it can protect the roof deck from water and other environmental factors during the installation process, especially when that installation comes with some sort of delay. As builders know, delays happen—funding issues, delays in roofing material shipments, weather slowing work, and so on. But even if there are no delays, things like rain and dew are reasons enough to install underlayment to keep the deck dry during a re-reoofing job.

As you can see, underlayment comes with a lot of valuable benefits. While it is technically possible to install a roof without it, all of these reasons combined illustrate how underlayment is an extremely important part of any roofing system.