• Rainy Day Exteriors

Roof Decking and Underlayment – What you Need to Know


As your go-to roofing company serving Port Orchard, Silverdale, Gig Harbor, and Tacoma, we are committed to helping our customers fully understand the intricacies of roofing.


Today, let’s dive into the world of your roof’s deck and underlayment. Here are four things we want you to know.


What is a Roof Deck?


A roof deck or sheathing is the plywood barrier that is under the shingles. Decking is attached to your roof’s framing and is a crucial part of the roof system.


Newer homes usually use plywood or OSB boards for the roof deck. But if your home was built more than about 30 years ago, you might have more traditional lumber boards for your roof deck.


What we often find is that these boards have gaps between them, which cause problems when installing shingles. This is because if the roof deck isn’t level or consistent, some of the shingle nails will miss the boards entirely causing you to lose shingles over time. If you own an older home, you will likely need to install plywood or OSB decking over the existing boards.


What is Underlayment?


The underlayment is the waterproof barrier that is placed over the roof deck. It’s installed over the roof deck and serves as an added layer of protection against severe weather like rain, wind, and snow.


There are three main categories of underlayment:

  • Asphalt-saturated felt (commonly referred to as tar paper)

  • Synthetic Underlayment

  • Rubberized asphalt underlayment

The asphalt-saturated and synthetic underlayment are both water-resistant. Rubberized asphalt underlayment is waterproof.


Synthetic underlayment is usually a roofer's go-to material because it’s tear-resistant and can be installed over the entire roof deck, which saves time.


Rubberized underlayment is also a great choice, but it is more expensive because it has a higher percentage of asphalt and rubber polymers, which make it waterproof. It can be used in the peaks and valleys of your roof, or at roof penetrations like chimneys or vents, which are more susceptible to leaks.


Why are they important?


The roof deck is the backbone of your roof system and provides the necessary support for the entire roof.


Underlayment is a roof saver. The underlayment protects the decking from chemicals in the shingles.


Did you know that underlayment doesn’t just protect the deck from water? It also protects the decking from chemicals in the shingles. In some climates, the shingles would essentially melt into the decking if underlayment wasn’t installed.


Another reason underlayment is essential is that if shingles coming off your roof, the roof deck is not immediately exposed. Thanks to the underlayment, the wood deck is protected until you install any missing shingles.


How do they work together?


Essentially the deck and the underlayment are the foundation for your roof system and work together to keep moisture out of your home. To work effectively, the roof deck and underlayment have to be in good shape. This is why you need to replace your roof deck if it’s recommended.


If you are considering a new roof, contact us today. We provide free inspections and estimates, which will include an assessment of your roof’s deck and underlayment.

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