Understanding Roof Underlayment And Why It Is Important

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Roof underlayment is an essential yet often overlooked component of a roofing system. Serving as the last line of defense between the roof and the elements, it plays a critical role in maintaining the durability and longevity of your roof. 

This article aims to shed light on the significance of roof underlayment, providing insights into its function, types, and why it’s vital for your home’s overall protection.

What is Roof Underlayment 

Roof underlayment, roofing felt, or tar paper is a thin material installed in between the deck of roof and the shingles. It’s typically made from  synthetic materials like polyethylene or fiberglass or asphalt-saturated felt.

The primary purpose of roof underlayment is to provide an extra layer of protection against water infiltration. Its function is to act as a secondary barrier, preventing any moisture that seeps through the shingles from reaching the deck of the roof and causing damage.

Types of Roof Underlayment 

There are three main types of roof underlayment: synthetic, asphalt-saturated felt, and rubberized asphalt. Each type offers different benefits and is suitable for specific roofing applications.

  • Synthetic Underlayment – Made from polyethylene or polyester, synthetic underlayment is lightweight, tear-resistant, and more durable than traditional felt. It also has a higher resistance to UV rays, making it an excellent choice for roofs with prolonged sun exposure.
  • Asphalt-Saturated Felt – Also known as tar paper or roofing felt, this type of underlayment is the most commonly used. It’s affordable and provides decent protection against water intrusion. However, it can be prone to tearing and is not as durable as synthetic underlayment.
  • Rubberized Asphalt Underlayment – This underlayment contains rubberized asphalt sandwiched between two layers of reinforced fiberglass or polyester fabric. It offers superior waterproofing properties and is ideal for areas with high levels of moisture or extreme weather conditions.

How to Install Roof Underlayment 

Roof underlayment is typically installed over the roof deck before the shingles are laid. It’s essential to ensure the underlayment is correctly installed to function effectively. Some key points to keep in mind during installation include:

  • Overlap and Sealing – The underlayment should be overlapped by at least six inches horizontally and twelve inches vertically. Additionally, all seams and joints must be sealed with roofing cement or adhesive tape to prevent water from seeping through.
  • Proper Fastening – Underlayment should be fastened securely to the roof deck using nails or staples. This ensures it stays in place during the installation of the shingles and also prevents wind uplift.
  • Valleys and Eaves – Extra care should be taken when installing underlayment in valleys and eaves, as these areas are more prone to water penetration. It’s crucial to create a double layer of underlayment in these areas for added protection.

Benefits of Using Properly Installed Roof Underlayment  

Properly installed roof underlayment offers numerous benefits, including:

  • Enhanced Waterproofing – As mentioned earlier, the primary purpose of roof underlayment is to prevent water intrusion. Proper installation ensures that your roof is protected against leaks and moisture damage.
  • Added Protection during Roof Repairs – In case of any damage to the shingles, having a layer of underlayment provides temporary protection until the repairs can be made.
  • Improved Insulation – Some types of underlayment, such as synthetic and rubberized asphalt, offer additional insulation to your home. This helps reduce energy costs and maintain a comfortable indoor temperature.

Common Problems with Roof Underlayment  

Improper installation or lack of underlayment can lead to several issues, including:

  • Water Damage – Without proper underlayment, water can seep into the deck of the roof and cause rotting or mold growth.
  • Premature Aging – Shingles not protected by underlayment can deteriorate quickly due to exposure to UV rays and other elements.
  • Voided Warranties – Many roofing manufacturers require underlayment to maintain product warranties. Failure to install it can result in voiding your warranty.


In conclusion, roof underlayment is a crucial component of any roofing system. It provides an extra layer of protection against water infiltration, enhances insulation, and adds longevity to your roof. 

It’s essential to understand the different types of underlayment and ensure proper installation for maximum benefits. Investing in quality underlayment can safeguard your home against potential damage and save money on costly repairs in the long run.

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