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Posted by McElroy Metal ● Jul 21, 2020 8:59 AM

Roof Underlayments Explained

When the average person thinks about roofing products, generally the one thing that comes to mind is the top/finish layer; the shingles or the metal. This is only natural, as it is this layer that everybody sees and is the part of the roof that aesthetically gives the structure its beauty.  However, there is a layer beneath the finish layer that is of equal importance. This layer is a protective barrier between the roof deck and the finish layer that is commonly referred to as underlayment. While used in both commercial and residential applications, underlayments generally have a minimum slope requirement of 2:12 to be most effective.  

McElroy HydraShell Roof Underlayment

For the last decade, the roofing world has seen an evolution in underlayment products being used in the field.  Historically, the preferred underlayment product was an asphalt-saturated felt; commonly referred to as felt paper or tar paper.  While this product was standard in the industry, it had its challenges.  Specifically, multiple rolls were required as each roll was only 2 square or 4 square in size, the product would buckle and wrinkle as it was rolled out, specifically if left exposed for any length of time, and because it was a paper product, it would degrade at a faster rate.

Read: What is the best underlayment for a metal roof?

Because of this, the market was ripe for an alternative that would give advantages over asphalt-saturated felt.  Thus, synthetic roof underlayments were born.  Synthetic underlayments are manufactured from high-performance polymers such as polyethylene or polypropylene.  They are much lighter weight.  They are generally manufactured in 10 square rolls; thus fewer rolls are needed on the job site.  They are easier to handle and maneuver, and they install quicker, easier, and smoother than the felt paper.  Maybe most importantly, they are safer than felt paper to walk on during installation, giving the contractor an added level of security.

McElroy Hydrashell Roof Underlayment

So why are underlayments important?  Their necessity is two-fold.  Not only do they allow the structure to be dried in during the construction phase, they ultimately provide a secondary barrier of weather protection to the structure.  If by chance water were to penetrate the top layer in any way, (i.e., improper installation, blow-offs due to storm damage, primary roof covering worn out, etc.) underlayments provide a protective layer that will shed water away instead of letting it come into the building.

Synthetic underlayments are generally manufactured in two different categories; mechanically fastened and self-adhered, which is often referred to as peel & stick.  Mechanically fastened underlayments are just that…fastened mechanically with a nail.  Various underlayments have differing requirements on how many fasteners to use and which fasteners are appropriate, but all ultimately require some type of fastener to hold them in place.  Self-adhered underlayments are true to their name as well.  They are manufactured with an adhesive on the back side that allows them to self-adhere to the roof deck without having to use a fastener.  While these products can be used to cover the entire roof, they are most generally used in the valleys and eaves and around penetrations such as skylights and chimneys.  Code requirements vary, but typically, a self-adhered underlayment should be used to at least 2 feet inside the exterior walls. 

Below are some questions that should be considered when purchasing synthetic underlayment:

metal roof underlayments1. What temperature thresholds should I install synthetic underlayment?

Mechanically fastened underlayments generally don’t have temperature limitations. However, self-adhered underlayments generally do.  Asphalt backed self-adhered underlayments should be installed between 40°F and 125°F.

2. How long can synthetic underlayments be exposed prior to putting on the final roof?

This will vary based on the underlayment, but generally synthetic underlayments have UV inhibitors added to the polymers which will allow it to be exposed between 90-180 days.  However, there are exceptions on both ends of the spectrum.

3. What is the warranty on synthetic underlayments?

Again, this will vary depending on the underlayment.  Warranties vary from zero to lifetime.  The buyer should pay particular attention to the warranty that they are getting when purchasing a synthetic underlayment.

4. Are there any special considerations for peel and stick underlayments for metal roofs?

Peel and Stick Underlayments have temperature ratings. When installed with metal roofing, use underlayments that are designed for metal roofing and have a temperature rating of at least 240 degrees.

5. How do I install synthetic underlayments?

Most synthetic underlayments will provide written installations either through the distributor or on the packaging itself.  However, you always start at the low point of the roof and run the product horizontally, overlapping each row in a shingling fashion.  When purchasing and installing a self-adhered underlayment, some products offer a split back release liner, which allows the film to come off in two pieces instead of one.  A split back release liner helps with the installation of a self-adhered underlayment, as it is easier to handle.

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About McElroy Metal

Since 1963, McElroy Metal has served the construction industry with quality products and excellent customer service. The employee-owned components manufacturer is headquartered in Bossier City, La., and has 14 manufacturing facilities across the United States. Quality, service and performance have been the cornerstone of McElroy Metal’s business philosophy and have contributed to the success of the company through the years. As a preferred service provider, these values will continue to be at the forefront of McElroy Metal’s model along with a strong focus on the customer.

Topics: Metal Roofing, Substructural Components, Best Practices, Installation

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