When you picture a roof, what comes to mind? Given your role as a contractor, you likely think more about tie-off points, difficult conditions, transitions, and steep pitches than a typical person. But chances are, your mind does not go beyond the surface. You know what’s below the final roof covering is equally important, but let’s face it, it’s certainly not the sexiest part of a roofing system.
This second layer of a metal roofing system is a protective barrier between the roof deck and the finish layer, commonly referred to as an underlayment. This important element has two main purposes: it ensures the structure stays dry during construction and provides a secondary reinforced layer of protection against the elements.
If, by rare occurrence, an extreme event was to damage the top layer of a metal roof, this additional barrier could defend the building by shedding water. While used in both commercial and residential applications, underlayments generally have a minimum slope requirement of 2:12 to be most effective.
For the last decade, the roofing world has seen an evolution in underlayment products. Historically, the preferred underlayment product was an asphalt-saturated felt, more commonly referred to as felt paper or tar paper.
While this product was standard in the industry, it presented challenges. Specifically, each project required multiple rolls because each roll was only 2 square or 4 square feet. Further, the product often buckled and wrinkled during installation, especially if left exposed for any length of time.
To complicate matters even more, it tended to degrade quickly because it was a paper product.
Read: What is the best underlayment for a metal roof?
Given the above, the market was ripe for an alternative to solve the challenges of asphalt-saturated felt products. Thankfully synthetic underlayments entered the market. Manufactured from high-performance polymers such as polyethylene or polypropylene, synthetic underlayments offer several advantages over asphalt-saturated felt.
Synthetic underlayments are much lighter weight, making them easier to handle and maneuver and, therefore, quicker and easier to install.
They are generally manufactured in 10 square rolls, so fewer rolls are needed on the job site.
Most importantly, they are safer than felt paper to walk on during installation, giving you and your install team an added level of security.
You’ll typically find two categories of Synthetic underlayments: mechanically fastened and self-adhered, often called 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 fastener to hold them in place.
True to their name, self-adhered underlayments include an adhesive on the back side that allows them to adhere to the roof deck without the use of a fastener for attachment.
While these products can be used to cover the entire roof, they are typically used in valleys and eaves, and around penetrations such as skylights and chimneys. Be sure to check your local code requirements for guidance.
Consider these questions when planning a roofing project:
1. How do I install synthetic underlayments?
Most synthetic underlayments will provide written installation instructions 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.
2. 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 specifically designed for metal roofing with a temperature rating of at least 240 degrees. It’s common to find these products marketed as “high temp.”
3. 
What temperature thresholds should I consider when selecting a 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.
4. How long can synthetic underlayments be exposed to the elements before installing the final roofing layer?
This will vary based on the underlayment, but generally, synthetic underlayments have UV inhibitors added to the polymers, allowing exposure for 90-180 days. However, there are exceptions on both ends of the spectrum, so consider carefully and do your research!
5. What is the warranty on synthetic underlayments?
Again, this will vary depending on the underlayment selection. Warranties vary from zero to lifetime. As with any other product purchase, request a copy of the warranty before purchase and installation and ask questions if something isn’t clear.
Summary
It’s important to remember that a good roof involves more than just the visible product. For more information about the best product for your project, contact our team!
If you want to learn more about installing metal panels, watch this video!
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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