Comparing Roof Underlayments

Even though it is hidden underneath a metal roof, an underlayment is a key component to ensuring a long life span of a metal building system.

Pros and cons to help choose the most appropriate underlayment

By Mark Robins

Photo courtesy of MFM Building Products Corp.

Essential to a metal roof, an underlayment serves as an extra weather-resistance barrier—providing greater moisture and thermal protection, and waterproofing. Without it, metal roofs would be loud when it rains, cold during the winter, and hot during the summer with high temperatures.

Entrapped moisture can accelerate the deterioration of any roof, even one comprised of metal roofing. During construction, once it is installed directly onto the roof deck, an underlayment protects buildings from water infiltration before the final roof covering installation. After construction, the underlayment sheds water that has found its way into the roofing system.

Three Types of Underlayments

Choosing the most appropriate underlayment is as important as choosing the right roofing material. When it comes to metal roofing, there are three main options for underlayment: felt, self-adhered and synthetic.

Felt. Felt underlayment—also called asphalt felt or tar paper—is commonly used on steep-sloped roofs. It consists of a base of natural fibers or cellulose and when treated with a waterproofing agent such as asphalt its water repellency is increased. Thirty pound felt underlayment—or two layers of 15-pound felt—are recommended for metal roof panels.

Self-adhered. Self-adhering membrane underlayment is a peel-and-stick product applied directly to roof decking. It usually includes either a rubberized asphalt or butyl-based adhesive that’s mounted on a polyethylene carrier sheet.

“Self-adhered underlayments provide the best protection against ice dams, wind-driven rain and moisture from infiltrating the building structures,” says David Delcoma, operations manager, MFM Building Products, Coshocton, Ohio. “Most selfadhered underlayments also self-seal around roofing fasteners to maintain this waterproof barrier. The absolute key when using a self-adhered underlayment is properly ventilating the roof because there are vapor barriers that do not allow air to flow through the membrane. Without proper ventilation, moisture can build up inside the structure, leading to potential mold growth and interior damage.”

Synthetic. Synthetic underlayment for metal roofing is a newer technology that’s grown in popularity in recent years. It’s manufactured by weaving or spinning together either laminated polypropylene or polyethylene plastic technology. It’s very durable because its thermoplastic polymers are more resistant to tearing, moving and other damage. It’s also very tolerant of high temperatures and delivers the longest underlayment life span.

All three of these underlayment types have their own pros and cons. To maximum roof performance and extend its life and the building it protects, consult underlayment manufacturers and industry professionals to better understand their attributes and which ones work best for different applications.

Photo courtesy of MFM Building Products Corp.