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
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.
Charting Underlayment Attributes
- Information supplied by MFM Building Products:
- www.mfmbp.com
- …and Polyglass U.S.A. Inc.:
- www/polyglass.us
- …and Drexel Metals:
- www.drexmet.com
- Limitations:
- Asphalt felt is not considered high-temp and in extreme heat asphalt can leak out the eaves and fail. Felt underlayment provides little resistance to air and moisture vapor migration, and does not serve as an effective air barrier. Synthetics should not be used in valleys or sidewalls.
- Installation:
- Felt and synthetics are the easiest to install; they are lightweight, easy to handle and only require roofing caps. Felt is usually available only in 2 to 4 roofing squares, so it can take many rolls to complete an average-sized roof. Self-adhered underlayments require a little more work due to the weight of the membrane, working with a split release liner and avoiding air pockets/wrinkles. Synthetics tend to come in rolls of 10 roofing squares. Most synthetics use cap nails as required by their warranty. Staples are usually not recommended because of leaking caused by walking that causes pulling against the staple. Synthetics offer a slip-resistant gripping surface to roof workers.
- UV Exposure:
- UV exposure can vary between all types of underlayments, depending on the top surface material composition. Even within one type of underlayment, the variance can go from 30days up to 180 days or even more. Know how fast the underlayment will be covered by the finished roofing system. This will determine the proper UV exposure to choose. UV exposure of synthetics allows roofs to stay uncovered for more extended periods during construction.
- Temperature Rating:
- For all installations under metal, the underlayment must be high-temperature rated. Due to the extreme heat build-up under metal, the underlayment must be able to withstand this heat to provide the highest level of waterproofing protection. For self-adhered underlayments, a temperature rating of at least 225 F will prevent the mastic from softening and seeping out causing the membrane to lose its waterproofing effectiveness.
- Durability:
- Synthetics and self-adhered underlayments are the most durable. The least durable are saturated felt products, which will breakdown due to moisture and UV exposure. Being a paper product, their material can degrade quickly.
- Slope Limitations:
- On roofs with a slope of 4:12 or greater, use a single layer of 15-pound, asphalt-saturated felt, starting at the eaves and lapping upper courses over lower by a minimum of 2 inches. Synthetics should not be used under a 2:12 slope due to them not being self adhering and leaking at the laps with wind-driven rain.
- Cost:
- Felt is the least expensive option. Synthetics and self-adheredare more expensive.
Photo courtesy of MFM Building Products Corp.
