According to a new Global Info Research study, the metal roofing market globally is expected to reach $18 billion in 2024, up from $14.5 billion in 2019. Furthermore, it is reported that the largest consumption of metal roofing is in North America. This 4.4 percent annual growth rate in this market simply cannot be ignored by roofing materials manufacturers and distributors.

As quoted from a recent article in Roofing Contractor magazine, Renee Ramey, executive director of the Metal Roofing Alliance (MRA), says:

"The metal roofing market in the United States and Canada is growing tremendously and there's just one thing holding back the opportunity for greater growth -- the lack of qualified contractors and installers to support the demand."

One main reason for the growing demand for metal roofing is their durability. The MRA website is full of examples of beautiful metal roofs that are also extremely durable. The truth of the matter is that the metal roof system installed over a premium modified bitumen peel-and-stick membrane is a perfect match: The metal roof protects the membrane from physical damage and deterioration from UV rays while the premium underlayment provides a secondary water barrier.

Metal roofers often bluntly ask manufacturers and distributors, "What underlayment is best under a metal roof?" A roofing products distributor would serve their customers well by offering them a range of choices as described in the "Good, Better and Best" discussion below.

The homeowner in the market for a metal roof is looking for a durable roof and will opt for the best metal roof underlayment available because a metal roof is a substantial investment. Premium underlayment products are especially attractive to contractors who install metal roofing because they are less likely to have to endure callbacks if they use premium products.

There is an expectation that a metal roof will last for many decades. Given the exceptional UV protection afforded by the metal roof, the life expectancy of the modified bituminous underlayment is quite long. These underlayment products are cousins to commercial roofing membranes and are typically made with fiberglass or polyester reinforcements. In the case of underlayments designed for use under metal roofing, the modified bitumen compound is specially formulated to withstand high temperatures.

A number of case studies of metal roofing applications can be found in the Case Studies section on the Tarco website. Premium self-adhering underlayment products offer many advantages. Most allow for relatively long exposure times and they also self-seal around fasteners. They do not suffer from the wicking of moisture through nail holes. When a fastener is driven through the modified bitumen layer, the SBS compound flows around the fastener, adheres to it and fully seals the membrane.

Specially Designed for Metal Roofing

Tarco offers an array of underlayment products specially designed for use under metal roofing.

Since a metal roof can have great longevity but typically does not provide complete moisture protection, it is very common to use a self-adhering modified bitumen underlayment beneath a metal roof. These membranes are more expensive than synthetic underlayments but the added protection is priceless, considering the consequential damage to the interior of the structure in the case of damage to or failure of the primary roof. If the deck is firmly attached to the building and the self-adhering underlayment is bonded to the deck then superior waterproofing protection is achieved even in the event of loss of the metal roof or metal roofing panels.

Self-adhered mineral-surfaced shingle underlayments are not suitable for metal roofing applications because the high thermal expansion of metal compared to most roof decks causes the metal to slide over the underlayment. Mineral granules are generally abrasive and would scratch through any corrosion resistant coatings and otherwise damage the metal panels and so they are to be avoided.

Another factor to consider is temperature resistance, considering the high temperatures that can be reached under metal. For example, metals such as zinc and copper generate a lot of heat. Underlayment designed for use in metal roofing applications must be able to withstand high roof temperatures.

In fact, there are two types of modified bitumen underlayments suitable for use in high temperature metal roofing applications, namely LeakBarrier PS200 ^HT and LeakBarrier PS200 ^MU . Both products are reinforced with a glass fiber mat. The differences are in the formulation of the modified bitumen compound, the overall thickness and weight of the underlayment, and the type and properties of the top surfacing.

PS200^HT has a top surface made from a polyester fabric, which allows for better walkability and cooler surface temperature. The specially engineered Hybrid Polymer System formulation allows the underlayment to withstand high temperatures. Of the two modified bitumen self-adhering metal roofing underlayment products, PS200^HT has better temperature resistance. It is rated to withstand temperatures up to 260° F. Interestingly, PS200^HT is also an excellent tile roofing underlayment.

PS200^MU is specifically designed for use in metal roof applications. It is formulated for temperature resistance up to 250° F. Thus PS200^MU is not recommended for extreme high temperature environments such as under copper or zinc metal roofing. PS200^MU has a polyolefin-based film on the top surface; the polyester top surface of PS200^HT is more rugged. Both PS200^HT and PS200^MU can be used as tile roofing applications but PS200^MU should not be used in adhesive (foam) set tile applications. One advantage of PS200^MU over PS200^HT is that it can adhere to itself in a two-ply system.

Increasingly, synthetic underlayment is also being used as a lower cost alternative to self-adhering modified bitumen underlayment. EasyLay UDL Basic, UDL 50 and UDL SA are suitable for use in metal roofing applications. UDL Basic and UDL 50 must be mechanically attached using fasteners. UDL SA is a self-adhering synthetic underlayment that is available for use in metal roofing. The top surface is a polypropylene fabric similar to that on mechanically attached UDL products.

When installing a metal roofing system that could last for 50 years or more, contractors and homeowners are more likely to specify a premium modified bitumen underlayment beneath the metal roof. However, depending on the type of metal roof, the climate and the building design, a synthetic underlayment or self-adhering synthetic underlayment may be chosen.

Pass on this information to your metal roofing customers.