Metal Roofing: Which Underlayment is best?

Another factor to consider when looking at getting a metal roofing system installed is what type of underlayment your contractor will be installing over your deck. There are three main types of underlayment that are typically installed under metal roofs.

30lb Roofing Felt: The standard acceptable underlayment for metal roofing systems. 30lb Felt is an asphalt-based product that is attached directly to the deck as an addition layer of protection. 30lb felt is typically thicker, stronger, than 15lb felt and may be less prone to tearing or ripping off during installation or weather events.

Synthetic Underlayment: A plastic based alternative to 30lb felt,
synthetic underlayment has gained popularity over the years. It offers several
advantages over 30lb felt. While 30lb felt tends to absorb moisture, most
synthetic underlayments can repel water, this can come in handy especially if your project requires it to be exposed to weather for a longer period.
Most synthetic underlayments also have some type of slip resistant coating making them a safer alternative to the crews working on your roof.
One thing you want to make sure you clear up with your contractor if they use synthetic underlayment on your system is that it is a high temperature synthetic that can be used with metal roofing systems.

High Temperature Ice and Water Shield: (Our preferred
underlayment) is a full coverage, fully adhesive
high temperature ice and water shield such as Polyglass MTS (Metal, Tile, Shingles). Its adhesive backing allows the contractor to minimize the
number of plastic cap nails needed to secure it to the deck,
(remember no hole, no leak) and allows them to ensure that in potential leak hot spots such as valleys that there are no perforations to the layer of protection at all. One thing we love about Polyglass MTS is that it has a self-healing aspect that heals around fasteners helping to prevent moisture infiltration. You want to make sure if your contractor is using ice and water shield that is is a high temperature product and that it is non-granulated. Ice and water shield also has a much higher exposure rating than either synthetic or 30lb felt.

Make sure you confirm with your contractor that the underlayment they are going to use is designed to be used with metal roofing systems and that it is a HIGH TEMPERATURE product.