Types of Roof Underlayment

As homeowners and homebuilders in Big Sky and Bozeman begin to evaluate the need for a reroof or roof construction, it is important to build a thorough understanding of all the components which make up a total roof assembly. One of the most vital components of this assembly is a product which is known as roofing underlayment.

What is roofing underlayment?

Roofing underlayment is a waterproof or water-resistant material which is installed between your roof deck and your roof’s finish materials. This material acts as a barrier that protects your home from numerous exposure elements including but not limited to rain, snow, ice, and wind. Though there are various forms of roofing underlayment on the market, we will focus on the two most common forms: Synthetic underlayment and Peel and Stick underlayment.

Synthetic Underlayment

A synthetic roof underlayment is a water-resistant sheet which is made from woven or spun polyethylene or polypropylene also known as synthetic polymers. This material is laminated into layers and installed over the roof deck using mechanical fasteners such as plastic cap nails. Though synthetic underlayment provides a cost effective, light-weight option for moisture resistance, this underlayment faces numerous challenges in high alpine environments including but not limited to tearing or puncturing under pressure and moisture infiltration from ice dam related capillary action and snow melt. The moisture infiltration can commonly be seen between the overlaps of the underlayment sheets, or through the mechanical fastener locations.

Peel and Stick Underlayment

Peel and stick underlayment is a type of roof underlayment which features an adhesive backing which is commonly derived from modified asphalt or butyl. This underlayment form provides numerous benefits including ease of installation (no fasteners needed depended on underlayment, slope, and roof design), malleability around critical roof areas, and durability under extreme pressure. Additionally, and most importantly for the Big Sky and Bozeman community, self-sealing and self-adhering underlayment provide superior moisture protection for your home through its strong bond to a roof’s deck, lack of fastener points, and adhesion between the overlaps of the underlayment sheets. Due to this, peel and stick underlayment are the preferred underlayment choice for alpine environments.

When is a butyl-based underlayment needed?

Butyl-based underlayment are specifically formulated to withstand temperatures up to 300-degrees Fahrenheit. This high temperature threshold allows for butyl-based underlayment to be a necessary option for unventilated roofing systems within a high alpine environment which experiences extreme UV exposure. Additionally, butyl-based underlayments are highly recommended for roofs which have metal panels created from copper, zinc, or cor-TEN which tend to readily conduct heat to the underlayment making them more likely to expose the membrane to extreme temperatures. Use of a non-butyl-based underlayment on an unventilated roof assembly or under copper, zinc, or cor-TEN metal panels can cause the underlayment to melt under extreme heat and not only create a sticky mess, but also create a potential failure in your roofing system.

For more information on underlayment needs for Big Sky and Bozeman’s high alpine environment, please reach out to Cornerstone Management Services, your local expert for high alpine roof assemblies.



Written by: 

Paul Williamson — Director of Business Development





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