Mastering Roof Inspections: Underlayment, Part 3

Various types of rubber-like materials are also used as underlayment and are generally referred to as “rubberized asphalt.” These typically have adhesive on one side, which is protected by a peel-off membrane, making them self-adhering. The rubber-like qualities of these underlayments also make them self-sealing, meaning that they seal well around fasteners, such as staples and nails.

Rubberized asphalt underlayments are manufactured to meet different requirements.

• They may have polyethylene or polyester bonded to the upper surface to provide non-skid and weather-resistant qualities.
• They may have a polymer film bonded to the weather surface to improve moisture resistance.
• They may be fiberglass-reinforced.
• They may have a mineral coating on the weather surface.
• They may be formulated for use in high-temperature settings. Some rubberized asphalt underlayments are designed to resist heat up to 250º F without degradation of the adhesive. This allows them to be installed under metal roofs and in harsh environments.
• The asphalt may be polymer-modified.

Polymer-Modified Bitumen

The term “modified bitumen” is often used when referring to asphaltic roofing materials. Sometimes, it’s shortened to “mod-bit.” The term “bitumen” is a generic name applied to various mixtures of hydrocarbons. One of these mixtures is the asphalt used in underlayment, asphalt shingles, and built-up roofing. It’s a common term in the roofing industry.

To improve various characteristics, such as strength and elasticity, bitumen is sometimes modified using polymers, which give it plastic- or rubber-like properties, depending on which process is used.

Polymers are materials made of molecules which are custom-designed to give the material specific properties. Polymers are used in many different types of roofing products to increase their resistance to damage and deterioration.

You may also hear the term “cross-linked polymer” used.  Molecules in cross-linked polymers actually bond to each at the atomic level; they share atoms, which greatly increases the strength of the material.

Selvedge Edge

Rolls of rubberized asphalt underlayment may come with a selvedge edge along one side of the roll. The selvedge edge is designed to create a strong, watertight seal along the edges where rolls overlap. The selvedge edge should always be along the top edge when the underlayment is installed in courses across a roof.

Ice and Water Shield