When the roof-covering material is being installed, the underlayment takes a beating and may be damaged by footfall or other materials.

In the future, asphalt-saturated felt underlayment will probably be used less and, by 2014, it will likely no longer be installed at all.  Asphalt is basically the residue left over from the process of refining crude oil.  As the price of oil has increased, refining techniques have been developed that extract the maximum amount of high-quality products from the crude.

These techniques, involving the use of  coker units, result in a residue of powder instead of the sludge from which asphalt is normally produced. With less asphalt being produced, an allocation program has been established for which the asphalt produced each year is allocated in limited amounts to manufacturers of asphalt shingles and underlayment.

Since shingles produce a higher profit margin than underlayment for the amount of asphalt used, most manufacturers are phasing out asphalt-saturated underlayments in favor of synthetic underlayments.   Although they fluctuate with raw material prices, as of 2010, prices for felt and synthetic underlayments were similar.

RUBBERIZED ASPHALT

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 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 situations.  Some underlayments are designed to resist heat up to 250° F without degradation of the adhesive.  This allows them to be installed under metal roofs an in harsh environments. 

The asphalt may be polymer-modified.

Polymer-Modified Bitumen

The terms "modified bitumen" is often used when referring to asphaltic roofing materails.  Sometimes, this term is 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-like 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.

NON-BITUMEN SYNTHETICS

Advantages

Like other underlayment materials, the use of synthetics has both advantages and disadvantages. 

Among their advantages include their light weight and high strength.  They are also typically non-skid.

Synthetics are resistant to fungal growth and are wrinkle-free, since they don’t absorb moisture. Although they can be designed as moisture-permeable, they are typically considered moisture barriers.

They’re also very resistant to UV damage and can be left exposed to weather for periods from six months to a year, depending on the manufacturer’s recommendations.

Disadvantages

As of 2010, there are some concerns with synthetic underlayment. According to the National Roofing Contractors Association:

• To date, there are no applicable ASTM standards for these products.
• Many synthetic underlayments don’t meet current building code requirements
• Use of these underlayments may void some manufacturers' material warranties for certain roof coverings (such as asphalt shingles).

• Wicking can be more of a problem than with felt underlayment. Installation along the roof eave is different with some types of synthetics.
• If the installer fails to read and follow the manufacture’s installation instructions and instead installs it like they would if they were using felt, they may create moisture problems.

Slope Limitations 

Slope limitations will vary by manufacturer. Some specify a greater overlap for low-slope roofs, and some don’t.

Roof Edges

To avoid problems from wicking moisture, many synthetic underlayments are designed to wrap around the roof edge and protect the edges of the roof sheathing. The edge metal is installed over the underlayment at both the eaves and rakes.

Fastening

 

Fall-Arrest Systems

Roofing (consumer-targeted)