There are three basic types of underlayment used beneath roofing materials:

  • asphalt-saturated felt;
  • rubberized asphalt; and
  • non-bitumen synthetic.


One of the most common types of underlayment used in residential, steep-slope applications is black, asphalt-saturated felt paper.  Felt underlayment may be made from either organic or fiberglass substrate, although the organic is much more common.  It's called "organic" underlayment because it has a cellulose base.

Felt underlayment is Water-resistive, but not waterproof.  It’s available in two thicknesses:  15-pound and 30-pound.  Fifteen-pound felt has a perm rating of about 5, although this number can rise in high-humidity conditions.

Thirty-pound felt is more resistant to damage during installation of the roof-covering material and will protect the roof longer if it should somehow become exposed to weather. The difference is obvious, once you see them together. Thirty-pound felt is much thicker and stiffer.

RUBBERIZED ASPHALT (self-adhering/self-sealing)

Various types of rubberlike 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 rubberlike 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 materials. 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.

You may also hear the term “cross-linked polymer” used.  Molecules in cross-linked polymers actually bond to each other at the atomic level; they actually 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.


Non-bitumen synthetic underlayments are made from polypropylene or polyethylene. These synthetic polymers are also used to make a huge variety of other types of products, from food-storage containers and rope, to long underwear.

  • 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.

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).

Concerns from other sources include the following:

  • 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.

As an inspector, you are not responsible for identifying the type of underlayment, but it’s a good idea for you to know what types exist and some of their properties.

Although companies who manufacture synthetic underlayment may also manufacture similar-looking housewrap, housewrap does not meet roofing underlayment requirements. Housewrap installed as underlayment is a defective installation. Underlayment is usually thicker than housewrap. In the photo above, you can see the difference between the two.

Installing Synthetic Underlayment

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 is generally done with plastic caps or roofing nails. The use of staples is discouraged because synthetics are not self-sealing.

In summary, roofing underlayment is an essential component to the roofing materials' ability to withstand the elements, protect a home's interior, and prolong its service life. The more an inspector understands about a roof's components, the better he can spot problems and deficiencies during an inspection.


Slope Limitations:

In low-slope roofs, which include 2:12 up to 4:12, felt courses should overlap a minimum of 19 inches. This will provide a double layer of underlayment across the entire roof.

In steep-slope roofs (4:12 and steeper), the upper courses of felt underlayment should overlap lower courses by at least 2 inches. You can see the difference between the underlayment overlapped 19 inches on the roof to the right and overlapped 2 inches on the roof to the left. In Figure 1 the lower roof is low slope with a 19-inch overlap and the upper roof is steep slope with a 2-inch overlap.



Felt is usually fastened with staples, but in high-wind areas, plastic windstrips may be used along the edges to prevent tearing.

Felt may also be attached in high-wind areas using plastic caps. Plastic caps offer better wind resistance than staples and help prevent leakage through the holes made by the fasteners.


Edge Metal Laps:

Felt underlayment should overlap the edge metal at the eaves and be overlapped by edge metal on the rakes. This is also the case for rubberized asphalt underlayment, but not necessarily for synthetics.


Asphalt-saturated felt may fail for a number of reasons:

Poor Quality:

A number of ASTM standards exist which offer specifications for asphalt-saturated felt.

Many manufacturers produce asphalt-saturated paper labeled “Underlayment,” “15-lb.” or “30-lb.,” which do not comply with any standards, and which are often saturated to a lower level than an ASTM-compliant underlayment. These underlayments typically absorb water more readily, and fail sooner. Water absorption can cause wrinkling as the product expands. These wrinkles may telegraph through to roof-covering products, such as thinner asphalt shingles.

Water from the felt may be absorbed by the roof deck, which can cause problems with expansion and contraction of the deck.

You won’t be able to tell by looking whether a product complies with any standards, but if you see what looks like premature failure or distortion of the underlayment, it may be caused by sub-standard underlayment.

Loss of Volatiles:

Over time, volatile compounds in the asphalt will dissipate, and the underlayment will become more fragile and moisture-absorbent. This will happen more quickly when felt is exposed to heat. The source of heat may be a warm climate, a particular type of roof-covering material, or poor roof-structure ventilation.

UV Exposure

Anywhere felt underlayment is exposed directly to sunlight, UV radiation will accelerate its deterioration. These poorly-bonded shingles were attached with staples on a home located in a high-wind area.

Installation Damage

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