PART II: The Basic Homeowner’s Guide to Roofing Underlayment – The Different Types of Roofing Underlayment

For a roof system to perform optimally and for its service life to be maximized, the underlayment product to be used must be carefully selected. Improper selection of the underlayment may allow water leakage and moisture entrapment which can both cause premature deterioration of the concealed components, affecting the performance of the entire roof.

Types of Roof Underlayment

Below are the three basic types of underlayment used in residential roofing.

Felt Underlayment (Asphalt-Saturated Felt)

This is the most commonly used type of underlayment in residential applications, especially for homes with steep-sloped roofs. It can be made from either organic substrate or inorganic (fiberglass) substrate. Although the organic substrate is more common, the inorganic variety usually lies flatter and stands up better than the former.

Here are some of the characteristics of felt underlayment:

•    It is water-resistant but not waterproof.

•    Generally available in two thicknesses: 15# (pound) and 30# (pound). The 15# has a perm rating of about 5 but can rise in high-humidity conditions. Meanwhile, the 30# felt is known to be more resistant to damage as it is thicker and stiffer.

•    Felt underlayment provides little resistance to air and moisture vapor migration, so it is not an ideal air barrier.

•    Best used in conjunction with ice dam protection.

Synthetic Sheet Underlayment (Non-Bitumen Synthetics)

Synthetic plastic sheet underlayment is typically made from polyethylene, polyolefin, or polypropylene, which are materials that are also used in the production of other products such as containers and ropes. Popular for their durability, synthetic sheet underlayment also has the following characteristics:

•    Comes in thicknesses of between 8 and 30 millimeters.

•    Lightweight yet possesses notable strength.

•    Less susceptible to wind damage and less sensitive to cold weather installation compared to felt.

•    Lies flatter after being subjected to wetting once installed.

•    Most synthetic underlayment on the market is vapor resistant, but does not qualify as an air barrier.

•    Resistant to moisture preventing fungal growth.

•    Resistant to UV damage.

Self-Adhering Membrane Underlayment (Rubberized Asphalt)

This is a type of underlayment commonly consists of rubberized asphalt, but may also use butyl-based adhesive. It is a self-adhering peel-and-stick product designed to adhere securely to the roof deck, and seals around nails, staples and other fasteners. Among its other characteristics, it may have the following:

•    Polyester or polyethylene bonded to the upper surface, providing non-skid and weather-resistant properties.

•    Fiberglass reinforcement.

•    A polymer film bonded to the weather surface that helps improve its moisture resistance.

•    Mineral coating on the weather surface.

Now that you already have a general idea about the basic types of roof underlayment, tune in for the next post, coming soon, to learn about the installation intricacies and recommendations for each of them.