What is Underlayment?

More roofing definitions

Most people know that every house needs a good foundation, but what about a roof? Underlayment, an extra layer of protection on top of the sheathing, has become so important that shingle manufacturers often require it to validate their warranties. Additionally, many state and city codes deem it necessary to meet fire, weather, puncture, and other protective standards. The main purpose of an underlayment is to provide an extra layer to safeguard a home from extreme weather and the elements.

Traditionally, underlayment has been constructed from felt. Also known as felt paper, it bonds with asphalt via coating or saturation in order to form an extra barrier between the roof and the house. Felt varies by weight and costs less than other underlayments. Although more widely used than some of the newer materials, felt is less resistant to moisture problems, overheating, and tearing.

Two new materials are growing in popularity in their use in underlayment (although they typically cost more than more felt paper.) The first is synthetic polymers, which are billed as holding up under all sorts of abuse and being safer for roofers to work with. They also resist mold and water damage and weigh less than other substances. The other roofing trend is the growing popularity of "complete roofing systems". These include an integrated weather barrier, underlayment, and shingles or tiles. Manufacturers reward buyers of these systems with longer warranties that offer better coverage.

A fourth option is the self-adhesive underlayment. It forms a water and ice barrier in areas affected by extreme weather conditions. It may be described as an underlayment beneath another underlayment, because a synthetic or felt layer is applied over its top and before the application of shingles or tiles. This layer is heavy duty and seals seamlessly with no nails or staples. It also closes tightly around any fasteners used to install the rest of the roof.

If you plan to build a new home or replace an old roof, you should carefully consider your options for underlayment materials and methods. Since there are more options available and the industry keeps growing, you'll want to make your decision based on your budget, the number of years you plan to live in the house, the climate, and the design of your house.