Roof underlayment is an essential part of the complete roof system that provides a weather-resistant backup barrier beneath the roofing surface materials. In this article, we’ll take a look at popular roofing underlayments and how they protect and contribute at every stage of your roof’s life cycle.
Roof Underlayments and the “Dry In” Phase
The underlayment plays an essential role early in new construction projects by repelling water and protecting the roof decks in the initial construction stage. Builders and roofers call this the “dry in” phase. That’s short for “dry inside”.
Ideally, underlayments are covered and protected immediately by installing the new roofing surface materials as quickly as possible. But that doesn’t always happen in real-world conditions. Inclement weather, contractor availability, and material backorder delays can all leave the underlayment as the only roofing protection for extended periods of time.
The majority of building codes require a fresh layer of new underlayment for re-roofing projects as well. Aging roofing materials or those lost to wind uplift such as asphalt shingles, wood shakes, and clay or cement tiles can leave the underlayment prone to tears and wind liftoff, exposing the roof deck below to water damage and leaks into the home.
Understanding Permeance and Underlayment
Underlayments continue to contribute to roofing resilience long after the “dry in” phase, by providing an essential “permeability” characteristic to the roof system. Permeance is the ability to pass water vapor while resisting liquid water penetration.
Permeance ratings measure the rate at which water vapor (as opposed to liquid water) passes through a material. In general, underlayments should have a rating of at least 5 perms.
Vapor permeable, or “vapor retardant” underlayments help enhance energy efficiency with the free flow of air between the roof material and the roof deck. Air contains moisture and the underlayment repels any condensation occurring in the air space under the roofing surface materials.
3 Common Types of Roofing Underlayments
- Asphalt-saturated felt, commonly known as “tar paper”
- Rubberized asphalt “peel and stick” membranes
- Non-bitumen (no asphalt) synthetics similar to modern house wraps (sometimes referred to as “roofing membranes”)
When energy savings are a priority, advanced underlayments are available that can replace the 3 basic types above to supercharge the energy efficiency of your roof by up to 218%.
Asphalt Felt Underlayment aka Tar Paper
You may have heard the terms “15-pound felt” or “30-pound felt” in reference to asphalt felt underlayments. These are holdovers from the days when asphalt-impregnated paper actually weighed 15- or 30-pounds per 100 square feet (1 square). Today they are much lighter and are more accurately called #15 (number 15) or #30 felt, or Type I and Type II. These are industry terms for what the general public refers to as tar paper.
In the construction industry, asphalt felt still sets the standard for underlayment. Most experienced builders and roofers still prefer asphalt felt over synthetics because it’s widely available, inexpensive, easy to install, and has a proven track record as an efficient water barrier.
A variety of rubber-like materials used as underlayment fall into the category of “rubberized asphalt”. These feature a peel-off film that covers the adhesive side that makes them self-adhering. Like rubber, these materials are “self-healing”, an excellent property for sealing well around fasteners. “Peel and stick” rubberized underlayments have many uses and special formulations for applications in different areas of the roof, including:
- Ice barrier underlayment at the eaves (edges) where ice can form.
- High water flow management in areas such as valleys where roof planes meet.
- Self-sealing around roof penetrations such as skylights, roof-to-wall intersections, and plumbing vents.
- Polyester or polyethylene bonding can provide a non-skid surface for better traction for the roofing crew.
- High-temperature formulated to resist heat up to 250 degrees without losing adhesion.
Synthetic Roof Underlayment
One of the major performance advantages of synthetic underlayments is that they have a much higher tear resistance than tar paper. This is important for slow-to-install, labor-intensive roofing materials such as slate and clay tile. Synthetic underlayments can maintain a “dry in” watertight condition for months when the roof can’t be installed immediately.
Additionally, some synthetics provide better traction for roofing crews working on steep-sloped roofs and your roofing contractor may choose it solely for the improved safety factor.
One of the drawbacks of synthetic roof underlayments is the cost. They can cost twice as much as #15 felt and vapor permeable synthetic underlayments can cost between 4 and 20 times as much as #15 felt.
Don’t be concerned that your roofer is cutting corners by preferring asphalt felt over expensive synthetic underlayments. Your roofing professional may have good valid reasons for sticking with the traditional felt underlayment as synthetics have many shortcomings, including:
- Prone to cause leaks when punctured by staples or common roofing fasteners due to capillary action.
- Allowing the “wicking” of moisture at overlaps which must be specially caulked rather than sealed with traditional black roofing cement.
- Degrading under exposure to UV light. Once this occurs, heat and oxidation continue to degrade the material long after the underlayment is covered by the roofing materials.
- Requiring special tools and additional labor, unlike asphalt felt that can be installed quickly with a common hammer stapler.
- Preventing “drying upward” of the roof so they should only be used over ventilated spaces such as cathedral ceilings or well-ventilated attics that allow downward drying.
Underlayments and DECRA Metal Roofing
Whatever underlayment is used as part of your DECRA roof, it’s important to ensure that you're in full compliance with local building codes. When underlayment materials aren’t covered in the code, always consult and follow the underlayment manufacturer’s instructions to avoid nullifying the warranty coverage.
Your roofing professional and our detailed DECRA Installation Guides can provide the expertise you’ll need to tailor your underlayment choices to your roof. If you have questions, contact us and an expert DECRA roofing advisor will be happy to help.