Asphalt shingles should not be installed directly on a wood deck roof substrate. Rather, the deck should first be covered by a roofing underlayment. This bulletin describes commonly available types of underlayment products, their respective industry specifications, and useful features and benefits. Always consult local building codes and the shingle manufacturer’s requirements regarding underlayment product selection and installation methods.
Asphalt-saturated organic felts:
Historically, asphalt-saturated organic felt has been specified and widely used as asphalt shingle underlayment. Although water-resistant due to its asphalt content, the nature of the organic felt reinforcement permits some degree of vapor permeability and moisture absorption.
ASTM D226, Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing. This standard defines requirements for both perforated built-up roofing felts (used in low-slope roof assemblies) and non-perforated steep slope underlayment. Perforated felt products are not suitable for use as shingle underlayment; non-perforated types were added to the standard to reflect the need for asphalt shingle and wood shake underlayment. The standard covers two types of underlayment, differentiated primarily by product weight: Type I, commonly known as No. 15 felt, and the heavier Type II, often referred to as No. 30 felt.
CSA A123.3, Asphalt Saturated Organic Roofing Felt. This Canadian specification, similar to the ASTM D226 standard, was originally intended to address perforated felts used in built-up roof construction, Type 1, but also includes non-perforated felt underlayment specifications, Type 2 and Type 3.
ASTM D4869, Standard Specification for Asphalt-Saturated Organic Felt Underlayment Used in Steep Slope Roofing. This standard was the first specifically addressing steep slope roofing underlayment. It covers four types (primarily differentiated by weight) and includes a water-shedding requirement (the “shower test”).
Self-adhering polymer-modified bituminous underlayment:
Self-adhering underlayment membranes are most frequently used as ice dam protection at roof eaves (see ARMA’s Technical Bulletin “Protecting Against Damage from Ice Dams” for additional information on ice dams). The polymer-modified asphalt compound in the product provides adhesion to the roof deck and at product overlaps and, most importantly, enables the membrane to seal around the shanks of roofing nails used in the overlying asphalt shingles. Should water from an ice dam “back up” and get beneath the shingle layer, it is unlikely to pass through the membrane layer even though numerous nails penetrate the membrane. These products are also excellent for use as secondary protection on ridges, hips and valleys, as well as around roof penetrations (e.g., vents, skylights, plumbing vent stacks, and chimneys). Ventilation becomes increasingly important when self-adhering underlayments are used over the entire roof deck. Self-adhering underlayments act as vapor retarders. The absence of adequate ventilation may contribute to premature deterioration of the roof system and roof deck, potentially resulting in water vapor condensation under the roof deck.
ASTM D1970, Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection. This ASTM standard covers ice and water eave protection materials.
CSA A123.22, Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection. This CSA standard is a Canadian adaptation of the ASTM D1970 standard.
Other underlayment types:
Although asphalt-saturated organic felts and self-adhering underlayments have been commonly used in steep slope roofing for decades, the following product standards reference alternative materials that are either relatively new to the industry or have been used in select markets and applications.
ASTM D6757, Standard Specification for Underlayment Felt Containing Inorganic Fibers Used in Steep-Slope Roofing. Similar in appearance and function to conventional asphalt-saturated organic felt underlayment, this product incorporates inorganic fibers in the reinforcement fabric. The use of inorganic fibers allows for a reduced asphalt content and may improve the product’s dimensional stability properties.
CSA A123.16, Asphalt-coated glass-base sheets. This is the CSA Canadian standard equivalent to the ASTM D6757 specification and is referenced in the National Building Code of Canada (NBCC) as an underlayment option for use beneath asphalt shingles.
ASTM D8257, Standard Specification for Mechanically Attached Polymeric Roof Underlayment Used in Steep Slope Roofing. Most commonly referred to as “synthetic” underlayments to differentiate them from traditional asphalt-saturated felts, these products are comparatively lighter than saturated felts yet typically cover a larger roof area per roll.
Benefits of underlayment:
Underlayment should always be used in new construction and reroofing as it provides many benefits to the roof system. Important reasons for installing underlayment when applying asphalt shingles over wood decks include:
- Most building codes require underlayment material meeting at least one of the industry standards referenced in this bulletin. Consult the relevant building codes to confirm roof system compliance. For example, section 1507.1.1 of the 2024 IBC and section R905.1.1 of the 2024 IRC contain specific requirements for underlayment materials, which vary based on the intended roof application. Some of the products listed in this bulletin may not be approved by applicable codes in certain areas. Section 9.26.6 of the NBCC 2020 addresses underlayment use in steep slope installation.
- Underlayment provides a secondary layer of protection beneath the shingles to shield the deck from wind-driven rain, helping prevent water from reaching the deck. This is why building codes require a double layer of mechanically fastened underlayment or a single layer of self-adhering underlayment on slopes of 2:12 up to 4:12.
- Underlayment helps keep the deck dry until shingles are applied, minimizing problems that may result if shingles are placed on wet decks. Shingles should be applied as soon after the application of the underlayment as possible.
- Underlayment is a required component in a fire-classified roof assembly.
- Installing underlayment helps reduce “picture framing” (the visible outline of deck panels caused by irregularities in roof decking thicknesses).
- Underlayment offers protection to the shingles from resins that may come from wood-based sheathing.
WARNING: Because it is dangerous to walk, climb, or work on a roof, ARMA recommends that only trained professionals engage in such activity. If you choose to do so, exercise extreme care, comply with all government safety regulations, and follow all safety work practices, precautions, and procedures, including but not limited to manufacturer’s instructions, labels, and warnings.
DISCLAIMER OF LIABILITY: This document was prepared by the Asphalt Roofing Manufacturers Association and is disseminated for informational purposes only. Nothing contained herein is intended to revoke or change the requirements or specifications of the individual roofing material manufacturers or local, state and federal building officials that have jurisdiction in your area. Any question, or inquiry, as to the requirements or specifications of a manufacturer, should be directed to the roofing manufacturer concerned. THE USER IS RESPONSIBLE FOR ASSURING COMPLIANCE WITH ALL APPLICABLE LAWS AND REGULATIONS.
Nothing contained herein shall be interpreted as a warranty by ARMA, either express or implied, including but not limited to the implied warranties of merchantability, fitness for a particular purpose or non-infringement. IN NO EVENT SHALL ARMA BE LIABLE FOR ANY DAMAGES WHATSOEVER, including special, indirect, consequential or incidental damages or damages for loss of profits, revenue, use or data, whether claimed in contract, tort or otherwise. Where exclusion of implied warranties is not allowed, ARMA’s liability shall be limited to the minimum scope and period permitted by law.
