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What is roof underlayment?

Roof underlayment is a water-resistant layer laid on the bare roof deck, under the shingles, as a second line of defense against wind-driven rain. It is most often asphalt-saturated felt, a self-adhering membrane or a synthetic sheet, and building codes ask for a second layer, or one self-adhering layer, on slopes of 2:12 up to 4:12.

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By InstaBids editorial team with AI research agents. Updated

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What does roof underlayment do?

Underlayment is a second layer of protection under the shingles: it shields the roof deck from wind-driven rain that gets past them, and it keeps the deck dry between the day it goes down and the day the shingles are applied. The Asphalt Roofing Manufacturers Association (ARMA) says so in its 2017 and 2025 technical bulletins: “It provides secondary protection for the roof deck from wind-driven rain and other moisture penetrations.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “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.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Underlayment helps keep the deck dry until shingles are applied, minimizing problems that may result if shingles are placed on wet decks.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30.

How does ARMA describe a roof underlayment?

Roof underlayment is the water-resistant sheet that goes between the bare wood roof deck and the shingles or other roof covering; ARMA, the trade association of asphalt roofing makers, describes it this way in the bulletins it posts for the public: “Asphalt shingles should not be installed directly on a wood deck roof substrate.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Rather, the deck should first be covered by a roofing underlayment.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Underlayment is water resistant.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30.

Is roof underlayment necessary?

Under asphalt shingles, yes: ARMA says it should always be used, most building codes call for it, and on slopes of 2:12 up to 4:12 codes want two layers or one self-adhering layer. A Charleston County, South Carolina form shows how the 2021 International Residential Code reads in a wind-design area: “Underlayment should always be used in new construction and reroofing as it provides many benefits to the roof system.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Most building codes require underlayment material meeting at least one of the industry standards referenced in this bulletin.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “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.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. In Charleston County, South Carolina, the county building department’s form for the 2021 code says: “Underlayment shall be two layers applied in the following manner: apply a 19-inch strip of underlayment felt parallel to and starting at the eaves.” Source: Charleston County, South Carolina, Building Inspection Services, read 2026-10-30. It continues with the 36-inch sheets: “Starting at the eave, apply 36-inch-wide sheets of underlayment, overlapping successive sheets 19 inches.” Source: Charleston County, South Carolina, Building Inspection Services, read 2026-10-30.

What is roof underlayment made of?

Traditional underlayment is paper or fabric saturated with asphalt, called felt; newer versions swap the reinforcement for inorganic fibers, which lets the maker use less asphalt. ARMA’s steep-slope bulletin describes the felt and fiber kinds: “Historically, asphalt-saturated organic felt has been specified and widely used as asphalt shingle underlayment.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Although water-resistant due to its asphalt content, the nature of the organic felt reinforcement permits some degree of vapor permeability and moisture absorption.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Similar in appearance and function to conventional asphalt-saturated organic felt underlayment, this product incorporates inorganic fibers in the reinforcement fabric.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “The use of inorganic fibers allows for a reduced asphalt content and may improve the product’s dimensional stability properties.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30.

What types of roof underlayment are there?

ARMA sorts shingle underlayment into three basic types, and felt comes in numbered weights such as No. 15 and No. 30 under ASTM D226; the self-adhering type has its own standard, ASTM D1970: “The most common type of asphalt shingle underlayment is asphalt-saturated felt.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “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.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “When applied as an underlayment, they are primarily used to help prevent water entry from ice dams at the eave areas of shingled roofs in cold climates.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “Self-adhering bituminous membranes are required by building codes to meet the requirements of ASTM D1970, and newer codes require these products to have a label indicating compliance with ASTM D1970.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30.

What is synthetic roof underlayment?

Synthetic underlayment is a polymer sheet fixed with fasteners rather than saturated paper; it is lighter than felt per roll and covers more roof per roll, and ASTM D8257 is the standard that names it. ARMA says: “There are three basic types of underlayment used under asphalt shingles: asphalt-saturated felt, self-adhering underlayment, and non-asphaltic synthetics (polymers).” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30. “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.” Source: Asphalt Roofing Manufacturers Association (ARMA), read 2026-10-30.

Does roof underlayment go under the drip edge?

It depends on the edge: at the eaves, underlayment goes over the drip edge, and at the rakes (the sloped gable edges) it goes under the drip edge, as the International Code Council’s 2012 residential code guide and the Texas Department of Insurance both explain: “Underlayment shall be installed over the drip edge along eaves and under the drip edge on gables.” Source: International Code Council (ICC), read 2026-10-29. “At the rake edges (the sloped edges that are referred to as “gables” in the code text), the drip edge must be installed over the underlayment.” Source: International Code Council (ICC), read 2026-10-29. “The roof underlayment must be installed over the drip edge along the eaves and under the drip edge at the rake edges of the roof.” Source: Texas Department of Insurance, Windstorm Inspections, read 2026-10-29.

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