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Home / Code Guides / Roof Underlayment & Ice Barrier

Ice Barrier & Roof Underlayment Code — Where It Goes and How Far

Two questions decide a shingle roof underlayment detail: how steep is it, and does the jurisdiction have a history of ice forming at the eaves. The first sets how many layers, the second adds a membrane.

RequirementModel Code StandardSection
Minimum slopeAsphalt shingles may not be installed on slopes below 2:12IRC R905.2.2
Low-slope underlaymentFrom 2:12 up to 4:12, two layers of underlayment are required — a 19 in. starter course at the eave, then 36 in. sheets lapped 19 in., with 4 in. end laps offset at least 6 ftIRC Table R905.1.1(2)
Normal slopeAt 4:12 and steeper, a single layer lapped 2 in. is permittedIRC Table R905.1.1(2)
Underlayment tablesUnderlayment type, application and fastening are set by material in Tables R905.1.1(1), (2) and (3)IRC R905.1.1
Ice barrierWhere the jurisdiction has a history of ice forming along the eaves, an ice barrier of two cemented layers of underlayment or a self-adhering polymer-modified bitumen sheet is required from the lowest edges of all roof surfaces to a point not less than 24 in. inside the exterior wall lineIRC R905.1.2
Steep roofsOn slopes of 8:12 and greater the barrier must also extend not less than 36 in. measured along the roof slope from the eave edgeIRC R905.1.2
TriggerWhether an ice barrier is required at all is a jurisdiction entry in Table R301.2, alongside frost depth and design temperaturesIRC Table R301.2
FasteningUnderlayment fasteners follow a 12 in. and 6 in. grid using annular or deformed-shank nails with 1 in. caps, penetrating at least 3/4 in.IRC Table R905.1.1(3)

Worth knowing

  • There is one extent dimension for the ice barrier, not two. The model text measures to a point 24 in. inside the exterior wall line and says nothing about a horizontal projection. Some jurisdictions amend the word “horizontal” in, which changes the answer on a low-slope overhang.
  • The 36 in. along-slope requirement at 8:12 and steeper is in addition to the 24 in. rule, not an alternative to it.
  • Detached accessory structures with no conditioned floor area are excepted from the ice barrier requirement.
  • The 2021 edition renumbered the climatic table from Table R301.2(1) to Table R301.2. Many jurisdiction handouts still print the old label, so a citation mismatch is not necessarily an error.
  • The three underlayment tables replaced the old per-material text starting in the 2018 IRC, so guidance written against the 2015 edition points at sections that no longer exist.

Common questions

What does the code say about minimum slope for Roof Underlayment & Ice Barrier?

Asphalt shingles may not be installed on slopes below 2:12. (IRC R905.2.2)

What does the code say about low-slope underlayment for Roof Underlayment & Ice Barrier?

From 2:12 up to 4:12, two layers of underlayment are required — a 19 in. starter course at the eave, then 36 in. sheets lapped 19 in., with 4 in. end laps offset at least 6 ft. (IRC Table R905.1.1(2))

What does the code say about normal slope for Roof Underlayment & Ice Barrier?

At 4:12 and steeper, a single layer lapped 2 in. is permitted. (IRC Table R905.1.1(2))

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

Related sections and guides

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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