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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.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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