Roofing Underlayment Requirements Under IRC 2018

Quick Answer

Yes — IRC 2018 requires underlayment beneath asphalt shingles per Section R905.2.7. For slopes of 4:12 and above, a single layer of ASTM D 226 Type I felt or ASTM D 4869 Type I or II synthetic underlayment is required. For slopes between 2:12 and 4:12, two layers of underlayment with the second layer lapped 19 inches over the first are required. In areas where the average daily temperature in January is 25 degrees Fahrenheit or less — indicated on the ice barrier map — ice and water shield (self-adhering polymer-modified bitumen membrane) must extend from the eave edge to a point at least 24 inches inside the exterior wall line. Ice and water shield is required as a substitute for standard underlayment in this ice-dam-prone zone. Other roofing material types have their own underlayment requirements in R905.3 through R905.10.

What R905.2.7 Actually Requires

Section R905.2.7 of IRC 2018 Chapter 9 specifies the underlayment requirements for asphalt shingle roofing. Underlayment is required under all asphalt shingle installations as a secondary water barrier that protects the roof sheathing and structure if water infiltrates behind a damaged or lifted shingle. Without underlayment, a single damaged shingle or a temporary wind-lifted course allows rainwater to contact the bare roof deck, which can cause sheathing deterioration and structural damage before the problem is diagnosed and the shingle is repaired.

For roof slopes of 4:12 or greater, the minimum underlayment is one layer of ASTM D 226 Type I felt (standard 15-pound felt) applied shingle-fashion with horizontal courses from eave to ridge, each course lapping the previous by at least 2 inches. The laps must be positioned with the upper course over the lower to shed water rather than trap it. Self-adhering synthetic underlayments meeting ASTM D 4869 Type I or II are widely used as alternatives to felt and provide improved tear resistance and moisture resistance during the construction period before shingles are applied.

For roof slopes between 2:12 and 4:12, the code requires a double-layer underlayment application where the first layer is applied as a starter course and the second layer is applied with 19-inch exposure — meaning the second layer laps 19 inches over the first course before the next course is started. This double-layer application provides significantly greater moisture resistance at the lower slopes where water has more opportunity to infiltrate under wind-driven rain conditions.

In ice-dam-prone areas — defined as locations where the January mean daily temperature is 25 degrees Fahrenheit or less — ice and water shield membrane must be applied at the eave area from the eave edge to a point at least 24 inches inside the exterior wall line. The ice and water shield serves as both the underlayment and an impermeable moisture barrier in the zone where ice dams form. Ice dams occur when snow melts at the warm roof surface near the ridge and refreezes at the cold eave overhang, creating a dam of ice that backs water under the shingles. The self-adhering membrane provides waterproofing even when water is forced upward under the shingles by hydrostatic pressure from the ice dam.

Why This Rule Exists

Underlayment provides secondary protection at the roof surface in the event that the primary roof covering is damaged, lifted by wind, or inadequately sealed at a specific location. Asphalt shingles are the primary weather barrier but they are a granular, tab-based product that can be lifted, cracked, or displaced by extreme weather conditions. The underlayment layer beneath the shingles provides a continuous moisture-resistant surface across the entire roof deck that maintains water-shedding function even when the shingle layer above it is temporarily compromised. In extended construction delays between sheathing and roofing, the underlayment also provides temporary weather protection for the sheathing and structural members.

Ice and water shield requirements address a specific cold-climate failure mode — ice damming — that can cause water infiltration even through a properly installed shingle field. The membrane provides impermeable protection at the eave zone where ice dam water tends to pool and back up under shingles. Without ice and water shield in these locations, ice dam water infiltration can cause significant interior damage that appears as ceiling stains and wet insulation in the rooms adjacent to exterior eaves.

What the Inspector Checks at Rough and Final

At the roof covering inspection, the inspector verifies that underlayment is installed across the full roof area under the shingles and that the application method matches the slope — single layer for 4:12 and above, double layer for 2:12 to 4:12. The inspector also checks that ice and water shield is installed at the eave area in applicable climate zones. Ice and water shield is typically a different color or texture than standard felt — many products are a dark polymer membrane visible at the eave edge. The inspector checks that the shield extends at least 24 inches inside the exterior wall line, not just to the wall face.

The inspector also verifies that the underlayment at the roof is a listed product — ASTM D 226 felt or ASTM D 4869 synthetic. Non-listed substitute materials such as house wrap, building paper intended for wall applications, or non-rated films are not acceptable as roofing underlayment.

What Contractors Need to Know

Install underlayment immediately after sheathing is placed on each roof section to provide temporary weather protection. A roof left as bare sheathing without underlayment through a rain event absorbs moisture that must be dried before roofing is applied. Apply underlayment in courses from eave to ridge with horizontal laps in the correct direction — upper course over lower. Nailing or stapling underlayment at the recommended frequency (approximately 12 inches at laps and 24 inches in field) prevents the underlayment from blowing off in the wind before shingles are applied. In high-wind conditions during roofing, use cap nails or cap staples that provide better wind resistance than standard roofing tacks at the underlayment.

Verify whether ice and water shield is required at the eave for the project climate zone. Use the January mean temperature map in Figure R301.2(1) or the ice barrier requirement designation for the county to confirm whether the requirement applies. In border counties near the 25-degree isotherm, confirm with the local AHJ. If ice and water shield is required, apply it before any felt underlayment at the eave and extend it at least 24 inches inside the exterior wall line measured from the interior face of the exterior wall, not from the exterior face.

For re-roofing over existing shingles where full underlayment replacement is not possible, the existing shingle layer acts as a secondary water barrier but does not technically satisfy the underlayment code requirement for new roofing. Most re-roofing projects install new underlayment in areas where existing shingles are removed and accept the existing layers as the underlayment equivalent in areas where no tear-off occurs. Verify the local AHJ policy on underlayment for re-roofing over existing shingles.

What Homeowners Get Wrong

Homeowners frequently assume that synthetic underlayment and standard felt are interchangeable in all applications. Synthetic underlayment products vary significantly in performance — some are rated equivalent to No. 30 felt while others are equivalent to No. 15 felt, and the ratings are not always clearly communicated in the marketing. Verify the ASTM D 4869 Type designation of the selected synthetic product and confirm it meets the requirement for the intended application.

Another common error is homeowners in borderline cold climates assuming ice and water shield is not needed because they rarely see ice dams. Ice dams form during specific weather events — warm spells followed by cold snaps that freeze meltwater at the eave — and the damage from a single significant ice dam event can far exceed the cost of ice and water shield that would have prevented it. In climate zones near the 25-degree threshold, installing ice and water shield is strongly recommended even where not strictly required.

Homeowners also sometimes leave underlayment exposed to UV for extended periods during a construction delay, assuming it will remain adequate protection. Standard ASTM D 226 felt loses its moisture resistance and physical integrity within 30 to 60 days of UV exposure — exposed felt tears easily, allowing water to penetrate through the deteriorated membrane. If underlayment must be left exposed for more than 30 days, use a UV-resistant synthetic underlayment product rated for extended exposure or cover the underlayment with a temporary tarp.

State and Local Amendments

IRC 2018 states including TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO adopted R905.2.7 underlayment requirements. The ice and water shield requirement applies in the colder portions of VA, NC mountains, KY, and MO based on the January mean temperature map. The warmer portions of these states — GA, SC, AL, MS, coastal TX, and lower elevations of TN — are generally in the no-ice-barrier zone. States adopting IRC 2021 should verify whether the ice barrier zone designation changed for specific counties, as the map may have been updated. Some coastal jurisdictions require enhanced underlayment specifications for high-wind applications — verify local requirements for coastal construction.

Some jurisdictions have adopted Energy Star Reflective Roofing requirements or cool roof requirements that may affect underlayment compatibility with specific shingle colors or products. Verify local energy code requirements for cool roof products if the project is in a high-cooling-load climate zone.

When to Hire a Licensed Contractor

Underlayment installation is part of the roofing contractor scope. A licensed roofing contractor should perform underlayment and shingle installation together as a coordinated system. For projects in ice-dam-prone areas, the roofing contractor must be experienced in ice and water shield application and integration with the felt underlayment courses above the shield. The transition from ice and water shield to felt underlayment above the shield must be weather-lapped correctly to prevent water infiltration at the seam between the two materials.

Common Violations Found at Inspection

• No underlayment installed beneath shingles — shingles applied directly to bare sheathing without the required moisture-resistant secondary barrier.
• Single-layer underlayment on a 3:12 slope where double-layer underlayment is required for slopes between 2:12 and 4:12.
• Ice and water shield not installed at the eave in a climate zone where it is required — standard felt used at the eave where self-adhering membrane is required.
• Ice and water shield installed only to the exterior wall face rather than extending 24 inches inside the exterior wall line as required.
• Non-listed material used as underlayment — house wrap, building paper rated for wall applications, or plastic sheeting applied under shingles without ASTM D 226 or D 4869 listing.
• Underlayment laps installed in the wrong direction — lower course over upper creates a water trap at each lap joint rather than the required shed-water geometry.
• Underlayment exposed to UV for extended periods and deteriorated to the point of inadequate moisture resistance before shingles are installed.