Understanding the 2021 Residential IRC Roofing Code (Auburn, AL)

Local note: The City of Auburn, Alabama uses the 2021 International Residential Code (IRC). This version became effective January 1, 2022 and applies to residential permits submitted on or after that date. Always verify project-specific requirements with the City’s Inspections/Building department.

Chapter 9 at a glance

• Chapter 9 governs residential roof assemblies: materials, application, weather protection, flashing, and special details.

• Roof coverings must have an appropriate fire rating (Class A/B/C) as required by local conditions and any wildland-urban interface rules.

• Roofs must shed water and be detailed to keep water out at all penetrations and intersections.
• Flashing: required at walls, chimneys/curbs, skylights, valleys as applicable; install to direct water to the exterior and integrate with underlayment & roof covering.
• Drip edge (referenced in R905.2 for shingles): at eaves & rakes; laps and fasteners per section; placed under eave underlayment and over rake underlayment for correct water path. (General outline drawn from IRC 2021 summaries.)

• Roofing materials must comply with the referenced standards (ASTM/UL, etc.) for that product type and be installed per manufacturer’s instructions and code.

R905 — Roof coverings (by material)

• Underlayment types are standardized by ASTM; install per slope, climate, and manufacturer.
• Ice barrier is required in ice-dam regions (self-adhered membrane at eaves to a specified distance upslope). Many Deep South jurisdictions do not require it, such as in Auburn, AL.

• Minimum slope: shingles permitted on slopes ≥ 2:12. From 2:12 up to 4:12, a special double underlayment (or equivalent) is required; at ≥4:12, standard underlayment applies.
• Nailing: per manufacturer and section; typical 4 fasteners per shingle (6 in designated wind zones). Place nails in the specified nailing zone—too high = lost seal & blow-offs.
• Valleys & flashing: closed/open valley methods allowed per section; integrate step/headwall/chimney flashing correctly.
• Drip edge: required at eaves and rakes; specific lap and fastening pattern improve wind/water performance (see R905.2.8.x).

• Applies to structural and architectural panels (e.g., standing seam, classic rib). Minimum slopes depend on profile and seaming; sealant/underlayment per manufacturer & code.
• Fasteners, clips, and substrates must match the approved system; mind dissimilar metals & coastal corrosion.

• Deck prep, membrane thickness, attachment (fully adhered, mech-fastened, or ballasted) must meet the listed standard and manufacturer’s details.

• Similar to EPDM: substrate, insulation, fastener patterns, seam welding, and edge metal follow the listing and manufacturer’s instructions; terminations and penetrations are critical.

• Clay/concrete tile, metal shingles, mineral roll, slate/wood shingles & shakes, modified bitumen, SPF, liquid-applied, PV shingles/panels each have specific standards and slope/underlayment rules—check the section for your product and follow the listing exactly.

R806 — Roof Ventilation (Attic/Ceiling Spaces)

• Baseline rule: Minimum net free ventilating area (NFVA) = 1/150 of the attic floor area (in square feet).
• Exception to 1/300: You may reduce to 1/300 if either:
  • a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling, or
  • the system is balanced with intake and exhaust (roughly 40–50% high, balance low) and high vents are at least ~3 ft above the eave vents.
• Balance: Split NFVA between intake (soffit) and exhaust (ridge/gable). A 50/50 split is a good target unless the manufacturer specifies otherwise.
• Baffles: Provide eave baffles at vented soffits so insulation doesn’t block airflow.
• Unvented attics: Allowed under R806.5 with air-impermeable insulation and specific detailing—different rules than above.

Quick NFVA Math (example)

• Attic footprint: 2,000 ft²
• At 1/150: 2,000 ÷ 150 = 13.33 ft² NFVA total = 1,920 in² (×144). Aim ≈ 960 in² intake + 960 in² exhaust.
• At 1/300 (if qualified): 2,000 ÷ 300 = 6.67 ft² NFVA = 960 in² total (≈ 480 in² intake + 480 in² exhaust).
• Convert to linear feet: Use the vent’s rated NFVA per linear foot (e.g., ridge vent 18 in²/lf → 960 ÷ 18 ≈ 53 lf ridge vent).

What homeowners should know

• Slope drives the rules. Low slope often means upgraded underlayment or different systems entirely.
• Underlayment isn’t optional. It’s the last water-shedder if a shingle or seam lifts.
• Flashing beats caulk. Proper metal flashings (and drip edge) are code-required because they last.
• Manufacturer instructions + code = the law. We install to both.

Sources & further reading

• IRC Chapter 9 Roof Assemblies.
• IRC Chapter 8 R806 Roof Ventilation.