Underlayment and Flashing, Roofing

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Underlayment Requirements and Standards

California Building Code Chapter 15 governs the installation of roof assemblies, specifically mandating underlayment as a secondary water barrier. For asphalt shingles, CBC 1507.1.1 requires underlayment to comply with ASTM D226 Type I or II, ASTM D4869 Type I, II, III, or IV, or ASTM D6757. On roof slopes between 2:12 and 4:12, a double underlayment application is mandatory. This involves applying a 19-inch wide strip of underlayment felt parallel to the eave, followed by full 36-inch wide sheets, each overlapping the previous by 19 inches. For slopes 4:12 and greater, a single layer of underlayment is sufficient, provided it is applied with a 2-inch head lap and 4-inch end lap. Fasteners must be corrosion-resistant and spaced to hold the material securely until the primary roof covering is installed. In areas prone to high winds, ASTM D226 Type II underlayment is preferred due to its higher tensile strength and resistance to tearing during the construction phase. Proper underlayment provides the critical moisture protection needed when wind-driven rain bypasses the primary shingles.

Vertical and Eave Flashing Mechanics

Flashing is the most critical component for preventing water intrusion at roof-to-wall intersections and penetrations. CBC 1503.2 requires flashing to be installed at wall and roof intersections, at gutters, and at changes in roof slope or direction. For vertical sidewall transitions, step flashing is the industry standard for shingles. Each piece of step flashing must be a minimum of 4 inches high and 4 inches wide, integrated into the courses of shingles as they are laid. This ensures that water is shed back onto the shingle surface rather than behind the siding or into the wall cavity. At the eave, a drip edge is required per CBC 1507.2.8.5. The drip edge must extend a minimum of 0.25 inches below the roof sheathing and extend up back onto the roof deck at least 2 inches. Drip edges must be mechanically fastened at maximum 12-inch intervals. Where the roof meets a chimney or other large masonry structure, counter-flashing must be regleted into the mortar joints to provide a redundant seal over the base flashing.

Valley Flashing and Cricket Transitions

Valleys are high-volume water channels that require specific flashing materials and dimensions to prevent erosion and leaks. CBC 1507.2.9.2 outlines the requirements for valley linings based on the material used. For open valleys lined with metal, the flashing must be at least 15 inches wide and made of corrosion-resistant metal, such as 26-gauge galvanized steel or 16-ounce copper. For closed-cut or woven valleys, an underlayment lining is still required, typically consisting of one ply of ASTM D226 Type II felt or a 36-inch wide layer of self-adhering polymer-modified bitumen. If using a mineral-surfaced roll roofing for a valley liner, it must meet ASTM D3909 and be at least 36 inches wide. Crickets or saddles are required on the ridge side of any chimney or penetration wider than 30 inches, as per CBC 1503.6. These diversions must be flashed similarly to the roof-to-wall intersections, ensuring water is directed away from the penetration and toward the main drainage path.

Ice Barriers and Penetration Flashing

In regions of California designated by the local building official as prone to ice damming, usually at higher elevations, an ice barrier is required per CBC 1507.2.8.2. This barrier must consist of at least two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet. The barrier must extend from the lowest edges of all roof surfaces to a point at least 24 inches inside the exterior wall line of the building. This prevents water that backs up behind ice dams from penetrating the roof deck and damaging the interior. Additionally, all roofing penetrations, such as vent pipes (CPC Chapter 5 and CBC 1503.2), must be flashed with a boot or lead flange. For plumbing vents, the CPC requires the vent to extend at least 6 inches above the roof surface and be sealed with a flashing that is integrated into the shingle courses to ensure a watertight transition.

Fastening and Safety Compliance Standards

The integrity of the roof system depends heavily on the fasteners and the substrate preparation. Shingles and flashing must be secured with corrosion-resistant nails, typically 12-gauge with a 3/8-inch head. Under CBC 1507.2.6, fasteners must be long enough to penetrate through the roofing material and at least 0.75 inches into the roof sheathing. If the sheathing is less than 0.75 inches thick, the fasteners must penetrate entirely through the sheathing. For C-39 Roofing Contractors, compliance with Cal/OSHA Title 8, Section 1724 is mandatory regarding fall protection and material handling. When working on roofs with slopes greater than 4:12 and a height exceeding 20 feet, contractors must provide personal fall arrest systems, guardrails, or safety nets. Furthermore, B&P Code 7058 defines the C-39 license as specifically for roofing, and any project exceeding $500 (labor and materials) requires a valid license. Failure to follow these codes can lead to CSLB citations and civil liability for water damage.

Specialized Underlayment for Wood Systems

Wood shake and wood shingle roofs have unique underlayment requirements compared to asphalt. According to CBC 1507.8 and 1507.9, wood shakes must be installed with an interlayment of 18-inch wide strips of ASTM D226 Type I felt. These strips are placed between courses, starting at a distance above the butt equal to twice the exposure. This "felt-shingling" technique ensures that any water bypassing the shake is directed back onto the course below. Underlayment for wood shingles on slopes 3:12 to 4:12 must be double-layered. For slopes 4:12 and greater, it is a single layer. Flashing for wood roofs must be at least 26-gauge galvanized steel. In fire-hazard severity zones, wood roofs must be treated to meet Class A, B, or C fire ratings, and the underlayment used must be part of the specific fire-tested assembly. This often requires specialized mineral-surfaced underlayments to achieve the necessary fire resistance.