8.2 Roofing Systems & Waterproofing Assembly

Roofing Systems & Waterproofing Assembly

Roofing assemblies form the top cap of the building envelope, protecting the structural framing and interior spaces from environmental precipitation, extreme solar radiation, and wildfire exposure. In California, roofing design and installation are governed by CBC Chapter 15, California Title 24 Part 6 (Energy Code), and California Civil Code §895 waterproofing defect standards. General contractors overseeing roofing operations must understand material slope limitations, flashings, underlayments, drainage hydraulics, and cool roof solar reflectance metrics.

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Steep-Slope Roofing Systems (Slope >= 2:12)

Steep-slope roofing applies to roofs with a pitch of 2 units vertical in 12 units horizontal (2:12 / 9.5 degrees) or greater. Steep-slope systems function on the watershed principle, relying on gravity to shed water rapidly across overlapping shingles or tiles.

Asphalt Shingles (CBC 1507.2)

• Slope Minimums: Asphalt shingles are approved for roof slopes of 4:12 or greater with standard underlayment. They may be installed on low slopes between 2:12 and 4:12 ONLY IF a double-layer of underlayment (or self-adhered polymer-modified bitumen membrane) is applied.
• Underlayment Standards: Compliant with ASTM D226 Type I (#15 felt) or Type II (#30 felt), or non-bituminous synthetic underlayments (ASTM D4869). On slopes between 2:12 and 4:12, underlayment must be applied in double coverage: a 19-inch starter strip at the eave followed by 36-inch wide sheets overlapped 19 inches.
• Fastening: Fastened with a minimum of four corrosion-resistant 11-gauge roofing nails per shingle (six nails in high-wind zones exceeding 110 mph or on steep mansard slopes over 21:12).

Concrete & Clay Tile (CBC 1507.3)

Concrete and clay roofing tiles provide superior fire resistance and thermal mass, making them extremely popular across California. However, their high dead weight (ranging from 9 to 14 lbs per square foot) requires structural engineering verification of roof rafters and trusses.

• Underlayment: Requires a minimum of one layer of Type II (#30) felt for slopes 4:12 and greater, or two layers for slopes between 2:12 and 4:12.
• Attachment Rules: On slopes up to 3:12, tiles are laid in mortar or attached with loose wire ties. On slopes between 3:12 and 7:12, tiles require mechanical fastening (corrosion-resistant screws or copper nails) at specified intervals. On slopes exceeding 7:12, EVERY tile must be individually mechanically fastened.
• Eave Closures (Birdstops): Eave edges must be fitted with prefabricated metal or concrete birdstops to prevent pest nesting and wind-driven wildfire embers from entering the cavity beneath the tiles.

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Low-Slope Roofing Systems & Waterproofing (Slope < 2:12)

Low-slope roofs (slopes less than 2:12 down to a minimum positive slope of 1/4 inch per foot [2%]) cannot shed water quickly via gravity alone. They are prone to standing water (ponding) and require continuous, impermeable, water-barrier membranes.

Single-Ply Thermoplastic Membranes (TPO & PVC)

Single-ply membranes are dominant in California commercial and flat-roof residential construction:

• TPO (Thermoplastic Polyolefin): Blends polypropylene and ethylene-propylene rubber. Highly resistant to UV radiation and chemical exposure, featuring heat-welded seams.
• PVC (Polyvinyl Chloride): Excellent resistance to animal fats, greases, and oils (ideal for restaurant roofs with kitchen exhaust systems). Seams are hot-air welded at 800°F to form a monolithic chemical bond stronger than the sheet itself.
• Installation Methods: Fully adhered with solvent-based contact adhesive, mechanically fastened with stress plates and screws into metal/wood decking, or ballasted with smooth river stone gravel.

Modified Bitumen Systems (SBS & APP)

Factory-fabricated composite sheets composed of asphalt modified with synthetic polymers:

• SBS (Styrene-Butadiene-Styrene): Modified with synthetic rubber, granting high elasticity and cold-temperature flexibility. Installed using hot asphalt mopping, cold adhesive, or self-adhered peel-and-stick backing.
• APP (Atactic Polypropylene): Modified with plastic polymer, providing high heat resistance and tough wear characteristics. Typically installed using a open-flame propane torch (torch-down application).

+-------------------------------------------------------------------+
|           LOW-SLOPE MEMBRANE WATERPROOFING COMPARISON             |
+-------------------------------------------------------------------+
| Material  | Seam Joining Method | Key Resistance Property         |
|-----------+---------------------+---------------------------------|
| TPO       | Hot-Air Welded      | High UV & Solar Reflectance     |
| PVC       | Hot-Air Welded      | Grease, Oils & Chemical Proof   |
| SBS Mod   | Hot Asphalt / Cold  | High Elasticity & Tear Strength |
| APP Mod   | Propane Torch-Down  | High Temperature Plasticity     |
+-------------------------------------------------------------------+

Flashing Details, Penetrations & Roof Drainage

Flashings guide water over construction joints and roof interruptions. Over 90% of roof leaks occur at flashings and penetrations rather than in the field membrane.

Step Flashing at Sidewalls

Where a steep-slope roof meets a vertical sidewall (e.g., a roof-to-second-story wall transition):

• Step Flashing Requirement: Continuous single-piece L-flashing (channel flashing) is strictly prohibited on shingle roofs. Instead, individual L-shaped step flashing pieces (minimum 4 inches high by 4 inches wide by 7 inches long) must be installed at every shingle course.
• Integration: The step flashing piece is placed over the end of the shingle and nailed to the wall framing (or roof deck). The next shingle course covers the horizontal leg, while the vertical 4-inch leg extends up the wall beneath the wall WRB in shingle-lap fashion.

Chimney & Parapet Flashings

• Chimney Crickets (CBC 1507.2.8.2): Where a chimney or vertical penetration width parallel to the ridge exceeds 30 inches, a roof cricket (saddle) must be built on the high side of the chimney. The cricket is a small inverted-V roof structure that diverts water around the chimney shoulders.
• Counter-Flashing (Cap Flashing): Masonry chimneys and concrete parapets require a two-part flashing assembly. The base flashing turns up the wall 6 to 8 inches. The metal counter-flashing is inserted 1 inch into a mortar joint reglet, sealed with polyurethane sealant, and laps DOWN over the top of the base flashing.

Roof Drainage Engineering & Overflow Systems (CBC 1503 & CPC Ch 11)

All roofs must be engineered to drain water efficiently. Standing water that remains on a roof surface 48 hours after precipitation is defined as ponding water, which voids manufacturer warranties and accelerates membrane breakdown.

+-------------------------------------------------------------------+
|                  ROOF DRAINAGE & OVERFLOW DESIGN                  |
+-------------------------------------------------------------------+
| Primary Roof Drain: Sized for local 100-year 1-hour rainfall rate |
| Overflow Drain / Scupper: Installed 2 inches ABOVE primary drain  |
| CRITICAL REQUIREMENT: Overflow must discharge INDEPENDENTLY to    |
| daylight where visible to building occupants (NOT tied into main) |
+-------------------------------------------------------------------+

• Primary Roof Drains: Primary drains are connected to interior storm drain piping sized per California Plumbing Code Chapter 11 based on roof square footage and local 100-year, 1-hour rainfall intensity (inches/hour).
• Secondary (Overflow) Drains & Scuppers: Under CBC 1503.4, if a roof is bounded by parapets or perimeter walls that could trap water if primary drains become clogged with leaves or debris, an independent secondary overflow system is mandatory:
  1. Overflow Drains: Secondary drain inlets placed with their rims 2 inches above the primary drain inlet.
  2. Parapet Scuppers: Openings cut through parapet walls. Scupper invert elevations must be set 2 inches above the primary drain and have a minimum length of 4 inches and height of 6 inches.
  3. Discharge Location: Secondary drains and overflow scuppers MUST discharge independently to daylight at an exterior location visible to building maintenance personnel (such as over a sidewalk or entrance canopy). They must NEVER be piped into the primary storm drain system!

Title 24 Cool Roof & WUI Fire Rating Requirements

Title 24 Cool Roof Standards (Part 6)

California mandates high-performance "Cool Roofs" to mitigate urban heat island effects and lower air conditioning electrical demand. Performance is measured by the Cool Roof Rating Council (CRRC) using three metrics:

1. Solar Reflectance (SR): The fraction of solar energy reflected by the roof surface (scale of 0.0 to 1.0).
2. Thermal Emittance (TE): The ability of the roof to radiate absorbed heat away from the building (scale of 0.0 to 1.0).
3. Solar Reflectance Index (SRI): A calculated value combining reflectance and emittance (0 to 100+).

Per Title 24 Prescriptive Standards for non-residential and residential low-slope applications:

• Low-slope roofs must achieve a 3-year aged solar reflectance of >= 0.63 and thermal emittance of >= 0.75, or a minimum SRI of 78.
• Steep-slope residential roofs in hot inland climate zones (CZ 10 through 15) require products with a minimum aged solar reflectance of 0.20 or an SRI of 16.

Wildland-Urban Interface (WUI) Fire Ratings (CBC Chapter 7A)

In California designated Fire Hazard Severity Zones and Wildland-Urban Interface (WUI) areas, roofing assemblies must meet strict fire propagation standards tested under ASTM E108 / UL 790:

• Class A Fire Rating: Highest fire resistance. Required on all structures built in WUI zones. Assembly must withstand severe exposure to external fire and embers for 1 hour without flame penetration into the attic space.
• Class B Fire Rating: Moderate fire exposure resistance.
• Class C Fire Rating: Light fire exposure resistance (standard untreated wood shingles are Class C or unrated and strictly prohibited in most California municipalities).