15.3 Steep-Slope Roofing & Below-Grade Waterproofing

Steep-Slope Roofing Systems

Steep-slope roofs are defined by IBC Section 1507 as roofs with a slope of 2:12 (17% slope) or greater, primarily shedding water through gravity overlap rather than hydrostatic barrier seals.

• Asphalt Shingles (Fiberglass Composition):
  • Slope Limits: Standard single-layer underlayment (one ply of ASTM D226 Type I/II felt or synthetic underlayment with 2-inch headlap) requires a minimum slope of 4:12. Low-slope applications between 2:12 and 4:12 are permitted only when installed with double-layer underlayment (two plies of felt overlapped 19 inches, creating a complete two-ply water-shedding membrane beneath shingles). Asphalt shingles are strictly prohibited on slopes below 2:12.
  • Fastening: Standard 4 nails per shingle (or 6 nails per shingle in high-wind regions >110 mph) placed below the adhesive sealant strip.
• Standing Seam Metal Roofing: Minimum slope typically 3:12 for architectural snap-lock seams, or 1/2:12 for structural standing seams with factory-applied hot-melt sealant in mechanically seamed ribs. Fastened with concealed sliding clips that accommodate longitudinal thermal expansion and contraction.
• Slate & Clay/Concrete Tile: Heavy gravity-shedding assemblies requiring robust structural framing (dead loads of 9 to 15 psf for tile; 10 to 25 psf for slate). Slopes generally require a minimum of 4:12; tile requires counter-battens over underlayment to promote cavity drainage and ventilation.
• Ice Barrier Protection (Ice-and-Water Shield): In geographical areas subject to freezing temperatures and snow (where average daily temperatures in January are 25°F or less, or where ice damming occurs), IBC Section 1507.2.8.2 mandates the installation of an ice barrier. The ice barrier must consist of a self-adhering polymer-modified bitumen membrane (complying with ASTM D1970). It must extend continuously from the lowest edge of the roof eaves to a point at least 24 inches (610 mm) horizontally inside the interior warm wall line (the interior finished face of the exterior wall). In valleys, the self-adhering membrane must be installed at least 36 inches wide (18 inches on each side of the valley centerline) beneath metal valley flashings.

Below-Grade Waterproofing Systems

Subsurface building foundations (basements, elevator pits, underground parking structures) face constant moisture exposure and hydrostatic water pressure. The architectural strategy must distinguish between dampproofing (which resists moisture vapor and capillary dampness in well-draining soils with no hydrostatic head) and waterproofing (which resists continuous lateral and upward liquid water under hydrostatic head).

BELOW-GRADE WATERPROOFING & DRAINAGE ASSEMBLY
  Native Soil Backfill
  ┌──────────────────────────────────────────────┐
  │ Granular Backfill / Filter Fabric            │
  │ ┌────────────────────────────────────────────┤
  │ │ Dimpled Polymeric Drainage Core Mat        │
  │ │ ┌──────────────────────────────────────────┤
  │ │ │ Continuous Waterproofing Membrane (60 mil│
  │ │ │ ┌────────────────────────────────────────┤
  │ │ │ │ Concrete Foundation Wall (Reinforced)  │
  │ │ │ │                                        │
  │ │ │ │               Footing Keyway ┌─────────┘
  │ │ │ │       Continuous Waterstop ──┼──┐
  │ │ │ │                              └──┴──────┐
  │ │ │ └────────────────────────────────────────┤
  │ └─┴─ Perforated Perimeter Drain (Weep Tile)  │ Concrete Footing
  └─── Washed Stone Envelope in Geotextile ──────┴─────────────────

Waterproofing Membrane Typologies

• Self-Adhering Rubberized Asphalt Sheet Membranes: Factory-controlled 60-mil (0.060") composite membranes composed of polymer-modified asphalt laminated to high-density cross-laminated polyethylene film (e.g., Bituthene). Requires primed concrete substrates, taped seams, and a protective drainage board. Offers uniform factory thickness and crack-bridging flexibility.
• Fluid-Applied Elastomeric Membranes: Polyurethane, polymer-modified asphalt, or hot rubberized asphalt applied seamlessly by squeegee, roller, or spray to a thickness of 60 to 120 mils. Eliminates field seams, conforms flawlessly to complex geometry (footing keyways, pipe penetrations, irregular soil nails), and bonds tenaciously to the concrete substrate, preventing lateral water migration behind the membrane if punctured.
• Bentonite Clay Systems: Sodium bentonite clay panels or geotextile-encapsulated blankets. Sodium bentonite clay swells up to 15 times its dry volume when exposed to ground moisture, expanding into a dense, impermeable gelatinous seal that automatically self-heals around small cracks and punctures. Must be installed under structural confinement (minimum 30 to 60 psf compressive backfill or slab pressure) to prevent unconfined swelling.
• Blindside Waterproofing (Pre-Applied Systems): Installed in zero-lot-line urban excavations where property lines prevent excavating around the foundation perimeter. The waterproofing membrane (thick HDPE sheets coated with pressure-sensitive adhesive or bentonite mats) is fastened directly to the excavation shoring wall (sheet piling, soldier piles with timber lagging, or slurry diaphragm walls) before the concrete foundation wall is cast. When liquid concrete is poured directly against the membrane, the heat of hydration and mechanical pressure chemically lock the membrane to the curing concrete face.

Hydrostatic Pressure Mitigation & Subsurface Drainage

No below-grade waterproofing membrane should bear hydrostatic water pressure indefinitely without structural relief. A comprehensive foundation moisture control system includes:

1. Dimpled Polymeric Drainage Mat: High-density polyethylene (HDPE) core with molded dimples faced with a bonded non-woven geotextile filter fabric. Installed against the exterior face of the waterproofing membrane. Relieves hydrostatic pressure by providing an unobstructed vertical drainage path for groundwater while shielding the membrane from puncture during backfilling.
2. Perforated Perimeter Foundation Drain (Weep Tile): A minimum 4-inch (100 mm) perforated rigid PVC (ASTM D2729 or ASTM F758) or corrugated HDPE pipe set along the outside perimeter of the footing. Pipe perforations must face downward (at the 4 o'clock and 8 o'clock positions) so groundwater rises into the pipe from below and drains away by gravity without washing soil fines into the invert. The drain pipe must be surrounded by a minimum of 6 to 12 inches of clean washed gravel (ASTM C33 No. 57 stone) wrapped completely in geotextile filter fabric to prevent silt clogging, sloped minimum 1/8 inch per foot (1%) to gravity discharge or an interior duplex sump pump basin.