While often called “flat” roofs, these structures are rarely perfectly horizontal. In the roofing industry, they are technically classified as low-slope roofs. Materials usually fall into one of two categories:
- Single-Ply Membranes: TPO (Thermoplastic Polyolefin), EPDM (Ethylene Propylene Diene Monomer), and PVC (Polyvinyl Chloride).
- Asphalt-Based Systems: Modified Bitumen (“Mod Bit”) and Built-Up Roofing (BUR).
The dividing line between a “steep-slope” and a “low-slope” roof is typically a pitch of 2:12 (2 inches of vertical rise for every 12 inches of horizontal run). Most building codes require a minimum slope of 1/4:12 (one-quarter inch per foot) to ensure positive drainage.
Any roof with a pitch less than 2:12 is considered low-slope. At this angle, water drains slowly enough that standard overlapping shingles (which rely on gravity) cannot be used; they would allow water to wick underneath and leak. Instead, a continuous, sealed membrane is required.
Because gravity is less effective at clearing water from these surfaces, low-slope roofs face unique challenges that must be engineered for:
- Ponding Water: The primary enemy of flat roofs. “Ponding” is defined as water that remains on the roof for more than 48 hours after rain. This creates hydrostatic pressure (forcing water into microscopic gaps) and adds significant weight to the structure.
- Drainage Systems: Unlike steep roofs that naturally shed water to the eaves, low-slope roofs require dedicated drainage designs, such as internal drains, scuppers (holes in the parapet walls), or crickets (diverters built under the membrane) to actively guide water to the exit points.
- UV Exposure: A flat surface receives direct sunlight for longer periods than an angled roof. This intense, direct UV exposure accelerates material degradation, making UV resistance a top priority for material selection.
- Thermal Shock: With nowhere for heat to escape quickly via airflow (as in an attic), flat roof surfaces experience extreme temperature swings, requiring materials with high elasticity to handle expansion and contraction without tearing.