Roof System Selection Is Both a Functional and Aesthetic Decision With Significant Structural and Thermal Implications
The choice between a low-slope roof (flat or near-flat, requiring a membrane waterproofing system) and a steep-slope roof (sloped enough for drainage without a continuous membrane, using shingles, metal panels, or tile) is one of the fundamental building system decisions the architect makes during schematic design. This decision affects structural system design (low-slope roofs typically require lighter structural systems but may need interior drainage; steep-slope roofs need more structural depth for the slope framing), waterproofing strategy, energy performance, daylighting potential, and the architectural expression of the building. The ARE PPD tests roof system selection as a building systems and performance design decision, asking candidates to evaluate which roof type and system is most appropriate for a described program, climate, and structural system.
Low-Slope Roofing (Flat and Near-Flat, Slope 0-3:12)
Low-slope roofs are defined by the IBC as roofs with slopes below 2:12 (approximately 9.5 degrees). At these slopes, gravity drainage is insufficient for shingle-type roofing; a continuous waterproof membrane is required. The major membrane types - TPO, EPDM, modified bitumen, built-up roofing - were discussed in the previous roofing comparison post. Low-slope roof drainage options: interior drains (drain set into the roof surface and connected to underground piping) preferred for large roof areas; perimeter scuppers (openings in the parapet wall) as secondary overflow protection; gutters and downspouts for smaller buildings without a parapet. Slope to drain: low-slope roofs must be sloped to drain (minimum 1/4 inch per foot, or the structural deck provides slope) to prevent ponding water. Insulation: typically continuous rigid insulation above the roof deck (protected membrane assembly or conventional assembly).
Steep-Slope Roofing (Above 3:12)
Steep-slope roofs drain by gravity and shed water before it can infiltrate through overlapping shingle or panel joints. Systems: asphalt shingles (residential, light commercial); metal roofing (standing seam, metal shingles); clay or concrete tile (durable, heavy); slate (premium; very heavy - structural design must account for 15-25 psf tile weight). Ice and water shield (self-adhered bituminous underlayment) is required in cold climates at the eaves (to prevent water infiltration at ice dam conditions) and at penetrations. Steep-slope roofs enable attic space (which can provide additional insulation depth and allow roof penetrations to occur in the attic rather than penetrating through the waterproof membrane), but require more structural framing depth and more complex geometry at hips, valleys, and dormers.
Green Roofs
Green roofs (vegetated roofs) use a low-slope structural membrane system as the waterproofing layer and add growing media and vegetation on top. Benefits: stormwater retention (30-75% of rainfall); urban heat island reduction; additional roof insulation layer (R-5 to R-15 depending on growing media depth); habitat. Structural requirement: growing media imposes significant dead load - extensive green roofs (shallow media, 2-6 inches) impose 15-30 psf; intensive green roofs (deeper media, 8-24 inches) impose 60-100+ psf. Structural system must be designed for this additional load at the schematic design stage.
Key Exam Points
- Low-slope roof (below 2:12): requires continuous waterproof membrane; interior drains or scuppers; minimum 1/4-inch slope to drain.
- Steep-slope roof (above 3:12): gravity drainage; shingles, metal, tile; ice/water shield at eaves in cold climates.
- Green roof: vegetated over low-slope membrane; 15-100+ psf additional dead load; structural design must account at SD.
- Interior drains vs. scuppers: interior drains preferred for large roofs; scuppers as overflow at parapet.
- Insulation: low-slope roofs use continuous rigid insulation above deck; steep-slope uses batts in rafter bays or cathedral ceiling assembly.
AREprep's PPD roofing and building systems content covers roof system selection, membrane systems, steep-slope systems, green roofs, and their energy performance and structural implications - giving ARE PPD candidates the roof system knowledge the exam tests in building performance and building systems coordination scenario questions.
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