Flat & Low-Slope Roofing

No roof is truly flat — all "flat" roofs need at least 1/4" per foot slope to drain. Low-slope is the technical term: any roof under 2:12 pitch. Same membrane systems apply: TPO, PVC, EPDM, modified bitumen. Typical residential flat roof sections (back porch, sunroom additions, modern flat-roof homes) and virtually all commercial are in this category.

PVC leads at 25–30 years. TPO is close at 20–25 years with proper install. EPDM at 20–30 years. Modified bitumen at 15–20 years. Actual performance depends heavily on install quality, ponding water avoidance, and annual maintenance. San Antonio heat is hard on all flat roofs — all ratings trend toward the low end without maintenance.

Per square foot for 2,000+ sq ft residential/commercial: TPO $6–$10, PVC $8–$13, EPDM $5–$8, modified bitumen $4–$7. Smaller residential sections (like a 400 sq ft back porch) run proportionally higher per square. Insulation upgrades, drainage modifications, and tapered insulation for ponding correction all add to base pricing.

Four common failure modes: (1) Seam failure — improperly welded seams separate over time; (2) Penetration flashing failure — HVAC curbs, drains, pipe boots; (3) Ponding water — prolonged standing water accelerates membrane UV damage; (4) Substrate failure — wet insulation below the membrane (often from an unaddressed small leak) that compromises the entire system.

Typical process: (1) Tear off existing or prepare substrate for recover; (2) Install insulation to required R-value with tapered insulation for drainage if needed; (3) Install cover board; (4) Install membrane (mechanically fastened, fully adhered, or ballasted per system); (5) Heat-weld or tape all seams; (6) Install flashing at penetrations and perimeter; (7) Drain installation and water testing; (8) Final inspection.

Minimum twice a year: spring and fall. Commercial buildings with heavy rooftop traffic (HVAC crews, satellite techs) benefit from quarterly. Post-storm inspections are important for hail and wind damage. Maintenance contracts (typically 1–2% of replacement cost annually) extend lifespan 5–8 years.

Depends on extent. Localized seam failure or flashing issue: straightforward repair, extends roof life. Multiple leaks in multiple locations: usually system-wide membrane degradation, suggests replacement. Wet insulation below membrane: problem is no longer surface-fixable — requires membrane removal and insulation replacement in affected areas minimum.

Three maintenance tasks: (1) Debris clearing — monthly in fall, anytime after storms; keeps drains flowing; (2) Seam probe inspection — every 3–5 years to catch separations early; (3) Penetration sealing — resealing around pipes and HVAC curbs every 5–8 years. All cheap relative to replacement cost.

Ponding = standing water on a flat roof 48+ hours after rain. Every flat roof should drain fully within that window. Ponding causes: UV damage acceleration (water magnifies sunlight), seam stress from freeze/thaw (rare in SA but real), membrane degradation at the exact points that sit wet longest. Ponding shortens any flat roof's lifespan significantly — ~50% on TPO in severe cases.

Three approaches: (1) Tapered insulation system — wedge-shaped insulation creates slope where there wasn't enough; (2) Additional drains — installing new drains at the low points; (3) Cricket construction — small built-up slopes diverting water to drains. Correcting drainage should be done at replacement; retrofitting on existing roofs is possible but more expensive.