How does roof ventilation work for flat roofs?

Common Georgia low-slope situations

• Porch roofs and carports attached to a pitched main roof.
• Additions and sunrooms covered with a modified bitumen or single-ply membrane.
• Mid-century and contemporary homes with low-slope main roofs.

The unvented approach

Rigid foam insulation, often polyiso, goes above the deck and under the membrane, sometimes with a cover board. Because the deck sits inside the insulation, it stays above the dew point and does not need airflow. This is how most low-slope roof systems are designed.

The vented approach

If insulation sits on the ceiling below and there is a cavity between ceiling and deck, that cavity needs airflow, usually from continuous soffit vents on opposite sides. Cross ventilation across a wide flat roof is weak, so these assemblies are prone to moisture problems unless the ceiling below is well sealed.

Relief vents

Some membrane roofs use one-way relief vents to let trapped vapor escape from the insulation layer. They are not attic ventilation, and their value depends on the system, so follow the membrane manufacturer.

Signs of trouble

Blisters in the membrane, soft spots, ceiling stains with no obvious leak, and mold in the cavity. Low-slope sections are also where water ponds, so drainage matters as much as ventilation. See common roof leak causes and roof ventilation basics.