Vented Cold Roof Design Basics

Two-Deck Basics

• When the outdoor temperature is freezing, heat escaping from attic insulation can melt snow on a roof, leading to the formation of ice at the gutter openings. When melting snow behind these ice dams penetrates shingles or laps of roof covering, it can cause leaks. A vented cold roof has two decks, a bottom or primary deck and a top or secondary deck. Vents prevent snow from melting by allowing cold air to move above the insulation on the warmer bottom deck but below the colder top deck.

Elements of Design

• The bottom deck is covered with felt saturated with asphalt or other waterproof membrane. The rafters between the bottom deck and top deck run parallel to the roof slope, allowing cold air to enter the bottom of the roof and exit at the top. To withstand the pressure of high winds rushing upward under the eaves, called wind uplift, the rafters between the decks are attached to the rafters supporting the bottom deck.

Vents

• A minimum of 1 inch of ventilation space should be between the rafters of the bottom and top decks. Vented cold roofs have continuous vent openings along the fascia or soffit. Fascia is an architectural term for the exterior, visible band running just under the eve of a roof. A soffit is the bottom of the eave. The net free ventilation area (NFVA) of cold roof vents is the same as attic and rafter vents. NFVA is open space of vents minus louvers and screen mesh. The standard of the Asphalt Roofers Manufacturing Association is at least 1 square foot of NFVA to 300 square feet of attic.

Other Design Considerations

• Vented cold roofs work better if the roof has a steep pitch. Chimneys should be located at the ends of a gable or at the ridge of the roof. Plumbing pipes should be vented at the ridge. Attics and rafters are ventilated to reduce the buildup of summer heat and reduce or eliminate water condensing in the attic and on rafters during the winter. A single roof can have both attic and cold roof ventilation systems.