Vented Flat Roof Assembly

6 2 vented and unvented roof assemblies vented roofs for pitches of 3 12 or above when detailed correctly vented roofs have a long successful track record in the pacific northwest.

Vented flat roof assembly.

In hurricane prone zones with frequent high wind events vented soffit collapse can pressurize a building which can cause windows to blow out and the roof to be blown off. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. Before constructing a flat roof assembly please refer to the unvented roof assembly comments under 6 2 vented and unvented roof assemblies and if applicable 6 3 parapet walls. 25 mm of ccspf against the underside of the roof deck to prevent air leakage through the roof assembly and to prevent moist indoor air from reaching the inside of the roof sheathing where it can condense.

Vented roofs provide the capacity for moisture to be removed from the underside of roof sheathing via convection currents in the vent cavity. That setup makes it easy to insulate the ceiling with thick fluffy insulation like cellulose or fiberglass. Similarly in coastal environments or in regions with lots of rain and wind moisture can be forced into the vents and into the roof assembly. Unvented flat roofs a technical discussion flat roofs have often been built with the insulation all on top of the vapor closed drainage plane as this avoids dew point condensation issues inboard of the roofing.

If wood shingles or shakes are used a layer of drainage mat e g. Control airflow install at least 1 in. However they help to not only stabilize the temperature within the roofing system and attic but also allow moisture to escape. This is a long slender aluminum vent that has a larger cover that is a domed shape.

Historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof. Let s take the example of a cathedral ceiling renovation project on older home in bc. However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options. Minimum 1 4 in.

The most common way to eliminate venting from a roof assembly is to use a spray foam spf insulation. Or 6 mm vented airspace should be installed between the roofing and the underlayment. It has the shape of a mushroom once it is completely installed. Another common flat roof vent is the mushroom vent.