Building Regulations on Roof Ventilation

Basics

• The spaces under the meeting point of the roof, such as enclosed attics or rafters require ventilation through openings that do not allow rain or snow to enter. The openings must be between 0.0625 to 0.25 inches. If the opening is larger, it needs a screen or hardware cloth that resists corrosion and contains openings between 0.0625 to 0.25 inches. These openings must vent directly to outside air. Code officials can make exceptions, if, for example, the atmosphere or climate does not require attic ventilation.

Minimums

• In spaces where material is used over vents, at least 1/150th of the space must be free of the material. This can be reduced to 1/300th of the space, if in harsher climates, a Class I or II vapor barrier is put on the warm-in-winter side of the ceiling. It can also be reduced if between 40 percent and 50 percent of the required vents are located in the upper part of the attic or rafter space. These “upper ventilators” must be within three feet of the highest point of the roof. The rest of the venting can then come from eave or cornice vents.

Unvented Spaces

• Attic and rafter assemblies may be unvented only under the following conditions. The unvented space must be totally within the structure’s thermal envelope. The ceiling side of the assembly must have no interior Class I vapor barrier. If wood shingles or shakes are installed, they require a minimum 0.25-inch vented air space between the shingles or shakes and the underlayment. And in harsh weather climates, air-impermeable insulation must either be a Class II vapor barrier or have a Class III vapor barrier coating.

Origins

• Municipal, state and some federal building codes on roofing come from the same source: the International Residential Code developed by the International Code Council. Founded in 1994, this non-profit organization consolidated the three sets of building codes in use at the time, and streamlined them into one collection of comprehensive and coordinated regulations. The ICC also developed other rules including the International Building Code, the International Fire code and the International Property Maintenance Code.