Continuous Ridge Attic Vents

By Paul Bianchina

While it's an acknowledged fact that an attic needs good ventilation, one of the biggest objections that builders and homeowners have to installing an adequate number of roof vents is that most of them can be unsightly. They don't match the color or style of the roofing, and can mar the beauty of many carefully designed roof structures.

One alternative to the traditional metal or colored plastic roof vents is to use a continuous ridge vent. Continuous ridge vents are easy to install, and they offer airflow right at the ridge - the highest point of the attic where it's needed most. The vent material is compatible with wood shakes and shingles, composition shingles, roof tiles, and metal roofing, and best of all, since the vent is covered over with the same roofing that's on the rest of the roof, it's much less visible. Combined with a sufficient number of low vents in the eaves or soffits, it offers a workable alternative for ventilating just about any type of attic.

How They Work

In normal roof construction and roofing, plywood or waferboard sheathing panels are installed over the rafters and meet snugly at the peak of the roof. The roofing is then installed over the sheathing, and the joint where the roofing material meets at the peak is covered over with a ridge cap to seal the joint and finish off the installation. No rain can get in, and no attic air can get out.

With a continuous ridge vent, the sheathing panels are installed with a gap at the ridge, typically about two to four inches wide, depending on the vent material being used. This gap allows hot attic air to escape, and is often large enough that no other vents are required on the roof.

To cover the gap and still allow air to move out of the attic, a special continuous ridge vent material is used, of which there are a couple of different types. Cor-A-Vent, one of the oldest and best know ridge vent manufacturers, uses strips of corrugated polyethylene - similar in cross-sectional appearance to a corrugated cardboard box - building up four to six plys of the material in each strip. The strips have a flexible fold and a v-cut profile along the centerline, allowing each strip to be folded over the ridge sheathing. This allows the vent material to cover the ridge gap, and air exits through the corrugations.

Cor-A-Vent offers a couple of different thicknesses of vent material - the thicker the material is, the more ventilation area it has, but the higher it protrudes up above the roof. Net free area for this material - the actual air space in the vent, abbreviated NFA - ranges from 9 to 18 square inches per linear foot, depending on thickness.

Another style, Celotex's Roll Vent, is a two-layer composite of nylon and polyester that creates a "fabric" with numerous small air gaps that allow air circulation. The flexible matt comes in 20 and 50-foot rolls, and conforms to roof pitches of 3/12 to 12/12. Here again, it is installed over a gap in the sheathing, covering the ridge while still allowing air to exit. Roll Vent has an NFA of 18 square inches per linear foot of material

With either material, no special tools or techniques are required for installation. For existing roofs, the ridge shingles are removed first, then a chalk line is snapped along each side of the ridge and the roofing and roof sheathing are cut out along the lines to create the gap. For new construction, the sheathing is simply stopped short of the ridge. The gap is stopped short of each end of the ridge by about six to 12 inches.

Starting at one end, the vent material is installed so that it is centered over the gap in the sheathing. The material is held down with standard roofing nails, and can be cut with snips or a utility knife. Finally, to make the installation weather-tight and cover the vent material so that it blends in with the surrounding roof, ridge shingles of the same material as the roofing are installed over the vent and fastened with nails.

Continuous ridge vents are available wherever you purchase your shingles and other roofing materials, and complete installation instructions are provided with the material. Talk with your contractor or roofing supplier to determine how much ventilation your roof requires, and whether you can achieve sufficient ventilation with a continuous ridge. They can also help you with special applications such as overly high or low pitches, clerestories, and other situations.

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