Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
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Do you have any instructions/tips for installing a ridge vent on an existing attic roof?
Attic ventilation is a system which requires air coming in and air going out. The air comes in at the eaves or soffit of the roof and comes out at roof vents, ridge vents or gable end vents. You need the soffit vents in order for the roof vents to work.
To put in a ridge vent in an existing asphalt roof is not that difficult. It requires removing the existing shingle cap at the ridge. These can be used over the new ridge vent if they can be saved. According to the vent purchased, an opening is cut out from the sheathing on each side of the ridge. If the roof is quite old and rafters were used the roof would have a continuous ridge board with the rafters nailed to this on each side. With trusses there is no ridge board. Set the depth of your circular saw to the thickness of the sheathing. Don't cut the rafters, ridge board or the trusses, just the sheathing. A small strip, about 3/4" to 1" is removed on each side of the ridge. The instructions with the particular vent should give this exactly. After the caps are removed, start with removing the shingles by using a utility knife. Snap a line the required distance from each side of the ridge line. Cut the sheathing out with a circular saw keeping clear of the asphalt shingles. Start this cut about 6" back from the edge of the roof, leaving the shingles intact for the first 6" and about 6" away from any chimneys, etc. After removing the strip of sheathing and shingles nail the vent into position. Keep the vent flush with the gable end of the roof, for looks, even though the vent cap starts about 6" from the edge. Nail the vent into position using 1 1/4" roofing nails. Some vents need end caps at the roof edge and some require covering them with either the old caps, if still in good condition, or left as is.
Another solution to ridge vents is adding roof vents. These are individual vents installed near the top of a roof, usually on the back side of the roof. The amount of ventilation used is dependent on the size of the attic ceiling. "The unobstructed vent area shall be not less than 1/300 of the insulated ceiling area for a slope greater than 4/12. For slopes less than 4/12 the vent area shall be 1/500 of the insulated ceiling area."
The code mentions unobstructed vent area. Make sure the soffit vents are not covered with the attic insulation which renders them useless. Styrofoam baffles should be installed at the insulation stage to prevent the insulation from being blown over the soffit vents. Check this out first.
Dave(Ask Dave) (About Dave)
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