Whether installing a shingle-over ridge vent or a metal ridge vent, the cap shingles and the wood sheathing must be removed. From there, it's a matter of reinstalling the shingles to hide the vent, as in the shingle-over model, or nailing the metal vent into place.
A ridge vent is placed along the ridge line, which is where the two halves of a peaked roof meet. Cap shingles are those that straddle the ridge line. These shingles must be removed along the entire length of the line. The center line should be marked, as well as lines noting the location of the wood trusses and the measurements for the wood sheathing removal.
A slot is measured and cut out along the ridge line for ventilation. The wood sheathing is removed by cutting each chalk mark with a power saw, being careful not to cut the trusses or into the ridge board, if one is installed on the roof.
To install the shingle-over ridge vent, the first section has one end plugged and is installed at the edge of the roof line. Open-ended vents are installed along the roof line, with each one nailed down in turn. The last section is cut to fit and has another end plug installed. The cap shingles are nailed over the vents.
The metal ridge vents also come in sections, but they are held together with connectors. The first section is plugged and installed at the edge of the roof line. The next one is connected, and this is repeated until the other end of the ridge line is reached. Tie-downs are placed over the connections, and the entire vent is nailed down.