The distance between two rafters determines whether you must cut a rafter to install a skylight. Standard rafters are on either 16-inch or 24-inch centers, meaning the distance from the center of one rafter to the center of the next measures exactly 16 or 24 inches. After allowing for the thickness of standard rafters, the actual space between the rafters is 14 1/2 inches or 22 1/2 inches, respectively.
If the skylight requires a rough opening that is equal to or less than the rafter space, you won’t have to cut a rafter to install the skylight, so you won’t have to install a double rafter unless the skylight is unusually heavy and the manufacturer specifies doubling a rafter for additional support. That is uncommon, however, and most small skylights already are sized to fit between standard rafters.
The rough opening is the wood frame in which you set the skylight. For standard-size skylights, you need to install headers, which are boards that run horizontally, between the rafters to provide a frame for the top and the bottom of the rough opening. Headers should be the same dimension as the rafters. For example, if the roof has 2-by-6 rafters, cut the headers from 2-by-6 lumber. The skylight manufacturer supplies dimensions for the rough opening.
If you cut a rafter to install a larger skylight, you have to transfer the weight load that rafter was carrying to the adjacent rafters. This requires the installation of double headers, the boards that run between the rafters, but not double rafters. The first header installs with nails or screws to the side rafters and to the end of the cut rafter. The second header attaches to the side rafters and to the first header. You install double headers on the top and on the bottom of the rough opening.
When you cut a rafter, you’re adding weight to the side rafters through the installation of headers. If you have to cut more than one rafter to install a large skylight, consult an engineer before doing so. Not only might cutting more than one rafter require doubling the side support rafters, it could require additional weight transfer. In addition to the size of the skylight, the roof’s pitch affects the need to add structural support. A roof pitch of 4:12 or steeper is less likely to require additional bracing than a roof with a 3:12, or lower, pitch.