Install a Skylight 7 - Determining Framing Requirements

To determine the framing needed for the opening, carefully examine the structure of the roof around your skylight opening. If your skylight fits exactly between two rafters, you'll need single headers, framing members running perpendicular to the rafters that support the sheathing. If it fits exactly between the rafters on either side of an opening spanning one or more rafters, you'll need double headers to support the sheathing and the cut rafters.

Skylights smaller than the space between the rafters require an opening framed with both headers and jack rafters, framing members running parallel to the rafters between the headers: one jack rafter if the opening abuts one rafter, two if it does not. Lumber used for headers and jack rafters should be the same size as the rafters. Be sure your framing will meet all local codes.

Cutting the Sheathing

Stretch the chalk line around the nails that were pushed through the roof, and snap the line between each pair of nails to mark the finished roof opening.

Mark another set of lines in the sheathing around these and parallel to them, depending on your framing requirements: mark a line at right angles to the rafters at both ends of the finished opening-1-1/2 inches away for a single header, and 3 inches away for a double one if you're using 1-1/2 inch-thick lumber. If you'll be installing jack rafters, mark lines parallel to the rafters 1-1/2 inches away from both sides of the finished opening.

Pull out any nails within 4 inches of the edges of the proposed opening. This avoids damaging the blades of your saw.

With a combination blade in your saw, set the depth of cut just to cut through the sheathing. Cut along the larger outline on the sheathing as described above.

Cutting the Rafters

Though rafters crossing the roof opening can be left in place, you may want to remove them for an unobstructed view of the sky. You'll then have to install double headers.

To remove a rafter, use a combination square to mark lines on the rafter to be cut and the rafters on both sides of the opening at a right angle to the cut edge of the roof sheathing; if the light shaft will be splayed or angled, mark the lines at the desired angle. The lines on the rafters on the sides of the opening indicate the placement of the headers.

The angle at which you secure the headers to the rafters depends on the angle of the light shaft you're building

Whether or not you've cut through any rafters, you'll need to frame the roof opening with headers and possibly jack rafters so the sheathing is supported on all four sides.

Installing Double Headers

After you've marked the angles of the headers, measure the distance between the rafters, cut four pieces to the length measured.

To secure the headers to the rafters, nail double joist hangers to the rafters using special hanger nails; make sure that the bottom of each hanger is aligned with the bottom of the rafter, and the outer side of the unshaped support is aligned with the line marking the header position.

If the header is not perpendicular to the top of the rafter, you may have to cut away any part of the hanger that protrudes above the header. Or you can use framing anchors instead of joist hangers.

Place a header into each facing pair of hangers and nail it to the cut end of the rafter with 16-penny common nails. Put a second header into each pair of hangers and nail it to the first header with 8-penny nails; space the nails 6 inches apart and stagger them along the length of the piece. Nail the joist hanger flanges to the headers. Repeat on the opposite side of the opening.

Installing a Single Header

Follow the same procedure for measuring as described for double headers; cut two, rather than four, pieces.

Installation is similar to that for a double header; you use only one header for each pair of single-size hangers.

Finishing the Framing

If your roof opening doesn't fit exactly between rafters you'll need to install jack rafters between the headers parallel to the rafters. Cut lumber the same size as the rafters to fit between the two headers. Install and nail these pieces in joist hangers nailed to the headers. Cut some plywood the same thickness as the sheathing you removed so it fits on top of the headers and jack rafters, if used. Nail it to the headers with 8-penny common nails.