Framing an Octagonal Roof, continued

Layout

After snapping an exact duplicate of the wall layout on our work platform, we framed a double-thickness 2x6 octagonal base, overlapping the corner joints for strength. To help with lifting, we slid 2-by blocks under the assembly to hold it 3 inches off the deck. Stand-blocks fastened to the platform held the double plate's outside dimension to the reference lines (Figure 2). (I had to subtract the thickness of the extra double plates from the common rafter height-above-plate to ensure that the fascia would align with that of the adjoining roof planes.)

Figure 2.A temporary work platform served as a base for a double construction plate built to the exact dimensions of the existing wall plates. Stand-blocks maintain its position; 2-by blocks elevate the structure to aid the final lift.

I marked the rafter layout directly on the plates. Instead of working to the blueprint layout, I simplified the framing by eliminating the ring blocking and centered "common" rafters. The spaces between hips would be framed with common jack rafters.

To support the framing at roof center, I glued together several layers of 1 3/4x18-inch LVL scraps to make a cube, then cut its corners at 45 degrees to make an eight-sided center block to catch the tops of the hip rafters (Figure 3).

Figure 3.A beveled center block laminated from 18-inch LVL cutoffs provides support where the hip rafters intersect at the roof center.

Building the Bell

To develop the shape of the bell, I had to start with a common rafter pattern, which would be recut later as jack rafters. I worked out the common length using the common run and a calculator -- just as I would for a straight 13-pitch rafter.

Cutting the rafters. I snapped lines on the work platform to represent the rafter in elevation as seated on the plate. The rafters were cut from 1 3/4x18-inch LVL stock. After cutting a rafter "blank" to length, I laid it on the pattern line and struck scaled-up radius curves using a site-made trammel stick (Figure 4). At the outer curve, the radius exceeded the width of the LVL, so I piggybacked a block of LVL on the rafter to fill the break and secured it with PL Premium construction adhesive (OSI Sealants, 800/321-3578, www.osisealants.com) and screws.

Figure 4.Where the rafter curves exceeded the confines of the 18-inch LVL beam stock, the author secured piggyback blocks with adhesive and screws.

Rather than going with the flat ceiling planes specified by the architect, Scott wanted to replicate the curves on the interior surface as well. After getting the go-ahead from the owners and the architect, we piggybacked additional blocking onto the undersides of the rafters to pick up the bottom curve. At first, we used a simple 12-inch gauge block to transfer the outer profile directly to the interior edge. But ever creative Scott wanted the ceiling line to travel horizontally out from the wall before turning upward.

To alter the ceiling profile, we used a thin poplar ripping and flexed it across the rafter face until we found a pleasing line. We traced the arc and carefully cut the rafter to shape with a saber saw. This rafter became the pattern for the rest of the commons. We initially cut 16 rafters, leaving the shorter jacks until later. Bosch #T144D blades proved to be excellent performers throughout this intensive cutting session.

Curvy hips. Although this roof was essentially no more than a fancy eight-sided hip roof, the unusual shape literally threw a curve into the hip rafter design. I don't know of any practical method for calculating the ratio of the common rafters' curved profile to that of the elongated hips. Fortunately, the common rafters provided a ready solution. We extended the wall line where the commons would sit, and set up a temporary ridge board parallel to the line, centered at the correct distance away (the common run distance, or 7 feet 5 3/8 inches). We built the ridge out to match the thickness of the center block and supported it at the proper height (Figure 5). Working directly on the platform, we installed four profiled common rafters against one side of the ridge and two against the other for bracing, as if framing a gable roof.

Figure 5.After cutting and accurately setting a hip rafter "blank" against a common at one end of a temporary frame, the author fashioned a scribing tool from an 8-foot length of 1-by pine that had been notched at one end to accept a carpenter's pencil. Held parallel to the ridge, the straightedge was dragged across the curved backs of the common rafters to transfer the correct profile to the hip.