Flanking sections. We framed the side sections using matching sets of triangular trusses, tacked to the architrave and supported on temporary legs on the inside. We tied the bottom chords together with a running 2x4, then sheathed the inside vertical face with five layers of 3/8-inch plywood, glued and stapled. The laps between layers are offset and all the joints are centered over the framing. We followed by screwing through the plywood into each truss with 3-inch Timberlok screws. The layered plywood unifies the trusses and becomes a laminated structural beam that carries the inside edge and center module of the truss assembly. One end bears on the steel I-beam and the other end is notched to bear on the top plate of the house.
A worker installs bevel-edged blocking between the trusses to provide nailing for the roof sheathing at the top edge, where the roof line transitions to a flat midsection. Here, the trusses have received the first of five layers of 3/8-inch UL plywood, forming a structural beam that supports the center module.
The porch-roof assembly joins the main roof at an acute angle. Instead of trying to calculate the angle mathematically, we extended the plywood on the truss module far enough to allow us to project and scribe the intersecting profile. To do this, we built a plywood mockup of the main roof and placed it on the building line snapped on the floor. We then used a long straightedge to transfer the pitch to the face of the plywood on the truss assembly. We allowed an extra inch of tolerance for fitting and shimming, then cut the plywood and reinforced it with framing.
A plywood mockup of the existing roofline furnishes the correct cut angle for the roof intersection.
Center section. With the three sloped sections completed, we next framed the roof’s flat-topped center section. This was a simple matter of custom-fitting sequential trusses to the existing gap. First, we made a batch of hold-offs to represent the eventual five layers of plywood sheathing we’d be laminating over the two sides. We screwed the hold-offs to the vertical 2x6 truss members, notched to receive the 2x4 top and bottom chords, and back-screwed them from inside the abutting roof sections for easy removal later. Then we cut the horizontal top and bottom chords to fit between the verticals and applied 3/8-inch plywood gussets to both sides of each truss.
The center roof module fills the progressively widening space between the side modules.
Hold-offs, back-screwed to the upright truss members, reserve the space that will be occupied by five layers of plywood once the center module is framed and pulled free for access.
With the sheathing and cap completed, the modules were ready for transport to the job site.
As we worked our way back out of the gap, we shimmed a series of shop dollies beneath the bottom chords. This allowed us — after removing the screws — to roll the completed center section clear and apply the plywood layers to its two sides.
With the framing completed, we sheathed the roof segments with a double layer of glued and stapled 3/8-inch UL plywood. On the conical surface, we cut pie-shaped segments and staggered the joints between layers. To introduce a slight, water-shedding crown to the flat roof section, we ran 3/4-inch rippings at mid-span across the top chords and bent the plywood over them.
Finally, I made the cap and completed the back of the cone. I made the cap from wedges of solid framing lumber, cut to the roof pitch and then band-sawn to radial tapers. I glued the wedges together with thickened, gap-filling West System epoxy and sanded the contours smooth after it hardened.
Valley lines. I used a simple method to determine the compound valley line where the cone overlaps the abutting roof. First, I set the cap in place, its back half supported down to the flat roof section by an offcut from the hub. Then, pivoting a long straightedge around the cap, I projected the cone’s pitch down onto the connecting roof and marked the valley line. Allowing for the thickness of the sheathing, we stepped back from the line and custom-fitted 2x4 framing in a radial pattern between the hub and the valley. With the sheathing applied, this last module ended up looking like a huge bat in flight.