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Framing a Round Tower, continued

Some of the shortening bevels were extreme angles, but they were easy to cut with a swing-table adapter (Pairis Products, Phelan, Calif.; 760/868-0973, www.bestconstructiontools.com) mounted on a wormdrive saw. I modified mine to accept a Linear Link chainsaw blade (Muskegon Power Tool, North Muskegon, Mich.; 800/635-5465, www.linearlink.com), made for timber framing or gang-cutting rafters.

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The author determined the bevel cuts for the rafters by drawing a full-size overhead plan view on plywood.

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Then he cuts the acute angles with a modified wormdrive saw mounted on a swing-table adapter.

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The interior ceiling was flat. We framed it with 2x8s, starting with a triple 2-by cross member and filling in with radial joists fanning around like spokes on a wheel.

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The tower's ceiling joists fanned out from a center triple-2x8 cross member.

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The birdsmouths were cut loose to allow the roof to slip into place over the sheathing lip.

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The eventual ceiling surface was backed with 1/2-inch plywood to stiffen the roof assembly for lifting, as well as to provide nailing for the radius crown molding.

We sheathed the underside with plywood to tie everything together and resist deformation when we craned the roof into place. The plywood also provided full nailing for the interior crown molding.

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One section of the roof sheathing was left off to allow for access inside the tower for installation. 

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The roof was finished with a bendable plastic fascia and cedar shingles.

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Compound-Curve Sheathing

To sheathe the roof, we paneled each rafter bay individually using 1/2-inch cdx plywood. We cut it to bend parallel to the face grain and used separate panels to cover the lower and upper curvatures. This way, we weren't fighting the plywood into a double inside and outside curve. I patterned the first set of panels by screwing blank stock directly to a bay and marking the rafters' centerlines. The rest of the panels were cut from tracings of the patterns.

We'd taken care to cut the rafters uniformly and space them equally, so the panels fit well with only minor adjustments needed. We applied a bead of construction adhesive to the rafter edges and used ringshank nails to hold the plywood down tight to the curves. We expected to have to bevel the rafter shoulders prior to sheathing, but it wasn't necessary.

While the roof was still on the ground, we applied the circular fascia board. I didn't want to back-kerf and bend a solid wood fascia or invest a lot of labor in a glue-laminated member. Instead, we used cellular PVC Azek (Vycom, Moosic, Pa.; 866/549-6900, www.azek.com). It wrapped easily around the roof's diameter. We used a double layer of 3/4-inch material, solvent-welded between layers and at the bevel-lapped butt joint.

Shingling in the Round

Like the rest of the roof, the turret roof was finished with Grade A 18-inch "Perfection" western red cedar shingles, installed over a self-adhering bituminous membrane. On top of the membrane, we applied Cedar Breather underlayment (Benjamin Obdyke, Horsham, Pa.; 800/346-7655, www.benjaminobdyke.com), a stiff nylon matrix that allows air to circulate under the shingles, and shingled directly over it, using hand-driven stainless-steel ringshank nails. The ever shrinking, concentric rings of the turret roof shingling required each shingle to be quite narrow and trimmed to a taper. This was a slow process. Our roofing sub had four of his guys working on this one little roof for three days, cutting, block-planing, and fitting shingles of increasingly smaller size as they neared the top. We saved some time by culling out all the narrowest shingles from the bundles and designating them for turret use. To cap the dome, Fred Sr. will fabricate a round, copper "beanie." The final touch will be a lead-coated copper finial, purchased from a specialty supplier.

John Seifert
is a principal owner of Seifert Construction in Mattituck, N.Y.