Framing a Round Tower, continued
Window headers. There were several windows in the tower. We used Kolbe & Kolbe wood units (Manawa, Wisc.; 920/596-2501, www.kolbe-kolbe.com), special-ordered with 6-foot-radius head jambs and sills. To keep the framing process simple, we framed the wall solid and cut the openings later. In a circular wall, all "sides" are load-bearing, so the windows required structural headers. We sandwiched a typical, double 2x8 header between single-thickness, 6-foot-radius, 3/4-inch plywood top and bottom plates. Short cripple blocks filled out the curved profile between plates.
Simple curved window headers were made with doubled 2x8s sandwiched between plywood plate segments.
Cripple blocking was added on the outside face where needed.
Curved sheathing. The 6-inch stud spacing helped create a smoothly curved profile for sheathing and provided plenty of nailing surface. We sheathed the turret walls with a double layer of 3/8-inch-thick cdx plywood, which we found easier to bend around the framing when applied vertically. We offset the seams in both directions on the second layer, making sure to align the long edges over framing centers for solid nailing. A single layer might have been adequate, but the double layer provided a solid nail base for the cedar shingle siding.
Plywood bends more easily parallel to the face grain, so the carpenters installed the double-layer 3/8-inch sheathing vertically.
A lip at the top of the tower wall captured the bottom plate of the roof.
The author measures for a cricket where the tower interrupts a valley.
We sheathed the topmost section first so we could continue with the roof framing, which depended on enclosing the turret. We brought the sheathing up 1 1/2 inches higher than the top plate to create a captive ring for the roof plate. When framing the roof, I left a 3/4-inch gap between the rafter tail and the plate to allow the wall sheathing to slip up in between.
We framed and sheathed a cricket in the valley behind the tower to drain water around the sides. Our father, Fred Sr., is skilled in sheet-metal work; he fabricates and installs all our flashings. He clad the cricket in lead-coated copper, with all seams triple locked and soldered.
Bell Roof Framing
We framed the turret roof on the ground, using site-made laminated plywood rafters. Its curvy shape was developed by combining two major arcs, specified by the architect, and was based on a 12/12 interior roof pitch. I drew the rafters full-size, directly on 3/4-inch cdx plywood, with the aid of a trammel stick.
Double-layer rafters, glued and nailed together with staggered joints, were laid out with trammel sticks on the deck.
A 4-foot radius formed the domed top, and a 9-foot radius defined the concave slope section. Each laminated rafter was 1 1/2 inches thick, with its layers made in segments to maximize the number of cuts per sheet — we chewed up 55 sheets of 3/4-inch plywood making the mudsill, plates, and custom-shaped rafters. I made two different layer configurations so we could stagger the butt joints.
Roof framing. I formed the basic roof outline with a pair of intersecting hips, using two full-span, plywood rafter "trusses," notched at midspan to interlock at 90 degrees, like a paper model where tab "A" inserts into tab "B".
Turret Roof Framing
Turret roof framing began with a pair of interlocking rafter pairs on a round plate (top left and right). Intermediate rafters were then filled in (left).
Inside each of the four hip intersections, I fit five additional rafters in a radial pattern at 15-degree intervals. I spaced the rafters along the plate by making the intervals equal with a tape measure. A shortening bevel cut allowed them to converge at the top of the dome.