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 (Figure 4).
Figure 4.Simple curved window headers were made
with doubled 2x8s sandwiched between plywood plate segments
(left). Cripple blocking was added on the outside face where
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 (Figure 5). 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.
Figure 5.Plywood bends more easily parallel to the
face grain, so the carpenters installed the double-layer
3/8-inch sheathing vertically (top left). A lip at the top of
the tower wall captured the bottom plate of the roof (top
right). Above, 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 (Figure 6).
Figure 6.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" (Figure
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
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.