by Thomas Buckborough
My design-build company recently completed a residential
addition that included a 16-foot-wide dormer with an arched
roof. Although the construction was challenging, the finished
copper-roofed dormer, complete with a curved plaster ceiling,
adds a dramatic element to the addition.
As I planned the dormer framing, I tried to anticipate the
best way to create a crisp edge where the curved dormer ceiling
intersects the flat sloping ceiling under the main roof.
Clearly, it would be difficult to create the curve of the roof
and the curve of the ceiling with the same rafters.
The solution was to frame the ceiling separately from the
roof. This allowed us to concentrate on getting the proper
bearing for the rafters without worrying whether the bottoms of
the rafters established the desired ceiling curve. To clarify
the design, I drew a section in both directions, as well as a
standard framing plan.
Fortunately, my lead carpenter, Kevin St. Onge, had experience
building curved structures on previous projects. The first
step, just as for any dormer, was to frame a rectangular rough
opening in the main roof. The LVL trimmer rafters and the LVL
header at the high side of the opening were sized by a
structural engineer. When we sheathed the main roof, we allowed
the plywood to run wild into the rough dormer opening, to
provide a surface for drawing the curve of the dormer roof at
the line where it intersected the plane of the main roof.
The dormer design included eight windows, all purchased from
Eagle Window and Door (800/453-3633;
www.eaglewindow.com). The two largest
windows were custom units with radiused tops.
The window header was assembled from five layers of CDX
plywood — four layers of 3/4-inch plywood and one of
1/2-inch. Because the 3 1/2 x 15 1/2-inch header has a post at
the midpoint of the 16-foot span, I felt confident that it was
To draw the curves on the plywood, we made a 19-foot-6-inch
trammel from scabbed-together 1-by boards. From the pivot
point, we measured two radius lengths — 17 feet 10 1/2
inches and 19 feet 2 inches — corresponding to the inside
and outside arcs of the header.
After scribing and cutting the plywood pieces, we assembled
the header with plenty of nails and construction adhesive,
staggering all the plywood joints. We nailed the header into
place, and then we turned our attention to the dormer