My business, Narragansett Housewrights, provides custom millwork and architectural components to builders here in southern Rhode Island. Last year, my best customer, Baud Builders, gave me the job of building a screened pavilion for a shingle-style waterfront home. The pavilion swept out in a curving wing from a corner of the building, terminating in a conical roof supported on a semicircular colonnade.
I only take on projects that I can build in the shop and deliver to the site in sections for assembly. If I had built this one according to the architect’s framing plan, it wouldn’t have been transportable. So I came up with an alternative method that I believed would also cut the build time by half. We bid the job accordingly, and after much discussion got approval from the architect and engineer to proceed.
The roof framing consists of five modules assembled on site, beginning with the half cone.
The four major modules all bear on the perimeter architrave beam and a steel moment frame at the cone’s midsection.
The fifth, nonstructural module (E) completes the top and back of the cone.
I typically skip shop drawings and begin instead by making a simple model, in this case a section of the conical roof. This gives me a fast, simple way to view the critical components at full scale in three dimensions. Basically, I cut short pieces of each element in the structure and stick them together with hot-melt glue.
Full-dimension components, quickly glued with hot-melt adhesive, provide a reliable, accurate way to check a proposed assembly. Note the crown molding and the plastic soffit vent material.
I envisioned the overall framing as four independent truss-framed modules joined together in the finished assembly: a half-cone, two sloping side modules, and a flat-topped center section. My trusses are nothing more than 2-by members connected with plywood gussets.
I designed the conical module around a series of identical triangular trusses fixed in a radial pattern around a central “hub.” The roof sections connecting the cone to the main house use a similar system of trusses arrayed along their respective framing lines.
The trusses’ tails bear on a curving architrave, a custom-made structural beam supported on six 4x6 posts concealed within hollow architectural columns. One of these columns was pre-existing and provided a point of departure for the new layout.
Moment frame. Two of the architectural columns surround steel posts, part of a welded moment frame needed to resist wind-racking and uplift. The roof modules fully encase the horizontal steel member, a W8x21 hot-dipped-galvanized I-beam.
Templated construction. To make sure that site and shop layout remained consistent with each other, I cut two identical sets of plywood templates to outline the architrave and pinpoint all the column locations. I brought the templates to the job site and laid them out on the masonry deck, then took lots of triangulated measurements off the building. I recorded these directly on the templates. Back at the shop, I used the triangulations to snap accurate layout lines on the floor so that I could position the templates exactly as they were on site. Meanwhile, one set of templates remained at the job site for locating and pouring the new column bases.
Identical plywood layout templates ensured a perfect match between site and shop efforts. The square cutouts pinpoint column centers.
Curvy architrave. I made the architrave in three segments — the first with a 28-foot radius, the second a semicircle with an 8-foot radius, and the third a straight length of LVL. To make the curved beams, I glue-laminated 11 layers of 3/8-inch plywood underlayment bent around shop-built forms. I used Excel One, a spreadable polyurethane adhesive (800/779-3935, excelglue.com), and a boatload of bar clamps. The adhesive sets up in about five hours.
Because plywood doesn’t bend with smooth, perfect uniformity, the laminated beam had a somewhat irregular face. To compensate, I used a router on a long trammel arm to cut slightly wider, perfectly accurate top plates from 3/4-inch MDO (medium-density overlay) plywood. We centered the plates on top of the beam segments and glued and screwed them in place. Later, after on-site assembly, I gauged the Azek fascia off the plates’ edges and shimmed the inner and outer pieces in perfect parallel.
Conical framing. I began framing the semicircular conical section by building a hub, a 2-foot-diameter half-cylinder made of staves cut from an LVL beam. It’s glued together with West System epoxy (866/937-8797, westsystem.com); for added mechanical strength, I bound the staves with four perforated steel straps. The staves — which are 1 3/4 inches wide on their outer face — provide faceted surfaces for attaching the 2-by trusses.
The cylindrical hub at the core of the conical roof section is made from staves of resawn laminated veneer lumber.
Each stave corresponds to a truss in a radial array.
The trusses are attached to the hub with structural screws. Intermediate lookout blocks provide firm backing for a cellular PVC fascia.
We set the trusses on the semicircular architrave and screwed them to the hub from the back, using 4-inch Timberlok screws (800/518-3569, fastenmaster.com), one each into the top and bottom chords.
The truss tails landed at about 20 inches on-center. To create more solid backing for the curved fascia, we glued and screwed lookout blocks on 6-inch centers to a length of 1-by cedar, then nailed this assembly to the truss tails, effectively tying them together.
We then sheathed the back of the half-cone with a single layer of 3/8-inch plywood, cutting holes between the webs for access and air circulation. We added cleats on top of the plywood to catch the lower-pitched abutting roof sections. (It would have eliminated compound valleys and simplified construction if the cone and the connecting roof were the same pitch, but I lost that argument.) I left the top of the cone flat, about a foot short of the peak, to be filled in after the adjoining sections were framed and sheathed.