Thinking Through a Curve


Thinking Through a Curve

A complex roof with multiple arcing panels provided design challenges both before and during construction.

In October of 2016, a series of graduated bowstring trusses were carefully transported to a high, narrow lot overlooking the Hudson River, the George Washington Bridge and the New York City skyline. It was a task that required both skill and patience. “The delivery had to be very specifically lined up with the builder,” recalled Tom Christensen, design manager at PDJ Components in Chester, New York. “We had to plan on the truck being there while they offloaded the trusses directly onto the roof because of the size of the property.”

The 8,500 square foot single family residence, in what Christensen describes as “a rather upscale section of New Jersey,” features a 2,774 square foot upper level with an inset radius porch overlooking the river. Shipping-wise, the roof trusses weren’t problematic. The longest, said Christensen, “was around 50 feet. So it wasn’t over-length or over-width,” in terms of stacking it on the truck or driving over the roads.

“It was just a matter of the closeness of the jobsite,” Christensen explained. “When you’re in a closed residential community, you need a driver who can maneuver the streets and get back into the jobsite.”

Fortunately, by the time PDJ was lining up a skilled driver and checking in on installation, Christensen’s biggest concerns about the project were well behind him. There had been two main challenges, he explained: first, getting everything aligned correctly in the design. Second, making sure everything stayed in alignment as the design went from paper to product.

“On one side, there’s a common fascia and the panels have to all start out – for the first, roughly, about six feet – with all the same series of pitches,” said Christensen. “The narrowest section was 24 feet, 7 and 1/4 inches, while the wider section was 43 feet, 9 and 1/4 inches.” The difficulty, he explained, came in maintaining the radius the architect had set while keeping “the first portion of the roof aligned so that as you look down the roof it’s the same plane for the first six feet.” There was no flexibility, Christensen added, in getting the series of roof sections to meet the architect-specified curve, because the radius steel roof panels needed to sit on plywood following the truss-established arc without bending out of shape.

“I’ve done quite a bit with curved trusses, both on internal and external curves,” said Christensen, but this project still proved challenging. “We all know that architects are good at coming up with ideas that look neat on paper,” he noted, but turning those two-dimensional drawings into three-dimensional structures isn’t always easy.

Even once the curves were set, there were still minor complications. For example: “There was getting the opening for a large, eight foot square skylight over the center of the stairs,” recalled Christensen. To accommodate that opening, he needed to ensure each affected truss was “stubbed at the right point.” Plus, because the side of the building jogged in with each roof section, Christensen needed to “create a series of mono gable-end trusses that cantilevered into the building to provide the location for the sheathing while maintaining structure stability.”

The uniquely stepped sides of the building combined with the unusual roof shape made for complicated design work. “With the curve and the difference of the setback, the cheek wall of the adjacent roof goes in a lot farther than the wall portion of the exterior wall,” said Christensen. “So you’ve got the jogs on the one side that you have to account for and then, on the other side, you’ve got three different width trusses that all had to have their initial planes aligned so it would be a smooth seamless look.”

The back of the house introduced another significant challenge: the hip roof at the end was also a curve. “So it’s basically half of a dome, and that had to line up with the curve of the 28 foot, 8 and 1/4 inch long girder truss that it hung off of,” explained Christensen.
It was no simple task, he said, to ensure “that the dome at the end aligned with the truss so that it smoothly flowed from one section to the other.”

The project wasn’t just a test of Christensen’s skill – the large, precise bowstrings were also a stretch for the production crew, he recalled. He did his best to simplify things for everyone, he said, by optimizing the trusses to need as few webs as possible. “You’re only allowed to have two unsupported joints in a panel, from the engineering standpoint,” he noted. Maintaining the curve while reducing the amount of wood in the structure by minimizing the vertical and diagonal members supporting the exterior top chord was therefore good in terms of design, material use and production complexity. As a bonus, using fewer webs was Christensen’s way of ensuring fewer problems down the line.

“One of the things that typically happens with a job with a curved roof, is you have a lot of webs in there to support each of the panel points,” he said. “And that creates nightmares, particularly for the HVAC guys running their ducts through the webbing.” Those nightmares can quickly come back to haunt a component manufacturer, Christensen added, because structural integrity isn’t an HVAC technician’s primary concern. “If a web is in the way, they just cut to get their product through and no one knows until there’s a problem later.” When it becomes a problem, said Christensen, his company needs to obtain an engineered repair for the damaged trusses. So what’s good for the HVAC crew is also good for Christensen’s design and production departments.

With the design complete and construction starting, Christensen faced the second of the major challenges in the project: making sure his designs accommodated the dimensions of the actual structure instead of those in the original building designs. “The biggest fear was what could take place when the structure was actually built,” Christensen said. “You can’t wait until the structure is totally up to have that information to do the design from; you’ve got to do it from the prints then adjust accordingly.”

With this unique roof, an accurate measurement of the physical building was particularly critical. “With a regular roof, whether it’s a hip or common trusses,” Christensen explained, “if a wall moves six inches it’s no big deal to change before they are built. It might make for a piggy back on the upper portion being taller,” but it’s nothing that framers can’t handle on the jobsite during installation. Curves, he said, are different.

“When you’re dealing with a curved roof, you’ve got to match every move on both sides to keep your truss having the same proportions.” For this house, with its multiple curved roof panels and asymmetrical side walls, exact measurements from the jobsite were particularly critical.

“Most jobs, we just get the final dimensions supplied by the builder,” said Christensen. It’s only the complex jobs – like this one – that demand a jobsite visit. Christensen says he can only imagine how difficult masonry work is, but the issue he sees most frequently is foundations that don’t match the plan dimension or that are out of square. In that scenario, he explained, the framer adjusts to square things up, and those slight changes carry through the rest of the structure.

On the one hand, there are drawbacks to waiting until the last minute to take final measurements and then adjust the truss design to ensure it works with the physical structure that’s been created on the jobsite. It’s extra time and there’s extra expense associated with a designer getting out of the office. Christensen says he sees the upside, too, in meeting with the builder and construction crew. “They took the measurements that I directed them to,” Christensen said, but his presence ensured they accurately captured the detail with the most immediate bearing on the truss design.

“We always want the builder to take the measurements,” said Christensen, “as this is part of their responsibilities and they will be the one that installs the trusses. Yes, I could have just emailed a drawing with the needed dimensions noted on it, but by going to the jobsite we are able to make sure the dimensions requested are the ones received because we see it being done. Additionally it creates a better relationship with the builder.” It sends the message, explained Christensen, “that we are a team and they are important to us. It also allows us to spot areas of concern that might go unnoticed just looking at a flat drawing.”

In this case, the extra attention was exactly what the job called for – it led to a perfect-fit design and an exceptionally smooth install. “It had its own little complexities,” Christensen said, but the work on this project is just good preparation for the next curve that comes his way. 

About the Author: Before joining the SBC Magazine team in 2015, Dale Erlandson wrote for a variety of publications in several different careers, including non-profit communications, teaching and technical writing.