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Roof trusses are engineered to sustain specific roof and wind force loads. Lumber lengths are specified and connector plates are used to tie all members together.
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After hurricanes ravaged Florida in recent years, building codes were strengthened to keep future damage to a minimum. Officials and builders have learned that keeping the lid on a house means forming a tight bond between the sail-like roof deck and the walls below. That job falls to the engineered roof truss system that holds it all together.
Trusses Surpass Traditional Framing
Carpenters used to use two-by lumber to frame into stringers and rafters. Engineers and architects now design roof trusses built of 2x4s in triangular configurations that are joined together with metal connector plates. The result is a cohesive roof truss that stands up to state, local, and national building codes. Trusses perform to such a high degree because the lumber is uniform in size, density, and quality, and metal connector plates ensure rigidity at joints.
Engineered trusses have been on the building scene for 35-years, a track record that impresses many builders and homeowners. Kirk Grundahl, executive director of the Wood Truss Council of America (WTCA), Madison, Wisconsin, says homeowners can be assured their roof trusses are engineered to exacting design standards nationwide since manufacturers must meet the standards set by WTCA and the Truss Plate Institute (TPI).
Sean O’Connor of Robbins Engineering, in Tampa, Florida—designers, plate fabricators, and truss system engineers—explains that roof trusses create a stronger roof structure because they are engineered using CAD (computer aided design) design techniques and computer analysis for worst-case scenarios.
“Because every one of the roof trusses are engineered, it literally takes into consideration all the forces acting on the truss from gravity loads to wind loads, seismic loads, and uplift loads,” O’Connor says.
Roof Trusses Allow Open Floor Plans
Trusses have many pluses, including their overall strength, ability to be placed quickly, and span capability. Since they’re built from shorter lengths of lumber, roof truss systems are typically less expensive to build than roofs with conventional framing.
Trusses are engineered to span larger distances than conventionally framed roofs. Since they transmit weight from the roof to the exterior walls, none of the interior walls needs to be load bearing. This opens up interior space and allows for many interior design options.
Wood, Steel, and Engineered Timbers
Roof trusses, historically composed of wood with metal connector plates, now have competition. As steel-framed homes catch on, so have all steel roof trusses. O’Connor says that to date steel roof trusses are typically reserved for the light commercial and industrial markets, with wooden trusses still dominating home construction.
Engineered wood products such as I-joists have also made a big surge in the market. “They can be used almost like framing lumber, but unlike conventional lumber, they’ll span up to 60 feet in length,” O’Connor says.
Wood roof trusses with metal connectors can also be treated with fire retardant and have an “excellent fire rating,” according to O’Connor. Wood trusses with metal connectors, from a budgetary and quality standpoint, are the price point winners,” according to O’Connor. When they are properly engineered and put together, the wood roof truss with low-cost connector plates will perform to engineered lumber parameters. “So they are a nice, low-cost solution to framing problems,” O’Connor says.
Coupled with hurricane straps for fastening the trusses to the walls, the roof system is typically better than any stick-built roof, according to O’Connor.
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A strong roof depends on its ties. Members must be tied to one another and to the outer walls so that loads can be transferred and building connections maintained.
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Strap it Down for Strong Structures
From a best practice standpoint, connecting roof to truss and truss to the rest of the house is critical to creating a storm-ready home. O’Connor says using the appropriate metal connector hardware to fasten the truss to the top plate of the wall, whether it is a block or wood-framed wall, will create a strong tie-in that can withstand almost any load.
The key is to look at the building as a complete system. “Usually you have one person designing the walls and another designs the roof and nobody ties the two together other than the building official,” Grundahl says. “It is imperative that these two come together properly.”
Installation is best left to professionals who come on site with a truck-mounted crane to place roof trusses. Once in place and connected, trusses may not be cut, notched, or altered in any way in order to maintain their engineered integrity.
Proof after the Storm
The stricter building codes have already stood the test of time. O’Connor says homes built after Hurricane Andrew came through subsequent storms much better than those built before. “Entire roofs or sections of roofs were not blown off. Most damage to the homes was caused by flying debris.”
“Whenever contractors are made to do things to more stringent codes, everybody complains at first, but then when they see the results and how it has saved buildings, they figure that making the change was the right thing to do,” O’Connor says.
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Text by Jim Flasch
© 2005 BobVila.com
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