The Ups and Downs of Wood Roof Trusses

The use of wood roof trusses in residential and commercial buildings alike has continued to accelerate in recent years, and for good reason. As technology improves, their design and fabrication have become even easier and more accessible for all builders and contractors. But they are not the best solution for every roof.

Here is what we like about the common wood roof truss:

• Highly structurally efficient. Most trusses are comprised of common dimensional lumber, often 2x4s and 2x6s. Trusses are designed to handle a great amount of load with a minimal amount of material.
• Longer spans. We have seen trusses that clear span 80 feet, much larger than any economical stick-built framing. Reduced numbers of interior bearings result in more open and flexible floor plans.
• Much faster to construct than stick-built. The trusses arrive on site usually in one piece (sometimes 2 or 3 pieces for larger trusses) and are lifted into place with a crane. Roofs can be framed in a fraction of the time compared to stick-built, accelerating the critical activity of weatherproofing the building.
• Can now handle crazy roof geometries. Thanks to 3D computer modeling, trusses can be both designed and manufactured digitally. If you can imagine a roof, chances are it can be framed with trusses.

The downsides:

• Trusses are non-redundant structures, which means that every piece is critical to the function of the truss. If a singular member is cut, damaged, or removed, the entire truss is compromised.
• More difficult to modify. Once constructed, modifying roofs framed with trusses is much more difficult than stick-framed. But it can be done, and we have developed some pretty creative solutions over the years to accommodate everything from new rooftop mechanical units to garage vehicle lifts.
• There are only a handful of truss manufacturers in North America and their design and fabrication techniques are not available for public use. The majority of structural engineers do not have access to these technologies. Manufacturers do not publish design information on their shop-applied connection plates, so design of these simple structures is limited to the manufacturer.

 

 

Trusses are efficient and economical, usually comprised of 2x4s and 2x6s. The steel mending plates are applied by machines in the shop – there is currently no common substitute for these connections

 

 

Framers assemble a set of 5 trusses, which will all be lifted at once. The metal bracing and roof sheathing is easily applied on the ground before lifting into place.

 

 

 

 

 

 

The set of trusses is installed with a crane and fastened to the walls below.