The Bottom Line
From the homeowner's perspective, modern truss roofs offer three practical advantages. Besides good looks that won't go out of fashion, modern truss-based designs offer more open interior space in a home because they don’t depend on support from walls and beams to stay up. The engineered pattern of triangular components makes trusses self-supporting over surprisingly long spans. It's entirely possible, for example, to build a roof over a 40-foot wide building without any interior posts or beams underneath, using only narrow, relatively-short pieces of lumber. And finally, modern truss roof construction yields more house for the dollar because of the inherent efficiencies in the construction process. Even a complicated roof on a three story house can be ready for sheathing a day or two after the truss truck rolls up.

On one hand, I have trouble giving myself complete permission to be excited about the revolution in truss roofs. After all, the trend works to displace some of the need for carpentry skills, inching the trade closer towards that of component assembly rather than a creative craft. And how can you get excited about the lowering of stakes like that in any trade? But then I think of all those plain-roofed homes that were the bread and butter of the homebuilding business back in the 1970s. Compare them to what’s going up now, even in entry-level developments, and you’ll see why it’s hard to resist the excitement. The great roofs that are showing up everywhere have done more to boost the curb appeal of new homes than any other single refinement in the business. And if that’s also happening at a cost that’s reasonable enough for many home buyers to afford, who can argue?

Sidebar: Not Just Any-Old Wood
When engineers sit down to design a roof truss, wood quality is something they have to be able to count on. Without some assurance that truss lumber will be consistent and flawless, what good is truss design when, say, a bad knot or some wavy grain can blow the whole thing out of the water? And since conventional lumber grading procedures couldn't be counted upon to supply the level of lumber consistency required, something else was needed.

That’s why the lumber industry developed a system of grading assessment call MSR -- machine stress-rated lumber. Equipment measures the stiffness and elasticity of each and every piece of candidate lumber and assigns a passing or failing result. Although the only way to fully assess the strength of a piece of wood is to actually break it, MSR stiffness testing is a trustworthy, non-destructive alternative, especially when coupled with conservative design safety allowances. Even lumber plants that determine MSR standards are regularly assessed themselves for machine calibrations, plant use practices, and maintenance of detailed test records as part of the process.

MSR lumber generally carries two measured values on its grade stamp: strength (Fb) and stiffness (E). These numbers side-step most important differences that exist between species, simplifying the truss construction process and any other part of the building trades where consistent, reliable wood strength is critical.

Sidebar: Floors Are Changing, Too
The same truss revolution that’s enhancing roof lines is also affecting floor framing. It’s now routine to design 24-inch deep floor trusses that can span 40-feet without support. Besides more usable interior spaces, trusses are easier to install subflooring onto because of the wider nailing surfaces presented by the top edge of the floor truss web. Freedom from the need for roof support is optimized when floor frames are also entirely self-supporting.

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