In traditional construction, the roof support system is frequently built out of trusses that are engineered, built, and certified offsite. Unlike the framing, which is often still built by hand on-location, the trusses are ordered up-front from a specialty company.
Why Engineered Trusses?
Engineered trusses are built in a controlled environment, individually inspected, and certified before being delivered to the jobsite. The wood is selected for highest quality and is often cut using CNC machines. Because of the techniques used, truss manufacturers are able to produce a product that is much stronger than a hand-built version using the same wood. The engineered truss is cheaper to build, uses less materials, and is quicker to install. Plus, having the trusses pre-certified simplifies the inspection and approval process, which makes for much less potential for re-work on the most perilous portion of the framing job.
One of the key features of a factory truss is the gang-nail plate (pictured, right). Gang-nails are a small rectangular piece of metal with dozens of sharp barbs punched out to one side. It both secures and supports the wooden joint through many angles of force.
While gang-nails can be purchased at home improvement stores and installed by hand, a factory lays out all of the pieces of a truss in place, positions the gang-nails, and rolls the entire fixture through a high pressure press to secure the plate evenly. The truss is then flipped and the process is repeated.
Making the Cut
Any newbie contractor who is handed a saw for the first time is admonished to “never cut a truss.” Each truss installed on our site includes bright yellow tags that clearly indicate they are not to be cut or modified. Doing so will invalidate the certification on that piece and will require re-certification by an engineer. This may seem bit extreme, considering any Tom, Dick, and Harry can theoretically slap together the framing that holds the roof up. However, each finished piece is an engineering and mathematical bijou delicately balancing tension and compression. It is designed to withstand great forces over lifetimes and through natural disasters.
As it turns out, one of the trusses in 1120 will need to be modified. We have a 4×4 skylight in the master bedroom and need the roof support and rafter re-formed to make an open viewing space. The framers will either independently modify the truss or the truss maker will send out a tech. The factory will then send out an engineer to sign off on the final product.
Modifying the truss after installation most certainly costs a bit more. This approach was perhaps part “oops”, but it also gave us the opportunity to place the window exactly where we want it. In the software world, we call this approach “agile.”
There is more progress than I can keep up with right now, so check back soon for more exciting updates!