Engineered Trusses

The traditional method of assembling the wood framing members that hold up a residential roof structure was a manual process of custom cutting rafters, ceiling joists and the various pieces required. For the last couple of decades, virtually all roof framing is built as factory manufactured components that are assembled on site. There are several advantages to this approach. Engineered designs allow greater loads to be carried with smaller dimension lumber; saving material costs. Greater uniformity in the components helps with the quality of construction. Factory automation provides labour savings on-site as trusses can be lifted into place as finished components. Less skill is required in the field and roof structures can be framed in less time.

Truss manufacturing companies serve builders by efficiently producing trusses at their factories and shipping them to the construction site. Smaller trusses are lifted by hand and larger ones are often lifted by crane to be installed. The process start with specialized design software that takes design parameters as input and creates truss designs that meet load, span and code requirements. In large part, trusses form a repetitious design element where each truss is offset parallel to the next by a standard amount.

A truss is made up of a top and bottom chord and supporting pieces between called webbing that are often arranged in a triangular pattern. There are truss designs to accommodate different requirements like cathedral ceilings, attic storage, tray ceilings and so on. Truss manufacturers cut several of each component and assemble the parts in jigs for speed and accuracy. At the joints, the pieces are held in place by a gusset, which is a plate with many fasteners. In earlier versions the gussets often were a piece of plywood covering the end of several parts held together with numerous nails. Now the most common gussets are galvanized sheet metal cut with many tangs that are pressed into the wood.

Laying out truss components

You can see how the workers assemble each part of the truss on a table according to the design. Gussets are laid on the joints and a press rolls into position and pushes the tangs down into the wood for a strong and secure joint. The process is repeated over and over until enough trusses have been produced for the house. Then they are shipped to the construction site. The first picture shows the layout of pieces and the second the press in action.

Pressing the gussets

In the example shown the trusses are relatively small and only form one half of the roof for a modular home. It’s not uncommon for trusses to span more than thirty feet in length and take the entire load for that distance. This removes the need for load bearing walls in the interior space of the house and facilitates open designs and subsequent interior renovations. Because trusses carry so much of the load in a house, it is critical that the design follows engineered specifications and that the plan is followed accurately on site. Municipal building officials help to ensure that code issues like nailing, bracing and other requirements are consistently applied. As home inspectors, we visually examine the trusses, typically from an attic hatch, to see if we can spot any damage to truss components or improper modifications. Given the limited view possible from this vantage point it can only be a cursory examination.


Author: Rob Cornish is a Home Inspector in Ottawa, Canada. © 2013 HomeXam Inc.
If you took the time to read this post, please take the time to Google+ it. Thanks.

Visit Us On TwitterVisit Us On FacebookVisit Us On YoutubeVisit Us On Linkedin