How to Read a Roof Truss Placement Layout

Understanding how to read a roof truss placement layout is an essential skill for builders, architects, and carpenters. A placement diagram is essentially a map of where each truss belongs, and knowing how to interpret it ensures that everything is installed correctly and efficiently.

Identifying the Trusses

Each truss in a placement layout is labeled with an identification code. These codes usually appear near the end of the truss, often marked with an arrow that indicates orientation. For example, a truss labeled D13 is unique to its location on the project, and no other truss will match it. These labels correspond directly to the truss drawings that show the details of how the truss is built.

This labeling system is critical because it ensures every truss ends up in the correct spot. You may also see other identifiers, like hanger labels, which tell you what hardware is required and how many are needed.

Reading the Diagram

A truss placement diagram is a plan view of the building, meaning you’re looking straight down from above. It shows where each truss should go, along with important details like spacing, dimensions, and orientation.

Dimensions are listed in feet, inches, and sixteenths. For example, a note that reads 10-6-0 means ten feet, six inches, and zero sixteenths. In addition to dimensions, the layout shows spacing, such as trusses set at two feet on center.

You’ll also see loading criteria on the diagram. These include roof snow load, dead load for the roof, live load for the ceiling, dead load for the ceiling, and wind speed or exposure requirements.

Spacing and Orientation

Trusses are often shown with starting points marked clearly. For example, the first truss in a section might be placed two feet in from a corner, followed by trusses every 24 inches on center. This makes it easy to measure out and mark the building before the trusses arrive, which helps delivery and installation go smoothly.

Wall Heights and Overhangs

Wall heights, also called plate heights, are another important detail on placement diagrams. A typical plate height might be nine feet, one and one-eighth inches, but some sections of the building may be taller, such as 11 feet, one and one-eighth inches. Differences in wall heights affect how trusses sit, so these measurements need to be clear and consistent.

The layout also shows overhangs, which are the portions of the truss that extend beyond the wall. Overhangs are measured in inches and fractions rather than feet and sixteenths. For example, an overhang might be listed as 15 ¼ inches. Sometimes trusses are cantilevered to create these overhangs, extending the bottom chord past the wall for a clean finished look.

Special Trusses and Loads

Not every truss in a layout is the same. Some may carry special requirements. For example, a drag truss transfers not only vertical loads from the roof but also horizontal loads from wind or seismic activity between walls. The diagram will note if a truss has a specific load requirement, such as a 2000-pound drag load.

Other types include girder trusses, which carry the weight of jack trusses that connect into them, and upset beams that certain trusses may hang from instead of sitting directly on a wall. Placement diagrams will clearly indicate these details, allowing framers to install them correctly.

Notes and Verification

Placement layouts often include notes from the designer. These may point out dimensions that need to be verified before trusses are built, such as unclear lengths on the building plans. Verifying these dimensions with the architect or builder ensures everything lines up properly on site.

The layout also includes manufacturer information and instructions. For example, Truss Components of Washington always asks that framers call before making adjustments so an engineered fix can be provided if something doesn’t line up.

Bracing Plans

Alongside placement diagrams, truss packages come with bracing plans—both temporary and permanent. Following the bracing plan is essential to maintain the structure's integrity while the trusses are being installed and long after the roof is complete.

Final Tips

To make the most of a roof truss placement layout, take the time to review it before the trusses arrive. Mark out locations on the building in advance, verify your dimensions, match labels carefully, and follow the bracing plan. And if you ever have questions, reach out to your truss manufacturer before making changes.

When read carefully, a placement layout is more than just a drawing. It’s the guide that ensures your roof comes together the way it was designed.