How to Read a Roof Truss Drawing: What You Need to Know
If you've ever looked at an engineered roof truss drawing, you know there's a lot of information packed into a single page. Between dimensions, lumber specifications, loading requirements, and engineering notes, it can be a little overwhelming.
Fortunately, understanding these drawings is easier than you might think.
In our latest video, Chad Johnson, owner of Truss Components of Washington, uses his 30+ years of industry experience to explain the most important parts of a roof truss drawing and what builders should look for.
Start With the Basics: Shape, Labels, and Dimensions
Before diving into the technical information, start by confirming that you're looking at the correct truss.
Every truss drawing includes identifying information, such as a project number and truss label. Labels like A01 or A02 correspond with the truss placement layout, helping builders determine where each truss belongs.
Next, look at the overall shape. Is it a common truss, scissor truss, or another design? Does the profile match what you expect for the building?
You'll also find important measurements, including:
Span: The horizontal distance indicated on the drawing.
Height: How tall the truss is from its specified reference point.
Pitch: The roof's slope, such as 5/12, meaning 5 inches of rise for every 12 inches of horizontal run.
Overhang: How far the truss extends beyond the exterior wall.
Truss dimensions are commonly written in feet, inches, and sixteenths. For example, 31-0-0 represents 31 feet.
Understanding Lumber and Loading Requirements
Roof truss drawings also identify the lumber used for the top chords, bottom chords, and webs. These specifications include lumber sizes and grades selected to meet the structural requirements of the truss.
Another important section is the loading information.
This tells you what loads the truss was engineered to support, typically measured in pounds per square foot (PSF).
You'll commonly see:
Top chord live load: Temporary loads such as snow.
Top chord dead load: Permanent roof materials, including roofing and sheathing.
Bottom chord loads: Loads associated with ceilings, insulation, and other applicable conditions.
Some trusses also have special loading requirements, such as equipment loads or forces transferred from other structural components.
These details help ensure the truss is designed for the specific building and its intended use.
Bearing Points, Connections, and Bracing
One of the most important things to identify on a truss drawing is where the truss is designed to be supported.
These locations are called bearing points. Depending on the design, a truss might rest on exterior walls, interior supports, or connect to another truss using a hanger.
Drawings may also include information about metal connector plates, fastening requirements, and permanent bracing locations.
These details matter because manufactured trusses are engineered systems. Their performance depends on proper support, connections, installation, and bracing.
Always follow the project's approved truss documents and applicable installation requirements.
How Truss Drawings and Placement Layouts Work Together
An individual truss drawing and a truss placement layout serve different purposes.
The truss drawing explains how an individual truss is designed. The placement layout shows where it belongs in the building.
The placement layout identifies truss locations, spacing, and orientation. For example, Chad demonstrates how a truss labeled A02 on its individual drawing corresponds with the A02 locations on the layout.
This is especially important during installation, when builders need to verify that the correct trusses are going in the correct locations.
Making Sense of Your Truss Drawings
You don't need to memorize every engineering detail to understand the basics of a roof truss drawing.
Start by checking the truss identification, shape, dimensions, and placement. From there, review the loading requirements, bearing points, connections, and bracing notes relevant to your project.
And if something doesn't make sense, don't hesitate to ask your truss manufacturer.
At Truss Components of Washington, we've been manufacturing engineered roof trusses, floor trusses, and prefabricated wall panels for builders throughout Western Washington since 1992.
Our team is always happy to help customers better understand their truss drawings and prepare for a successful project.
Have questions about an upcoming project? Contact Truss Components of Washington to speak with our team or request a quote.