How to Read a Roof Truss Placement Diagram
A roof truss placement diagram is a plan view of the building, meaning you're looking at the structure from directly overhead. It shows where the individual trusses in a roof package are intended to be placed.
Each truss is typically assigned an identification such as D11, D12, D13, J01, or another designation depending on the manufacturer's labeling system. Those labels allow you to match the trusses shown on the placement diagram with the individual truss drawings included with the package. Pasted text
Some trusses may look similar while being designed for different locations or loads, so correctly matching the truss identification to the placement diagram is important.
1. Start With the Truss Labels
One of the first things to identify is the truss designation.
The placement diagram will show the label for each truss and where it belongs within the structure. The physical trusses will also be labeled so they can be matched with the diagram and corresponding truss drawing.
Depending on the project, you may see groups or "series" of trusses with labels such as J01, J02, J03 and J04.
However, don't assume two trusses are interchangeable simply because they look similar. A particular truss designation may be unique to one location in the building. Pasted text
2. Check the Truss Orientation
Placement diagrams will typically include an orientation marker, such as a triangle or arrow at one end of the truss.
This identifies which end of the truss corresponds with the orientation shown on its individual truss drawing. Pasted text
Orientation becomes particularly important on more complicated roof systems where trusses may have different profiles, bearing conditions, overhangs, or loading requirements.
Before setting a truss, make sure you have both the correct truss and the correct orientation.
3. Understand the Dimensions
A roof truss placement diagram can contain a lot of dimensions in a relatively small area.
A common dimension format on truss drawings and placement diagrams is:
Feet – Inches – Sixteenths
For example, a dimension shown as 10-6-0 represents:
10 feet, 6 inches, 0 sixteenths of an inch. Pasted text
Reference lines on the diagram indicate where those measurements begin and end.
Pay close attention to these dimensions rather than assuming the truss locations based solely on the visual representation of the building.
4. Find the Truss Spacing and Starting Point
The placement diagram should also identify the required on-center spacing.
For example, a roof might call for trusses at:
24 inches on center
That doesn't necessarily mean every space throughout the entire roof will be exactly 24 inches. Complex roof systems can include irregular spacing where different truss sections intersect.
The example discussed in our video includes a series of trusses spaced 24 inches on center followed by an unusual one-foot space before the next series begins. Pasted text
This is why both the spacing and the starting point matter.
Whenever possible, it's a good idea to review the roof truss placement diagram before the trusses arrive so the locations can be marked ahead of time. This can also help make the delivery and installation process smoother. Pasted text
5. Look for Hangers and Hardware
Trusses aren't always simply bearing directly on a wall.
A placement diagram may also identify hangers and other hardware required in certain locations.
For example, a hanger designation may appear next to a group of trusses along with a quantity indicating how many are required. Because manufacturers can use different labeling conventions, it's important to distinguish between:
Truss identification
Hanger or hardware identification
Quantities
Placement notes
On complicated layouts, several of these pieces of information may appear very close together. Pasted text
6. Pay Attention to Wall and Plate Heights
Wall height can directly affect how a roof truss is designed and how it bears on the structure.
Placement diagrams may show a typical plate height as well as areas where walls are higher or lower than the rest of the building.
For example, a project may primarily use a 9-foot-1⅛-inch plate height but include an entry wall that is two feet higher. Pasted text
Those differences need to be clearly communicated between the builder, framer, designer, and truss manufacturer.
A seemingly small misunderstanding about wall height can affect how the trusses meet the walls and how the roof system fits together.
7. Identify Overhangs and Cantilevers
You may also see portions of a truss extending beyond the exterior wall. This is the overhang.
One distinction worth knowing is how these measurements may be written.
While many truss dimensions are shown in feet, inches, and sixteenths, overhangs and heel heights are commonly referenced in inches.
For example, a two-foot overhang may simply be referred to as a 24-inch overhang. Pasted text
The placement diagram can help clarify where different overhang conditions occur throughout the roof.
8. Review the Design and Loading Information
A truss placement diagram may contain important design criteria for the roof system, including information such as:
Roof snow load
Roof dead load
Ceiling live and dead loads
Wind speed and exposure
Roof pitch
Eave conditions
Top chord size
These aren't just miscellaneous notes. They describe some of the conditions the roof truss system has been designed around. Pasted text
Certain trusses can also have special loading requirements.
For example, a placement diagram may identify a drag truss, which can be designed to transfer lateral forces associated with wind and seismic loading in addition to vertical loads. The example in our video identifies a drag truss with a 2,000-pound drag load. Pasted text
Special conditions like these are another reason it's important to follow the placement diagram rather than assuming trusses that appear similar can be moved or substituted.
9. Watch for Beams and Special Bearing Conditions
Not every truss will necessarily bear directly on a wall.
Some may be supported by beams or other structural components.
For example, the placement diagram may identify an upset beam supporting certain trusses while neighboring trusses bear on a wall. That information needs to be communicated to the framer so the supporting structure is positioned as intended. Pasted text
You may also encounter several different truss types within the same area, including jack trusses, girder trusses, and other specially designed trusses.
The more complicated the roof becomes, the more important the placement diagram becomes.
10. Don't Ignore "Verify" Notes
Sometimes the information provided to the truss designer isn't completely clear.
Rather than guessing, the designer may mark a dimension or condition with a note requesting verification.
That means the builder, architect, or other appropriate project contact needs to confirm the information before the trusses are manufactured.
Resolving these questions before production can prevent a relatively small uncertainty on the plans from becoming a much larger problem at the jobsite. Pasted text
Before Installing Your Roof Trusses
Before installation, take the time to compare the placement diagram with the building plans and the trusses that were delivered.
At a minimum:
Verify the dimensions.
Match the truss labels to their locations.
Confirm the orientation of each truss.
Review wall heights, bearing conditions, and special notes.
Follow the required temporary and permanent bracing information.
Contact the truss manufacturer before making adjustments if something doesn't match.
Truss packages commonly include information regarding temporary and permanent bracing, which should be followed to help maintain the integrity of the structure. Pasted text
Most importantly, don't improvise a field modification to an engineered truss because something doesn't appear to fit. Contact the truss manufacturer so the issue can be evaluated and, when necessary, an appropriate engineered repair or solution can be provided.
Questions About Your Roof Truss Package?
A roof truss placement diagram can look overwhelming the first time you see one. Once you understand the labels, orientation, dimensions, spacing, bearing locations, wall heights, and other common callouts, it becomes much easier to understand how the roof system is intended to come together.
At Truss Components of Washington, we work with builders and contractors throughout Western Washington to manufacture roof trusses, floor trusses, and prefabricated wall panels for projects of all sizes.
If you have questions about a TCOW truss package or are planning an upcoming project, contact our team and we'll be happy to help.