How to Read a Roof Truss Drawing
Roof truss drawings are packed with numbers, labels, and notes that can be overwhelming at first glance, but once you know what each section means and how everything connects, it becomes a lot easier to read. Whether you’re on a job site, in the office, or just getting familiar with these layouts, this guide will walk you through what you need to know.
Start by Identifying the Truss Type
The first thing to look at is the shape of the truss. Every truss drawing will show a profile view of that specific truss. Common trusses have a basic triangle shape. A scissor truss will have a vault built into it. Gable end trusses, which are often used at the ends of a building, may have vertical studs instead of interior webs. Before diving into numbers, make sure the shape of the truss matches what you expect for the part of the building it’s meant to go in.
Project and Truss Labels: How to Match Your Drawing
At the top of the drawing, you’ll find a project number. This should match the number found on the truss layout sheet. Matching project numbers confirm that you're looking at the right set of plans.
Each truss is also labeled with a unique identifier, like A01 or B02. These labels match up with the layout plan to show where each truss is placed in the building. For example, if the layout indicates that eleven trusses labeled A02 go along the back wall of the building, that A02 drawing will show exactly what those eleven trusses look like and how they’re built.
Understanding the Span and Height
Truss span and height are two of the most important measurements. They are listed in feet, inches, and sixteenths of an inch. For example:
A span listed as “31-0-0” means the truss stretches 31 feet, 0 inches, and 0/16ths.
A height listed as “8-8-14” means 8 feet, 8 inches, and 14/16ths from the base (bearing point) of the truss to its peak.
These dimensions help verify that the truss will fit within the building design. If the measurements don’t match the layout, something is off.
Pitch: Interpreting the Roof Slope
The pitch of the truss refers to the slope of the top chord and is expressed in rise over run, like 5/12. That means for every 12 inches of horizontal run, the truss rises 5 inches. A steeper pitch gives a higher roofline and may be used for aesthetic reasons or to shed snow and rain. A 6/12 pitch is common and practical. Lower pitches, like 4/12, are more cost-effective but may require extra waterproofing to prevent water pooling on the roof.
Overhangs and Why They Matter
Overhangs are the parts of the truss that extend past the exterior walls. These are typically listed in inches, such as 1-3-0, which equals a 15-inch overhang. Overhangs direct water away from the walls and foundation, protecting the structure from erosion and moisture damage. It’s also important to know the overhang length to ensure the fascia board and siding line up correctly.
Lumber and Sizing Details
Truss drawings include the size and type of lumber used for the top chord, bottom chord, and webs. For example, you might see all members listed as “2x4 DF No. 2.” (DF = Douglas Fir) These material specs matter for a few reasons. They help with estimating weight, choosing compatible components like fascia, and understanding the structural performance of the truss.
Load Information: What the Truss Can Hold
Load values are usually listed in pounds per square foot (psf). You’ll often see:
Top chord live load: The temporary weight the truss must support, like snow. In Western Washington, this might be 25 psf.
Top chord dead load: Permanent weight such as roofing material, sheathing, and even solar panels.
Bottom chord dead load: The weight of drywall, insulation, electrical fixtures, and the truss itself.
Bottom chord live load: May be listed as zero or noted in the drawing notes, unless additional loads (like HVAC or storage) are expected.
Some trusses will also show special loads, such as drag loads, which transfer forces across a structure, or concentrated loads where additional weight from mechanical units is expected.
Bearing Points and Connectors
A key part of reading the drawing is identifying where the truss is supported. These are called bearing points. One end might sit directly on an exterior wall, while the other might connect to another truss using a hanger. This will be shown with symbols or notes. For example, a triangle symbol typically marks the left end of the truss on the layout plan. Understanding which side is which helps ensure correct installation orientation.
Plate Size and Ply Connections
Look for notes on the metal plates used to hold the truss together. You might see something like “5x5 vertical teeth.” That tells you the size and tooth orientation of the connector plates. If it’s a multi-ply truss (a truss made from more than one layer), there will be instructions on how to fasten the plies together, such as with nails or bolts.
Permanent Bracing Locations
Some drawings show where permanent bracing should be installed. These might be marked with small boxes or notes along the web members. Permanent bracing ensures long-term stability and needs to be installed during construction, not just during temporary setup.
Truss Spacing: How Far Apart They’re Placed
Truss drawings often include spacing information, typically listed in Feet OC. That just means how far apart each truss should be. Two feet on center (2 Feet OC) is common, but some trusses may need to be spaced closer or further apart. In buildings with unusual dimensions, spacing may vary at certain points to ensure everything fits properly. This spacing directly affects the loads each truss is designed to carry.
Engineering Notes: Special Instructions and Deflection
Toward the bottom of most truss drawings, you’ll find detailed engineering notes. These can include:
Nailing patterns for chords and webs
Special loads (like drag or point loads)
Ply connection instructions
Deflection limits
Deflection tells you how much the truss can bend under load. For example, building code allows deflection up to 1/240 of the span under live load. But many trusses are designed to deflect far less, like 1/999 of the span, which is almost imperceptible. Lower deflection means a more rigid, stable truss.
Who Uses What Information
Every person on a construction team uses this drawing differently:
Builders check the overall shape and ensure the truss fits the design.
Carpenters look for orientation, placement, and bracing details.
Engineers verify the loads, deflection, and structural integrity.
Manufacturers use the full drawing to build and label each truss correctly.
Bottom Line
The bottom line is this: a truss drawing might look complicated, but it’s really just a collection of very specific, helpful details. It tells you the size, shape, spacing, lumber, loads, and how to install and connect the truss. And if you ever find yourself unsure about something on the drawing, don’t guess. Call your truss manufacturer. We’re always happy to walk through it with you.
