Measuring a roof truss comes down to capturing seven key dimensions: the span (bottom chord length), overall height, heel height, overhang length, pitch, wall-to-wall distance, and any offset if the peak isn’t centered. Whether you’re replacing damaged trusses, matching an addition to an existing roofline, or ordering custom trusses for new construction, getting these numbers right is the difference between a truss that drops into place and one that doesn’t fit.
Parts of a Truss You Need to Know
Before you start measuring, it helps to know the vocabulary. A truss has two main structural elements: chords and webs. The top chords are the sloped members that follow the roofline. The bottom chord is the horizontal member that spans from wall to wall, forming the base of the triangle. The webs are the interior members connecting top and bottom chords, arranged in diagonal and vertical patterns to distribute weight.
The peak is the highest point where the two top chords meet. The heel is the corner where the top chord meets the bottom chord at each end of the truss. Gusset plates are the metal or plywood connectors at each joint holding the members together. The overhang (also called the tail) is the portion of the top chord that extends past the wall to form the eave.
The Seven Measurements That Define a Truss
1. Bottom Chord Length (Span)
This is the most fundamental measurement. Run your tape along the full length of the bottom chord from one end to the other. On an installed truss, this includes any portion that extends beyond the exterior walls. The span determines the structural load the truss must carry, and even a half-inch error can cause problems during installation.
2. Overhang Length
Measure from the outside edge of the wall’s top plate to the end of the top chord on each side. Do not include the fascia board in this measurement. Check both sides of the truss independently, because they aren’t always equal, especially on older buildings or additions.
3. Heel Height
This is the vertical “thickness” of the truss at the heel, measured from the bottom of the bottom chord straight up to the underside of the roof sheathing (or the top of the top chord if sheathing is removed). Heel height determines how much insulation you can fit above the exterior wall and affects the overall look of the roofline at the eave.
4. Overall Height
Measure from the bottom of the bottom chord straight up to the peak. If you can access the peak directly, a tape measure works fine. If you can’t reach the peak safely, you can calculate this from the pitch and half the span, which is covered in the next section.
5. Peak Location
On a symmetrical gable truss, the peak sits at the exact center of the span. But on many designs, it doesn’t. Measure horizontally from each end of the bottom chord to the point directly below the peak. If those two numbers aren’t equal, you have an offset peak, and your truss manufacturer needs both measurements.
6. Wall-to-Wall Distance
If the bottom chord extends past the exterior walls (creating a cantilever on the interior side), you need a separate measurement from the outside face of one wall to the outside face of the opposite wall. Note whether the walls have siding, brick veneer, or other cladding, since this affects where the truss actually bears.
7. Cantilever Distance
If the truss extends past a wall on the interior side (common in bump-outs or bay windows), measure from the outside of the supporting wall to the end of the truss body, not to the end of the overhang. This is a separate number from the overhang.
How to Measure Roof Pitch
Pitch describes how steeply the roof rises and is expressed as a ratio of vertical rise to horizontal run. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. You can measure pitch several ways depending on what you can access.
The simplest method: hold a level horizontally against the underside of a rafter or top chord so that 12 inches of the level extends from your starting point. Then measure straight down from the end of that 12-inch mark to the rafter. That vertical distance is your rise, giving you a direct x:12 reading. If you measure 6 inches, you have a 6:12 pitch.
If you know the total rise (overall height minus heel height) and half the span, you can calculate pitch mathematically. Divide the rise by the run (half the span) and multiply by 12 to get the x:12 ratio. To convert to degrees, take the inverse tangent of the rise divided by the run. A 6:12 pitch, for example, equals about 26.6 degrees. A digital angle finder placed directly on the top chord gives you degrees instantly, which you can then convert.
Pitch matters for structural loading. Building codes treat pitches of 3:12 and steeper differently from low-slope roofs when it comes to how the ridge bears weight and how rafter ties function.
Tools That Make the Job Easier
A standard 25-foot tape measure handles most truss measurements on the ground or in an accessible attic. For installed trusses where you can’t easily reach the peak, a laser distance meter is far more practical. Models with built-in angle sensors can take indirect measurements using a single reference point, calculating heights you can’t physically reach by combining distance and angle readings automatically.
A digital angle finder (sometimes called an angle locator) clamps directly onto a chord and reads the angle in degrees. This is faster and more accurate than the level-and-tape method for pitch, especially on steep roofs. A standard speed square or rafter square also has pitch markings and works well for quick checks.
For accuracy, always measure in inches rather than feet-and-inches when recording numbers for a manufacturer. Rounding errors compound across multiple measurements, and truss fabricators work in precise increments.
Account for Actual Lumber Sizes
Trusses are typically built from 2×4 or 2×6 lumber, but nominal sizes don’t match actual dimensions. A 2×4 actually measures 1.5 by 3.5 inches, and a 2×6 measures 1.5 by 5.5 inches. This matters when you’re measuring heel height or overall height on an existing truss, because the chord thickness affects those numbers. If you’re measuring to the outside of a chord, note whether you’re including the chord’s depth or measuring to its near edge. Being explicit about this with your manufacturer prevents misunderstandings.
What Your Truss Manufacturer Needs
If you’re ordering replacement or new trusses, your seven measurements are the starting point, but manufacturers need more. Expect to provide architectural or structural drawings if you have them, along with wall and ceiling heights, the planned on-center spacing between trusses (commonly 24 inches for residential), and floor joist specs if the trusses tie into the floor system.
You’ll also need to supply local load requirements. Snow load varies dramatically by region, and your local building department can tell you the design snow load for your area in pounds per square foot. Wind load matters too, particularly in coastal or high-wind zones. These numbers drive the engineering behind web configuration and chord sizing, so a truss designed for a mild climate won’t be approved for a heavy snow region.
Any unusual features need to be communicated clearly: cathedral ceilings require scissors trusses with a pitched bottom chord, and the bottom chord pitch depends on the top chord pitch, span, and heel height. Oddly placed windows, skylights, or mechanical chases that pass through the truss plane all affect the design. Building codes prohibit cutting, notching, or drilling holes in manufactured trusses unless the manufacturer specifically allows it or a structural engineer signs off, so any penetrations need to be planned before fabrication.
Finally, trusses require a minimum of 1.5 inches of bearing surface on wood or metal framing, and 3 inches on masonry or concrete. If your walls are unusual in any way, measure the available bearing width so the manufacturer can confirm the truss design works with your structure.
Measuring Trusses Already in Place
Measuring installed trusses is trickier than measuring on the ground because access is limited. In an attic, you can usually reach the bottom chord and heel areas directly, but the peak may be difficult to access safely. Start with the bottom chord length and overhang on each side, then measure heel height at both ends. Use a laser distance meter aimed at the peak to get overall height if you can’t reach it with a tape.
Check for consistency by measuring at least two or three trusses across the roof. Older buildings sometimes have trusses that vary slightly due to settling, moisture damage, or original construction tolerances. If you find variation, note the range and share it with your manufacturer so they can design for the actual conditions rather than an idealized measurement. Photograph each measurement point as you go, since it’s much easier to clarify a question with a photo than to climb back into the attic for a second round of measurements.