How To Measure Trusses 7 Key Dimensions To Know?

Measuring roof trusses correctly comes down to seven specific dimensions: span, rise, pitch, chord length, web member length, heel height, and overhang. If you get any of these wrong, the entire roof structure can be off. This guide walks through each one with the actual numbers and methods builders use.

What Are the 7 Key Dimensions When Measuring Trusses?

The seven dimensions are not a random list. They come from standard truss design and engineering requirements used by the Truss Plate Institute and structural engineers across the US. Each one plays a specific role in how the truss carries load and fits the building.

Here is the complete list with what each dimension actually tells you:

• Span – The horizontal distance from one outer wall to the opposite outer wall. This is the total width the truss must cover.
• Rise – The vertical height from the top of the wall to the peak of the truss. This determines roof slope and attic space.
• Pitch – The roof slope expressed as a ratio of rise over run. A 6/12 pitch means 6 inches of rise for every 12 inches of horizontal run.
• Chord Length – The actual length of the top or bottom sloping board from end to end. This is not the same as span because of the angle.
• Web Member Length – The internal triangular braces that transfer loads. Each web member has its own specific length based on the truss design.
• Heel Height – The vertical distance from the top of the wall plate to the top of the top chord at the outer edge. This affects insulation clearance and ventilation.
• Overhang – The horizontal distance the truss extends past the outer wall. This provides eaves and protects siding from rain.

Each dimension is measured from specific reference points. The span is measured from outside of bearing to outside of bearing. The rise is measured from the top of the top chord at the bearing point to the peak. Mixing these reference points causes errors that compound across the entire roof.

How Do You Measure Truss Span Correctly?

Span is the most critical dimension because it determines whether the truss will fit the building at all. The standard method is to measure from the outside edge of one bearing wall to the outside edge of the opposite bearing wall. This is called the clear span or design span in engineering drawings.

For a simple gable roof on a 24-foot-wide house with 2×6 walls, the span would be 24 feet plus the thickness of both walls. If each wall is 5.5 inches thick, the total span is 24 feet 11 inches. Many builders measure from inside wall to inside wall and add wall thickness, which works but introduces rounding errors.

The Truss Plate Institute specifies that span measurements should be taken at the bearing points, not at the ridge or ends of the truss. If you measure at the ridge, you get the wrong number because the truss angles outward. Always measure at the bottom chord where it sits on the wall.

A common mistake is assuming the span equals the building width. It does not. The span is the distance between bearing points, which may be different from the total building width if there are overhangs or cantilevers. Check the architectural plans for the exact bearing locations before cutting or ordering.

What Is the Correct Way to Measure Truss Pitch and Rise?

Pitch is always expressed as a ratio like 6/12 or 4/12. The first number is the vertical rise in inches per 12 inches of horizontal run. A 6/12 roof rises 6 inches for every foot of horizontal distance. This ratio is used to calculate rafter lengths, material quantities, and load capacities.

To measure pitch on an existing truss, place a level horizontally on the top chord and measure the vertical distance at the 12-inch mark. That number is your pitch. If the level shows 5 inches of rise at 12 inches of run, the pitch is 5/12. This method works on any truss as long as the level is perfectly horizontal.

Rise is the total vertical height from the top of the bearing wall to the peak. You calculate it by multiplying the pitch by half the span. For a 24-foot span with a 6/12 pitch, half the span is 12 feet. Multiply 12 feet by 6 inches per foot, and the rise is 72 inches or 6 feet. This assumes the ridge is centered, which it is on most standard gable trusses.

Some trusses have an offset ridge, especially on shed or asymmetrical roofs. In those cases, you calculate rise separately for each side using the individual run distances. The total rise is the same on both sides, but the runs are different. Measure each run from the bearing point to the ridge point horizontally, then apply the pitch formula to each side.

How Do You Determine Top and Bottom Chord Lengths?

Chord length is not the same as span. The top chord runs at an angle from the bearing point to the ridge. Its actual length is longer than the horizontal distance because of the slope. The bottom chord runs horizontally from bearing to bearing, so its length equals the span in most cases.

To calculate top chord length, use the Pythagorean theorem. The horizontal run is half the span. The vertical rise is the rise you calculated earlier. Square both numbers, add them together, then take the square root. That gives you the top chord length from bearing to ridge. For a 24-foot span with a 72-inch rise, the top chord length is about 13 feet 5 inches per side.

Bottom chord length is simpler. It equals the span minus any overhang extensions. If the span is 24 feet and the overhang is 12 inches on each side, the bottom chord is still 24 feet because overhangs are separate pieces on most trusses. Some truss designs integrate the overhang into the bottom chord, so check the drawings.

Chord lengths must account for the heel height. The heel is the vertical space between the top and bottom chords at the bearing point. A taller heel means the top chord starts higher, which slightly changes its angle and length. Most standard trusses have a 4-inch to 6-inch heel, but energy-efficient designs can have 12-inch heels for insulation clearance. Adjust your chord calculations accordingly.

What About Web Members and Internal Bracing?

Web members are the diagonal and vertical boards inside the truss that transfer loads from the top chord to the bottom chord. Each one has a specific length based on its position in the triangular pattern. You cannot measure them all the same way because their angles and endpoints vary.

The standard approach is to measure from intersection point to intersection point. Each web member connects at the centerline of the chords, not at the edges. Use a framing square to find the angle and a tape measure to get the distance between the centerline marks. Write down each length separately because they are rarely the same.

Common web member configurations include Warren, Pratt, and Howe patterns. In a Warren truss, the web members form equilateral triangles and all have similar lengths. In a Pratt truss, the vertical members are shorter than the diagonals. The pattern determines which lengths repeat and which are unique.

If you are building a custom truss, you need to calculate each web member length using trigonometry. The angle of each web member depends on the pitch and the spacing of the panel points. Most builders use truss design software for this because manual calculation is tedious and error-prone. For existing trusses, simply measure each one individually and label them.

How Do You Measure Heel Height and Overhang Accurately?

Heel height is measured vertically from the top of the bottom chord to the top of the top chord at the bearing point. Place a square against the bottom chord and measure straight up to the top of the top chord. This dimension is critical for insulation depth and roof ventilation clearance.

Building codes in most US jurisdictions require a minimum heel height for energy efficiency. The International Residential Code specifies that attics with insulation must have at least 1 inch of ventilation space above the insulation at the eaves. A heel height of 4 inches is common for standard trusses, but high-performance homes often use 8 to 12 inches.

Overhang is measured horizontally from the outside face of the bearing wall to the end of the truss tail. This is not the same as the rafter tail length because the tail is angled. The horizontal overhang determines how far the eaves extend and affects water runoff and shading.

Standard overhangs range from 12 to 24 inches. Longer overhangs provide more protection but add weight and require stronger connections. Measure from the wall face outward, not from the bearing point inside the wall. If the wall thickness is 5.5 inches, the overhang measurement starts at the exterior surface, not the interior.

Common Mistakes When Measuring Trusses

The most frequent error is measuring span from the inside of the walls instead of the outside of the bearings. This mistake shortens the truss by the thickness of both walls, which is usually 9 to 11 inches total. A truss built 10 inches too short will not sit properly on the walls and can cause structural instability.

Another common mistake is confusing pitch with rise. Pitch is a ratio, rise is a total measurement. If someone asks for a 6/12 pitch and you give them a 6-foot rise on a 12-foot span, that is actually a 12/12 pitch. The numbers look similar but the result is completely different. Always clarify whether you are discussing pitch ratio or rise height.

Measuring chord lengths without accounting for heel height also causes problems. A truss with a 12-inch heel has a different top chord angle than one with a 4-inch heel, even if the pitch is the same. The chord must be longer to maintain the same slope from a higher starting point. Use the actual rise from the top of the heel, not from the bottom chord.

Overhang measurements are sometimes taken from the bearing point inside the wall rather than from the exterior wall face. This adds the wall thickness to the overhang, making the eaves longer than intended. The extra length can cause the truss to overhang the fascia or interfere with gutters.