How to Calculate Roof Truss Dimensions: Span, Rise, Pitch, and Chord Length

Calculating roof truss dimensions starts with a few basic measurements: the building span, roof pitch, rise, and overhang. These figures help establish the roof’s geometry and support accurate planning, but they do not determine whether a truss is structurally safe. Truss member sizes and connections must account for loads, materials, and local building requirements. The formulas below explain how to calculate common dimensions and what to confirm before ordering or building a truss.

What Roof Truss Dimensions Describe

A roof truss is a structural framework, usually made of interconnected top chords, a bottom chord, and internal web members. Its dimensions describe the truss’s overall shape and how it fits between the building’s supports. For a typical symmetrical gable roof, the principal measurements are span, run, pitch, rise, and top-chord length.

These terms are related, but they are not interchangeable. For example, a 24-foot truss span does not mean each sloping top chord is 12 feet long. The horizontal run is half the span on a symmetrical roof, while the top chord is longer because it follows the slope.

• Span: The horizontal distance between the truss’s bearing points, as defined on the project drawings.
• Run: The horizontal distance from one bearing point to the roof’s centerline on a symmetrical gable roof. It is usually half the span.
• Pitch: The roof’s rise compared with its horizontal run, commonly stated as inches of rise per 12 inches of run.
• Rise: The vertical distance from the bearing level to the roof peak, or ridge, for a symmetrical gable truss.
• Top-chord length: The sloped distance along a top chord. Its exact endpoint depends on the truss design and whether an overhang is included.

Drawings and manufacturers may define measurements differently, particularly at the bearing seat, wall plate, and eave. Before using a calculation to order a truss, confirm the measurement points with the designer or truss supplier.

Measure the Building Span and Run

Begin with the distance between the truss bearings, not automatically the outside-to-outside width of the building. The correct span depends on how the truss bears on the walls or other supports. A plan may show the bearing-to-bearing span directly; if it does not, the building dimensions and bearing details must be checked.

For a symmetrical gable roof, calculate the run by dividing the span by two:

Run = Span ÷ 2

For example, if the bearing-to-bearing span is 24 feet, the run on each side is 12 feet. This half-span rule applies to a symmetrical gable, not necessarily to a shed roof, an asymmetrical roof, or a truss with offset supports. Those layouts require their own horizontal measurements.

Keep the units consistent throughout the calculation. A span measured in feet can be converted to inches by multiplying by 12. Mixing feet and inches without converting can produce a rise or chord length that is off by a factor of 12.

Calculate Roof Rise From Pitch

In common U.S. residential construction, roof pitch is often written as a ratio with a 12-inch horizontal run. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. It does not mean a 6-degree angle.

When the run is in feet, calculate the rise in feet with this formula:

Rise = Run × (Pitch ÷ 12)

For a 24-foot span with a 6:12 pitch, the run is 12 feet. The rise is:

12 × (6 ÷ 12) = 6 feet

The ridge is therefore 6 feet above the bearing level, assuming a symmetrical truss and a level bearing line. The building’s total height also depends on the wall height, bearing elevation, heel configuration, and any other details shown in the plans.

For a roof with an angle specified in degrees instead of a pitch ratio, use the tangent of the angle:

Rise = Run × tan(angle)

Angle-based calculations are less common in residential plans. A calculator set to the correct angle mode is essential when using this method. A 6:12 pitch corresponds to an angle of about 26.6 degrees, but the pitch ratio is generally easier to apply in framing calculations.

Calculate the Top-Chord Length

For a symmetrical gable roof, the sloped top-chord length from the bearing point to the centerline can be estimated with the Pythagorean theorem:

Top-chord length = √(Run² + Rise²)

With a 12-foot run and 6-foot rise, the calculation is √(12² + 6²), or √180. The sloped distance is about 13.42 feet, which is approximately 13 feet 5 inches. This is the geometric distance from the bearing point to the peak, not necessarily the cut length of a truss member.

Another way to calculate the sloped length is to use the pitch factor:

Top-chord length = Run × √(1 + (Pitch ÷ 12)²)

For a 6:12 roof, the factor is √1.25, or about 1.118. Multiplying a 12-foot run by 1.118 gives approximately 13.42 feet. The formula is useful for estimating roof geometry, but construction drawings govern actual member lengths and joint locations.

Truss top chords may be assembled from multiple pieces or meet at a ridge joint. Their specified lengths can differ from the simple bearing-to-centerline measurement because of connector plates, end cuts, bearing-seat details, and the designer’s joint layout.

Include the Eave Overhang Correctly

An overhang is the portion of the roof that extends beyond the exterior wall. It is often specified as a horizontal distance, such as 12 or 18 inches, but the sloped top chord extends farther than that horizontal measurement.

To estimate the additional sloped length for a horizontal overhang, use the same pitch factor:

Sloped overhang length = Horizontal overhang × √(1 + (Pitch ÷ 12)²)

For an 18-inch horizontal overhang on a 6:12 roof, the horizontal distance is 1.5 feet. The sloped extension is 1.5 × 1.118, or about 1.68 feet. Added to the 13.42-foot bearing-to-peak distance, the approximate slope distance from the eave end to the peak is 15.10 feet.

This estimate describes the geometry, not necessarily the truss order length. Some trusses include the overhang in the top-chord design; others use a separate outlook or extension detail. Plans may also define the overhang from the wall face, fascia, or another reference point. Confirm what the stated overhang measures before ordering materials.

Account for Heel Height and Ridge Elevation

The heel is the area where the truss meets its bearing support near the eave. Heel height refers to the vertical depth or clearance at that connection, but the precise definition can vary between drawings and manufacturers. It is important because it affects how the roof connects to the wall and how much space is available for insulation near the eaves.

Heel height is not automatically added to the calculated roof rise. The rise formula measures from the bearing level to the ridge based on the roof’s pitch and run. A plan may show a raised heel, a specific bearing detail, or a different reference elevation, so the truss drawing should state how the dimensions are measured.

For basic layout, if the bearing elevation is known, the ridge elevation can be estimated as:

Ridge elevation = Bearing elevation + Rise

For example, if the bearing level is 9 feet above a reference floor and the calculated rise is 6 feet, the ridge is about 15 feet above that same reference. This estimate does not include adjustments required by the project’s detailed framing or architectural drawings.

Worked Example: A 24-Foot-Wide, 6:12 Gable Roof

Consider a symmetrical gable roof with a 24-foot bearing-to-bearing span, a 6:12 pitch, and an 18-inch horizontal overhang on each eave. The following values describe its basic geometry:

These figures are useful for understanding the roof shape and checking rough estimates. They are not a complete truss layout. The truss drawing may specify different member dimensions, bearing points, top-chord segments, plate locations, or overhang construction.

Measurements Needed for a Truss Order

A truss manufacturer needs more than the span and pitch to produce a suitable design. Requirements differ by project, but the plans or order information commonly include the following:

• Building span and bearing locations: The exact distance and support conditions used for the truss design.
• Roof profile: Pitch, symmetry or asymmetry, and any changes in slope.
• Overhang details: Length, reference point, and whether the overhang is built into the truss or framed separately.
• Heel and bearing details: Required seat dimensions, wall or beam conditions, and elevation information.
• Design loads: Dead load, roof live load, and applicable snow, wind, or other loads based on the site and building requirements.
• Special conditions: Ceilings, attic storage, solar equipment, mechanical equipment, openings, or other concentrated loads that affect the design.

Local building codes, adopted design standards, and site conditions affect the required design. The building department or project design professional can identify applicable requirements; the truss supplier needs accurate project information to prepare the truss design and installation documents.

Geometry Does Not Determine Structural Capacity

Basic roof truss dimension calculations establish shape, not strength. They cannot determine the required lumber grade, member size, web arrangement, connector plates, bracing, or allowable loads. Two trusses with the same span and pitch can require different designs because of different materials, spacing, loads, support conditions, and building locations.

Roof trusses are engineered components. In the United States, the truss design and installation information should be prepared or reviewed by appropriately qualified professionals under the applicable codes and project requirements. A general geometric formula is not a substitute for a truss design drawing or sealed engineering where required.

Do not cut, drill, notch, or alter a truss member unless the modification is approved in writing by the truss designer or a qualified building design professional. Changes can disrupt the load path through the truss and create a structural hazard. If a truss arrives damaged or does not match the project, pause installation and contact the supplier or responsible designer.

Common Calculation Errors to Avoid

• Using the wrong span: Outside wall width may not equal the bearing-to-bearing span shown on the truss design.
• Confusing pitch with degrees: A 6:12 pitch is a rise-to-run ratio, not a 6-degree roof angle.
• Forgetting to halve the span: On a symmetrical gable roof, each side’s run is half the total span.
• Mixing units: Convert feet to inches or inches to feet before calculating.
• Ignoring the overhang reference: A stated overhang may be measured from the wall face or another point, and it may be horizontal rather than sloped.
• Treating an estimate as a cut list: Geometric chord length does not account for joints, plates, cuts, or manufacturer details.
• Assuming every roof is symmetrical: Shed roofs, offset ridges, and complex roof forms require different geometry.

Quick Formula Reference

These formulas provide a reliable way to calculate the basic geometry of a symmetrical gable roof. For construction, the project drawings and engineered truss documents remain the controlling source for bearing locations, member dimensions, connections, loads, and installation requirements.