How to Calculate Roof Truss Span

Published September 2026 · 8 min read · By AIBuildCalc

A truss span is not the length of a rafter, and it is not the width of the roof. It is the horizontal distance the truss carries between two supports, measured before pitch, overhang, or surface area enter the picture. Get that number right and a manufacturer's span table becomes readable.

Quick answer: Measure from the outside face of one bearing wall to the outside face of the opposite bearing wall. For a gable roof over a rectangular house, that is normally the full building width. Then confirm the load the truss must carry and the spacing you plan to use, because a truss rated for a 30-foot span at 24 inches on center may be rated for only 24 feet under a heavier snow load. Run the numbers with our Roof Truss Calculator once you have both.

What span actually measures

A truss span is a bearing-to-bearing measurement taken horizontally. Three things it is not:

Not the slope length. A truss spanning 24 feet at a 6/12 pitch has top chords about 13.4 feet long, not 12, because each chord runs diagonally to the ridge.
Not the roof width including overhang. The eaves cantilever past the wall, and that overhang is specified separately as a top-chord cantilever.
Not the interior room width. Measuring inside face to inside face ignores wall framing and sheathing. On a truss near its limit, those two or three inches can decide whether the profile is rated at all.

Where trusses bear on a center beam, the building width splits into two spans.

Step 1: measure the clear span

Identify every bearing wall the truss will sit on, then measure from the outside face of one to the outside face of the opposite wall. Bearing walls carry load to the foundation; partitions added later do not.
Measure at both ends of the building and use the larger figure if the two differ.
Record the result in feet and inches. Rounding 23 ft 8 in. up to 24 ft is safe; rounding down is not.

Step 2: work out slope length, ridge height, and truss count

The run of each top chord is half the span, and the rise follows from the pitch.

Span (ft) = distance between outside faces of the two bearing walls Run (ft) = Span / 2 Rise at ridge (ft) = Run x (Pitch / 12) Slope length (ft) = Run / cos(atan(Pitch / 12)) Truss count = (Roof length (ft) x 12 / Spacing (in)) + 1

Worked example: a 24-foot span at a 6/12 pitch. Run is 12 feet, so the rise is 12 x (6 / 12), or 6 feet. The pitch angle of about 26.6 degrees makes each top chord 12 / cos(26.6), roughly 13.4 feet. A 40-foot roof with trusses 24 inches on center needs (40 x 12 / 24) + 1, which is 21 trusses.

Step 3: confirm the load the truss carries

A span table is valid only at the load it was calculated for. Residential trusses across much of the United States are designed for a top chord live load of 20 pounds per square foot, a top chord dead load of 10 psf for the roofing, and a bottom chord load allowing for ceiling drywall and insulation. Where ground snow load is significant, the required top chord live load rises to 30, 40, or 50 psf and the rated span drops accordingly. Attic and storage trusses add load beyond those assumptions.

Step 4: choose the spacing

Trusses are economical because a deep, triangulated assembly carries a roof at wider spacing than a single rafter of the same material.

Spacing	Typical use	Effect on cost and span
24 in. on center	The residential default for asphalt shingle roofs under normal loads	Fewest trusses, lowest material and labor cost, shortest rated span for a given profile
19.2 in. on center	Between the two, often so a sheathing module lines up	Slightly more trusses; modest span gain
16 in. on center	Heavy coverings such as clay or concrete tile and slate; higher snow load; stiffer ceilings	Roughly 50 percent more trusses; allows a longer span or a lighter profile
12 in. on center	Very heavy coverings, high snow regions, or where the profile cannot be deepened	Double the trusses; maximum span for a given depth

Reading a span table the right way

Span tables are published per truss profile and per design condition. A table header might specify a Fink profile, a 20 psf top chord live load, and 24 inches on center, and every one of those qualifiers is part of the answer. Step down the table to the largest span that is greater than or equal to your measured dimension, then confirm the spacing and the load in the header match your project exactly. Two traps cause most of the confusion: comparing a rating taken at 16 inches on center against a plan drawn at 24, and comparing a table calculated for 20 psf against a site that needs 40.

Common mistakes

Assuming a steeper pitch increases allowable span. It does not. Pitch changes height and slope length, not the capacity of the profile.
Forgetting to tell the supplier about overhangs, which are fabricated into the truss and cannot be added on site.
Ordering without checking delivery access. Trusses arrive on a long truck and are placed by crane; narrow streets and low branches change the plan.
Mixing profiles from two manufacturers, or two span tables, on one roof.

When to stop calculating and call an engineer

Spans beyond the published table, roofs that are not rectangular, point loads such as a rooftop air handler, vaulted ceilings without a bottom chord, and any project where an existing interior wall comes out all need a design professional. Price the roofing material next with the Metal Roofing Calculator, check the geometry with the Roof Pitch Calculator, or browse the roofing calculators index.