Is the truss count my order quantity?

It is a preliminary count. The truss supplier's layout sets the final number, including gable ends, girders and any doubled trusses.

Can I use this to build my own trusses?

No. Manufactured trusses are designed by the supplier's engineering process for your loads and location. Use this tool for planning and checking quotes only.

Construction Materials

Roof Truss Geometry Calculator

Work out the geometry of a symmetrical gable roof and a preliminary truss count. Enter the building span, either the pitch (inches of rise per 12 inches of run) or the rise in feet, the horizontal overhang, the building length and the truss spacing. You get the rise, roof angle, sloping top-chord lengths with and without the overhang, and how many trusses fit along the building. This is a geometry and quantity tool only — it does not size members, check loads or confirm code compliance.

How do I calculate roof rise and truss count?

Rise = half the span × pitch ÷ 12. A 24 ft span at 6/12 rises 12 × 6 ÷ 12 = 6 ft at an angle of arctan(6/12) ≈ 26.57°. The sloping length from wall to peak is 12 × √(1 + 0.25) ≈ 13.416 ft, or 14.534 ft with a 12 in horizontal overhang. For a 40 ft building at 24 in on center: 480 ÷ 24 = 20 spaces, so 21 trusses with one at each end.

Key takeaways

Pitch is rise per 12 inches of horizontal run; angle = arctan(pitch ÷ 12).
A horizontal overhang becomes longer when measured along the slope.
Count = ceil(length ÷ spacing) + 1, assuming a truss at both end walls.
Structural design of trusses is the manufacturer's or engineer's job.

Enter your details

Roof shape

Building span (outside of walls, ft)

Enter slope as

Pitch (inches rise per 12 in run)

Overhang, horizontal (inches)

Measured level from the wall face, not along the slope.

Layout

Building length (ft)

Truss spacing (inches on center)

Rise at the peak

6 ft (6′ 0″)

6/12 pitch · 26.57°

Sloping length incl. overhang (each side): 14.534 ft
Preliminary truss count: 21

Geometry (per side)

Half span (horizontal run): 12 ft
Rise: 6 ft (6′ 0″)
Roof angle: 26.57°
Slope factor √(1 + (pitch/12)²): 1.118
Sloping length, wall line to peak: 13.416 ft (13′ 5″)
Overhang: horizontal: 12 in
Overhang: sloping: 13.42 in
Sloping length incl. overhang: 14.534 ft (14′ 6.375″)

Truss layout

Spacing: 24 in on center
Spaces along the length: 20
Trusses (spaces + 1, one at each end): 21
Last space (closes at the end wall): 24 in

Sloping lengths are geometric lines to the outside of the wall and peak — not cut lengths. Heel height, ridge details and fascia cuts change actual member lengths.
Gable-end trusses, girder trusses and bracing are specified by the truss designer. Manufactured trusses require supplier or engineering design.

Your result is an estimate based on the information entered.

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HomeCalc USA — Roof Truss Geometry Calculator

Generated

Inputs

Building span (outside of walls, ft): 24
Enter slope as: Pitch (x in 12)
Pitch (inches rise per 12 in run): 6
Overhang, horizontal (inches): 12
Building length (ft): 40
Truss spacing (inches on center): 24 in

Results

Rise at the peak: 6 ft (6′ 0″)

Half span (horizontal run): 12 ft
Rise: 6 ft (6′ 0″)
Roof angle: 26.57°
Slope factor √(1 + (pitch/12)²): 1.118
Sloping length, wall line to peak: 13.416 ft (13′ 5″)
Overhang: horizontal: 12 in
Overhang: sloping: 13.42 in
Sloping length incl. overhang: 14.534 ft (14′ 6.375″)
Spacing: 24 in on center
Spaces along the length: 20
Trusses (spaces + 1, one at each end): 21
Last space (closes at the end wall): 24 in

Assumptions

Symmetrical gable roof with equal pitch on both sides.
Lengths measured to geometric lines, ignoring heel height and member thickness.
A truss at each end wall; no openings or hips.

Estimates are for informational purposes only and may differ from actual costs, lender offers, tax assessments or contractor quotes.

How to use this calculator

Measure the span across the outside of the bearing walls.
Enter pitch, or switch to rise if you know the peak height.
Enter the eave overhang as a horizontal distance.
Enter building length and spacing to get a preliminary truss count.

What each input means

Input	What it means
Span	Outside wall to outside wall, measured level across the building.
Pitch / rise	Inches of rise per 12 in of run, or total rise from wall plate line to peak.
Overhang	Horizontal distance the eave projects past the wall face.
Spacing	Center-to-center distance between trusses.

Formula used

Rise = (Span ÷ 2) × Pitch ÷ 12 · Angle = arctan(Pitch ÷ 12)

Sloping length = Horizontal run × √(1 + (Pitch ÷ 12)²)

Trusses = ceil(Length × 12 ÷ Spacing) + 1

Overhang is entered as a horizontal projection; its sloping length is longer by the same pitch factor as the rest of the top chord.

Worked example

Span 24 ft, 6/12 pitch, 12 in horizontal overhang, 40 ft long, 24 in on center. Run = 12 ft; rise = 6 ft; angle = 26.565°; slope factor = 1.1180; wall-to-peak = 13.416 ft; overhang sloping = 13.42 in; total sloping length per side = 14.534 ft. Trusses: 480 ÷ 24 = 20 spaces → 21 trusses. At 41 ft the count becomes 22 with a 12 in final space.

Common mistakes to avoid

Treating overhang as a sloping length
Mixing a sloped measurement into horizontal math under-counts roof area and fascia length.
Dividing length by spacing without adding one
20 spaces need 21 trusses when one sits at each end.
Reading geometry as a structural check
Rise and lengths say nothing about snow, wind or dead loads.

When the estimate may differ from reality

The truss supplier's drawings will differ from these geometric lines because of heel height, top-chord depth, ridge and fascia details, and the engineered layout may add gable-end, girder or doubled trusses.

Assumptions and limitations

What this calculator assumes

Symmetrical gable roof with equal pitch on both sides.
Lengths measured to geometric lines, ignoring heel height and member thickness.
A truss at each end wall; no openings or hips.

What it does not account for

Does not calculate structural adequacy, allowable loads, lumber sizes, connector plates, bracing or code compliance.
Hip, mono-pitch, scissor and attic trusses are not modelled.

Frequently asked questions

Is the truss count my order quantity?: It is a preliminary count. The truss supplier's layout sets the final number, including gable ends, girders and any doubled trusses.
Can I use this to build my own trusses?: No. Manufactured trusses are designed by the supplier's engineering process for your loads and location. Use this tool for planning and checking quotes only.

Sources and references

Roof Pitch, Rise, Span, and Truss Spacing ExplainedDefinitions and formulas for roof pitch, rise, run, span, sloping length, overhang and truss count, with a labelled worked example. Geometry only — not structural design.