Does this design a safe roof truss?

No. It describes a reference profile and layout positions. Structural truss design includes loads, members, connections, bracing and supporting building design, none of which this calculator checks.

Why add one to the number of bays?

A bay is an interval between positions. One bay has two endpoints; twenty consecutive bays have twenty-one positions when both ends are counted.

What if the building length is not divisible by spacing?

The tool rounds bays up and shortens the final bay. For a 41-foot layout at 24 inches, it shows 22 positions with a 12-inch last bay. An actual engineered layout may distribute spacing differently.

Does heel height change the calculated top-line length?

Not in this reference model. It raises the top reference line at the bearing and the peak by the same amount. Span and pitch determine the sloped line between them.

Preliminary truss layout

Roof Truss Calculator

Estimate symmetric gable truss geometry and preliminary positions from span, pitch, heel height, layout distance and specified spacing. Export the assumptions.

Gable reference geometry
Endpoint-aware position count
Final bay shown

Geometry and layout planning

Plan truss geometry and positions

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Use a symmetric gable profile. Results describe reference lines and preliminary positions, not lumber sizes, joint details, load capacity or safe installation spacing.

Truss geometry

Bearing-line spanftBetween the two bearing reference lines of a symmetric gable profile.Pitch rise per 12 in run/12Equal pitches only. Geometry does not determine load capacity.Heel reference heightinVertical distance from bottom-chord datum to the top reference line at bearing.Horizontal overhang per sideftUsed for the sloped extension length; not included in the bearing span.

Layout

First-to-last truss line distanceftAlong the ridge between first and last planned truss centerlines, not roof length including gable overhangs.Specified maximum spacinginUse the spacing from the building design. 24 inches is an example, not a structural recommendation.

Preliminary truss positions

21 positions

Includes both endpoint lines. Does not specify truss types or an engineered order.

Scaled reference triangle only. Heel, joints and overhang are not drawn.

Geometry and layout details

Rise above bearing top reference: 6 ft; Half-span × pitch / 12.
Peak above bottom datum: 6.5 ft; Geometric rise plus entered heel reference height.
Top reference length per side: 13.4164 ft; Bearing to peak; joint geometry and lumber dimensions excluded.
Sloped overhang per side: 1.118 ft; Outside the bearing span.
Layout bays: 20 bays; Number of intervals between first and last position.
Final bay spacing: 24 in; Earlier bays use the specified spacing; the last bay may be shorter.

Before construction, use an approved framing design or manufacturer’s truss drawings. These results do not calculate member sizes, webs, plates, bracing, connections, birdsmouths or building-code compliance.

Transparent

Formula, intermediate values and rounding rules are visible.

Private by default

Saved project measurements stay in this browser unless you copy a share link.

Purchase-aware

Inputs, units and limitations stay visible so you can verify each estimate.

What this roof truss calculator measures

A symmetric gable profile can be represented by two right triangles. Half the bearing-line span is the run for one side. Multiply it by pitch divided by 12 to find rise above the top reference line at bearing. Add the entered heel reference height to express the peak above the bottom-chord datum.

For a preliminary layout, divide the distance between the first and last truss centerlines by the specified maximum spacing. Round the number of bays up, then add one position for the starting endpoint. Earlier bays use the entered spacing; the final bay takes the remaining distance. A distance shorter than one spacing still has two endpoint positions in this model.

Position count is not a manufactured-truss order. Gable-end units, girder trusses, openings, hips, concentrated loads and end-wall details can change quantities and types. The tool does not calculate webs, plate connections, member lengths for fabrication, bracing or structural adequacy. A truss designer must define the actual system.

Reference rise

span ÷ 2 × pitch ÷ 12

Equal pitches on both sides; heel height is separate.

Peak above datum

rise + heel inches ÷ 12

All output heights use the defined bottom reference datum.

Truss positions

ceil(layout ft × 12 ÷ spacing in) + 1

Includes positions at both endpoints. Layout distance excludes gable overhangs.

How to use this calculator

1Enter bearing-line span and equal-sided gable pitch.
2Enter the heel reference height and horizontal overhang per side.
3Measure first-to-last centerline distance along the ridge, excluding end overhangs.
4Use the spacing specified by the building design, then review the count and final bay with the truss supplier.

Common mistakes

• Forgetting the first position when converting bays to truss count.
• Using roof length including gable overhangs instead of centerline layout distance.
• Assuming the example 24-inch spacing is suitable for the building loads.
• Using the top reference line as a fabrication length for a chord with joints and member depth.

Use this calculator when

You want a preliminary equal-sided gable outline and a transparent layout count.
You have an independently specified spacing and can verify the endpoint convention.

Use another method when

You need web design, member sizes, plates, bracing or load checks.
The building needs hip, scissor, attic, girder or asymmetric trusses.

Worked example

24-foot span and a 40-foot layout

At 6/12 pitch, the 12-foot half-span rises 6 feet. A 6-inch heel reference height places the peak 6.5 feet above the bottom datum. A 40-foot first-to-last distance at 24-inch spacing contains 20 bays.

20 bays + 1 starting position = 21 preliminary positions