Roof Truss Calculator
Estimate truss quantity, roof rise, top chord length, layout spacing, roof area, and preliminary ordering cost.
Plan a simple symmetrical roof truss layout
Enter the building span and length, roof pitch, truss spacing, and horizontal overhang. The calculator estimates how many trusses are needed when one truss is placed at each end, along with the roof rise and sloped top chord geometry.
Calculate roof truss dimensions by using the roof span, roof pitch, and overhang length. For a roof with a 24-foot span and a 6:12 pitch, each truss half has a 12-foot run and a 6-foot rise. Calculate rafter length using the formula √(run² + rise²). Truss design must also account for snow loads, wind loads, and local building code requirements.
These are preliminary layout quantities, not an engineered truss design. Internal webs, lumber grades, connector plates, bracing, bearing, uplift resistance, and other structural requirements must come from approved construction documents or a qualified truss designer.
Required truss layout
Order -- including extras, with a maximum actual spacing of --.
Required trusses
--
Includes both end trusses.
Suggested order
--
--
Roof rise
--
Bearing line to ridge.
Top chord slope
--
One side, bearing to ridge.
Horizontal run
--
Pitch angle
--
Roof surface area
--
Estimated truss cost
--
Truss planning breakdown
| Item | Result | Planning use |
|---|
Structural note: Do not fabricate, alter, drill, notch, or field-repair a truss from these geometry results. Use the truss manufacturer's sealed design, placement diagram, permanent bracing details, bearing requirements, and local approvals.
How to use the roof truss calculator
- Measure the span: Use the horizontal distance between the outside faces of the supporting walls unless the truss supplier specifies another bearing-line dimension.
- Enter the building length: Measure the full layout distance from the first end truss to the last end truss.
- Set pitch and spacing: Enter the roof rise per 12 units of run and the maximum on-center spacing shown by the plans.
- Add the eave overhang: Use the horizontal projection beyond each bearing wall, not the sloped tail length.
- Review the order count: The result includes a truss at both ends and adds your selected spare quantity.
- Confirm with the supplier: Send the span, pitch, building length, loading, bearing, heel height, overhang, and opening requirements to the truss manufacturer.
Roof truss formulas
For a simple symmetrical gable roof, the horizontal run is half the building span. The roof rise follows from the pitch, and the top chord slope is the hypotenuse of the run-and-rise triangle.
Run = span ÷ 2
Rise = run × pitch ÷ 12
Top chord slope = √(run² + rise²)
Trusses = ceil(building length ÷ spacing) + 1
The count formula rounds the number of bays upward, then adds one end truss. This ensures the calculated actual spacing does not exceed the requested maximum. If openings, girder trusses, step-down trusses, hips, valleys, or different end conditions exist, the final placement plan can require additional or specialized units.
How to measure roof truss inputs correctly
Small differences in where a measurement starts and ends can change the truss count, ridge height, roof area, and supplier quote. Use the approved plans when available, and keep bearing dimensions separate from exterior finishes and roof projections.
Building span
Measure horizontally between the outside faces of the supporting wall plates, or use the exact bearing-to-bearing span specified by the truss supplier. Do not include eave overhangs.
Truss layout length
Measure from the centerline of the first truss to the centerline of the last truss. Exclude a gable-end overhang unless an end truss is actually located at its outer edge.
Roof pitch
Use rise per 12 units of horizontal run, such as 6:12. Confirm the design pitch from drawings rather than measuring shingles or an uneven existing roof surface.
Eave overhang
Measure the horizontal projection from the wall bearing line to the fascia. Do not enter the sloped top-chord tail length; the calculator derives that distance from pitch.
On-center spacing
Use the maximum spacing shown on the structural or truss placement plan. On-center means centerline to centerline, not the clear space between truss members.
Heel height
Heel height is not a calculator input, but the supplier still needs it. A raised energy heel changes overall truss height, web geometry, siding, blocking, and insulation space.
Field measurement check
Measure the span and length in at least two locations, check whether the supporting walls are parallel, and record any change in bearing height. For an existing building, also locate chimneys, stair openings, vaulted areas, mechanical equipment, and interior bearings before requesting a truss package.
Measurement and heel-height reference: APA – Raised-Heel Trusses.
Worked example: 24 ft × 40 ft gable roof
Consider a 24-foot building span, 40-foot truss layout length, 6:12 pitch, 24-inch on-center spacing, and a 1-foot horizontal eave overhang. One extra truss is added to the preliminary order.
Span
24 ft
Length
40 ft
Pitch
6:12
Spacing
24 in
Overhang
1 ft
| Step | Calculation | Result |
|---|---|---|
| Horizontal run | 24 ft ÷ 2 | 12 ft |
| Roof rise | 12 ft × 6 ÷ 12 | 6 ft |
| Top chord to ridge | √(12² + 6²) | 13.42 ft |
| Required trusses | ceil(40 ft ÷ 2 ft) + 1 | 21 trusses |
| Suggested order | 21 required + 1 extra | 22 trusses |
| Roof surface area | 2 × 14.53 ft slope × 40 ft | About 1,163 ft² |
The roof area includes both 1-foot eave overhangs but excludes gable-end overhang, roofing waste, valleys, and material laps. The 22-truss order is only a quantity example; gable trusses, girder trusses, and other specialty units must be identified separately.
How roof truss type changes the estimate
The calculator models the exterior envelope of a simple symmetrical gable truss. The repetitive spacing count may still help with other systems, but specialized shapes can change member geometry, end conditions, quantities, and price.
| Truss type | Can the count help? | What changes | What to confirm |
|---|---|---|---|
| Fink or common gable | Usually, for a regular rectangular building. | Internal web pattern and member sizes are engineered. | Span, pitch, heel, loads, bearing, and end trusses. |
| Howe or modified gable | Yes, as an initial repetitive count. | Web direction, panel points, and reactions differ. | Design drawing and permanent restraint locations. |
| Attic or room-in-attic | Only as a rough quantity. | Floor loading, room width, knee walls, and stairs affect design. | Habitable loads, openings, insulation, and mechanical routes. |
| Scissor truss | The spacing count can help. | Sloped bottom chords change interior height and reactions. | Exterior pitch, ceiling pitch, heel, and wall thrust design. |
| Mono-slope truss | Count only; do not use the gable geometry results. | The roof rises across the full span instead of half the span. | High and low bearings, full-span rise, drainage, and uplift. |
| Hip or girder set | No complete takeoff from a common-truss count alone. | Girders, jacks, step-down trusses, and multiple plies are added. | Manufacturer's placement diagram, hangers, and load transfer. |
Order from the placement plan: Roof intersections, offsets, large openings, valleys, hips, cantilevers, and concentrated loads can replace common trusses with specialty units. A total count without truss marks and types is not a complete purchase list.
Truss configuration reference: Structural Building Components Association – Truss Configurations.
Interesting fact
Raised-heel trusses can influence energy performance as well as roof geometry. APA – The Engineered Wood Association reports that homes using raised-heel trusses could achieve HERS Index ratings 4 to 6 points lower. The additional heel height leaves more room for full-depth insulation above exterior wall plates and can simplify attic ventilation. The actual benefit depends on the complete sheathing, air-sealing, ventilation, and insulation design rather than the truss shape alone.
What this estimate includes and excludes
A useful takeoff separates measurable roof geometry from structural design decisions. The calculator handles the envelope and repetitive layout; the project drawings and truss package control the load path.
Included in the estimate
Truss quantity, ordering allowance, roof run, rise, pitch angle, sloped chord geometry, approximate roof surface area, and optional purchase total.
Requires engineered information
Web arrangement, member sizes, lumber grade, plates, reactions, uplift, heel height, deflection, permanent bracing, point loads, and repair details.
Engineering standards reference: Truss Plate Institute – Standards Development.
Truss order checklist
Building geometry
Confirm span, length, pitch, overhang, heel height, bearing width, roof shape, and exact truss spacing.
Loads and openings
Identify local snow and wind criteria, roofing weight, ceilings, attic storage, HVAC loads, chimneys, stairs, and large openings.
Delivery and installation
Plan access, unloading, lifting, temporary restraint, permanent bracing, dry storage, and inspection of damaged pieces.
Typical spacing scenarios
| Nominal spacing | 40 ft building length | Use | Important check |
|---|---|---|---|
| 12 in on center | 41 trusses | Dense layout for project-specific requirements. | Decking spans, loads, blocking, and plan notes. |
| 16 in on center | 31 trusses | Closer structural module where specified. | Exact end-bay adjustment and supplier layout. |
| 24 in on center | 21 trusses | Common planning interval for many trussed roofs. | Approved sheathing, ceiling, and load design. |
Examples assume one truss at each end. Do not select spacing from this table; use the approved plans and truss placement diagram.
Frequently Asked Questions
How many roof trusses do I need?
Divide the building length by the maximum truss spacing, round up to a whole number of bays, and add one unit for the opposite end. A 40-foot building at 24-inch centers has 20 bays and therefore needs 21 units before extras or specialized girders are considered. The roof span does not change this repetitive count, but it does affect the size and engineered capacity of each assembly.
Does the calculated top chord equal the lumber cut length?
No. The result is the hypotenuse of the run-and-rise triangle from the bearing point to the ridge. Actual cuts depend on heel height, joint geometry, the bottom chord, each web member, overhang details, and connector locations. Use the roof truss calculator for preliminary geometry only; the fabrication drawing controls every member length.
Why does the calculator include both end trusses?
On-center dimensions describe the bays between assemblies, so a layout with 20 spaces needs 21 units to enclose them. The unit at each end may be a purpose-built gable truss rather than a common interior model. Confirm its roof framing, outlookers, sheathing support, and end-wall connection details with the supplier.
Can I use it for an attic truss, scissor truss, king post, or queen post?
The repetitive quantity can provide a rough starting point, but the geometry results represent a simple symmetrical gable envelope. Attic rooms, scissor ceilings, king-post and queen-post layouts, hips, valleys, mono slopes, and girder sets have different internal force paths. Whether made from lumber or steel, these systems require a project-specific design and placement drawing.
How do roof pitch and overhang affect the result?
Enter the eave projection horizontally from the wall bearing line to the fascia or tail endpoint. The entered pitch determines the roof slope, which converts that projection into a longer diagonal distance and changes the estimated surface area. A gable-end projection is not included in this calculation.
Can a damaged truss be repaired or modified on site?
Only with an approved repair detail from the manufacturer, truss designer, or responsible engineer. Cutting a member, drilling a chord, changing a support, or installing an unapproved gusset plate can alter forces at every joint. Repairs must preserve capacity for dead and live load as well as the specified snow load, wind load, and uplift conditions.
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Disclaimer: This roof truss calculator is a preliminary estimating and educational tool. It calculates simplified geometry and repetitive layout quantities from user-entered dimensions, but it does not design a structural truss, verify code compliance, calculate structural capacity, or provide construction approval. Results do not account for lumber species or grade, connector plates, joint eccentricity, dead load, live load, snow drift, rain load, wind pressure, uplift, seismic forces, unbalanced loading, concentrated loads, ceiling loads, storage loads, mechanical equipment, deflection limits, bearing reactions, heel height, lateral restraint, temporary bracing, permanent bracing, girder connections, corrosion exposure, fire requirements, transport limits, erection methods, or site conditions. Truss configurations and end conditions vary, and field measurements may differ from nominal plan dimensions.
Use approved architectural and structural drawings, local building requirements, and the truss manufacturer's individual design drawings and placement diagram. Do not cut, drill, notch, splice, alter, overload, or repair a truss without a written detail from the responsible truss designer or licensed design professional. Confirm all quantities, dimensions, pricing, delivery constraints, lifting plans, bracing, bearing, fasteners, and specialty trusses with the supplier and contractor before purchasing or installing materials.
Last updated: June 15, 2026