Roof Truss Calculator

How to Use the Roof Truss Calculator

Our roof truss calculator is intended for geometry, layout and quantity planning. It supports a common gable, a two-slope gambrel or barn profile, and a mono / shed profile in both imperial and metric units.

1. Choose the roof profile. Select Common Gable, Gambrel / Barn or Mono / Shed.
2. Enter the truss span. Use the wall-to-wall bearing span for the geometry you want to model.
3. Enter the building length. This is the direction in which the trusses repeat.
4. Select the maximum on-centre spacing. Choose 16, 19.2 or 24 inches in imperial mode, 400, 450 or 600 mm in metric mode, or enter a custom spacing.
5. Enter the roof slope. Common gable and mono modes accept pitch or degrees. Gambrel mode uses separate lower and upper pitches plus the break-point position.
6. Review the geometry and quantity. The calculator returns rise, chord lengths, angles, actual repeated spacing and the number of truss positions.

If you only need a conventional rafter length rather than truss geometry, use the Rafter Length Calculator. If your pitch is unknown, start with the Roof Pitch Calculator.

Roof Truss Calculator Formula for a Common Gable

A symmetrical common gable has a centred peak, so each roof side uses half of the bearing span as its horizontal run.

For pitch written as x:12:

The straight-line top-chord distance from bearing to peak is:

The equivalent roof angle is:

These equations describe the outer roof geometry. A manufactured truss can still have a raised heel, different chord depths, structural webs and connection details that are not represented by these simple centre-line dimensions.

Roof Truss Height Calculator

For a common gable, the basic geometric roof truss height is the rise from the bearing line to the peak.

Example: a 30 ft span at 6:12 has a 15 ft horizontal run:

So the geometric rise is 7.5 ft. If you enter a 4 in heel reference, the calculator also shows a simple planning high-point figure of:

Roof Truss Top Chord Calculator

The top chord is the sloping outer member of a conventional pitched roof truss. For a simple gable, its geometric bearing-to-peak length is the hypotenuse of the rise/run triangle.

For the same 30 ft span at 6:12:

If a horizontal overhang is entered, the calculator extends that roof-slope geometry using the same pitch factor. This is useful for preliminary dimensions, but it is not a timber cutting schedule for a manufactured truss.

Roof Truss Angle and Pitch Calculator

Pitch and angle are two ways of describing the same roof slope. A 6:12 roof rises 6 units for every 12 horizontal units.

For a symmetrical gable, the included angle at the peak is:

At 6:12:

If you need a wider set of pitch conversions, see the Roof Pitch Chart.

Roof Truss Spacing Calculator

The spacing section answers a practical planning question: how many repeated truss positions fit along the building if a chosen on-centre spacing is not to be exceeded?

The extra 1 represents the truss at the opposite end of the building.

For a 48 ft building with a maximum spacing of 24 in (2 ft):

If the building length is not an exact multiple of the selected spacing, our calculator rounds the number of spaces up and then reports the resulting even spacing. That keeps the actual layout at or below your selected maximum rather than leaving one oversized end bay.

How Many Roof Trusses Do I Need?

If the approved plans already specify truss spacing, the repeated quantity is straightforward.

These are repeated layout positions only. Special end conditions, girder trusses, valleys, openings, attic trusses and other project-specific truss types can change the actual order.

Roof Truss Span Calculator: What “Span” Means Here

The phrase roof truss span calculator can mean two very different things.

Maximum allowable span depends on far more than roof pitch. Design loads, truss depth/profile, lumber species and grade, member sizes, connector plates, deflection, bearing, bracing and the full truss configuration all matter.

The American Wood Council notes that metal-plate-connected wood trusses are designed using structural software that evaluates the required load conditions and are manufactured under the ANSI/TPI 1 framework. In other words, a span entered into this calculator should come from the building design or truss supplier, not from a generic geometric rule.

Roof Truss Design Calculator: Geometry vs Structural Design

Searchers often use the phrase roof truss design calculator when they really need dimensions for a roof profile. That part can be calculated with straightforward geometry.

Structural truss design is different.

The Truss Plate Institute describes ANSI/TPI 1-2022 as the national design standard for metal-plate-connected wood truss construction. It covers design requirements for truss members and joints, metal connector plates, responsibilities in the design process and manufacturing quality assurance. TPI states that the 2022 edition is referenced in the 2024 International Building Codes.

Gambrel Roof Truss Calculator

Gambrel roofs are one of the strongest separate search intents in the keyword research, so the calculator includes a proper Gambrel / Barn mode rather than treating a gambrel as a normal gable.

A gambrel side has two different slopes:

• a steeper lower slope; and
• a shallower upper slope.

The calculator lets you define where that slope change occurs by entering the break point as a percentage of the half-span.

Each slope then gets its own rise and chord calculation:

12 Foot Gambrel Roof Truss Calculator Example

Suppose a 12 ft gambrel profile uses a 50% break point on each half, with an 18:12 lower slope and a 6:12 upper slope.

Half-span:

At a 50% break:

The rises are:

That does not mean every manufactured 12 ft gambrel truss should have those slopes. It simply shows how the calculator models the profile you enter.

16 Foot Gambrel Roof Truss Calculator Example

With the same 18:12 lower pitch, 6:12 upper pitch and a 50% break point, a 16 ft span has an 8 ft half-span:

Use the calculator above to change the slopes and break point to match the actual gambrel profile you are planning.

Shed Roof Truss Calculator

A shed, lean-to or mono-pitch truss has one primary slope from the low bearing to the high bearing. The important geometry difference is that its horizontal run is normally the full bearing span, not half the span.

This mode is useful for searches such as shed roof truss calculator, mono roof truss calculator, single-slope roof truss calculator and lean-to roof truss calculator.

12 Foot Shed Roof Truss Example

For a 12 ft mono span at 3:12:

Again, this is the external geometry. It does not determine the web system or whether a particular manufactured truss can carry the project loads.

16 Foot Shed Roof Truss Example

At the same 3:12 slope:

If the project uses a different slope, change the pitch in the calculator rather than relying on a pre-set shed-truss table.

Roof Truss Cost Calculator

The optional cost section is deliberately simple. Once you have a real supplier price for the truss type being ordered, enter that quoted cost per truss plus any known delivery/setup allowance.

We do not publish a fixed “price per truss” inside the calculator because real truss pricing depends on span, profile, pitch, loading, lumber, connector plates, quantity, location, fabrication and delivery.

Roof Truss Weight Calculator: Why Weight Is Not Estimated Here

A genuine roof truss weight calculator needs the actual engineered truss, not just the span and pitch.

Total truss weight changes with:

• top- and bottom-chord sizes;
• web-member lengths and sizes;
• lumber species, grade and moisture content;
• metal connector plate sizes and locations;
• raised heels, cantilevers and special details; and
• the final engineered profile.

Because those details are not known from simple roof geometry, the calculator does not invent a weight. If truss weight matters for lifting, handling or structural planning, use the truss supplier's engineered/shop documentation.

Roof Truss Load and Snow Load Calculators

Keywords such as roof truss load calculator, roof truss load capacity calculator and roof truss snow load calculator describe structural engineering tasks.

A roof truss must be checked for the actual design load combinations that apply to the project. Those can include dead load, roof live load, snow, wind uplift and other effects. Snow itself can involve roof slope, exposure, thermal conditions, drifting and unbalanced loading rather than one universal pounds-per-square-foot number.

The American Wood Council's own span-calculator guidance notes that roof snow loads require appropriate load determination rather than being inferred from pitch alone. Its broader Wood Frame Construction Manual includes structural provisions that account for dead, live, snow, seismic and wind actions.

Can I Use This to Build My Own Roof Truss?

You can use the calculator to understand the outer geometry of a proposed roof profile, but that is not the same as designing a safe structural truss.

A real metal-plate-connected wood truss is an engineered assembly. ANSI/TPI 1 addresses the design of truss members, joints and connector plates and the responsibilities involved in the design process.

So searches such as build your own roof truss calculator or how to build a roof truss calculator should be treated carefully: geometry can be planned here, while structural design and fabrication details should come from qualified truss design/manufacturing documentation.

Fink, King Post, Howe and Other Roof Truss Types

Names such as Fink, king post and Howe usually describe the internal web arrangement as well as the outer profile. That is why the calculator does not simply draw a few diagonal lines and call them a structural design.

Scissor, Attic and Raised-Tie Trusses

A scissor truss, attic truss or raised-tie truss adds important internal geometry that affects usable ceiling space and structural behaviour.

They are not represented by the calculator's basic gable outline:

• Scissor truss: has sloping bottom chords and an interior ceiling pitch.
• Attic truss: creates a room/storage opening within the structural web system.
• Raised-tie truss: moves the bottom tie above the normal bearing line.

The outside roof rise may still be calculated from span and pitch, but usable room dimensions and structural web/chord design require the actual engineered truss configuration.

Hip Roof Trusses and Girder Trusses

Hip roofs usually involve more than one repeated standard truss type. A hip-truss package can include common trusses, truncated trusses, jack trusses, hip girders and other project-specific components.

For that reason, the simple repeated-count formula should not be used to create a final hip roof truss order. The building's overall roof geometry can be calculated separately, but the actual truss package should come from the truss layout supplied for the project.

Wood vs Steel Roof Truss Calculators

The geometry formulas for span, rise and roof angle do not care whether the structural system is wood or steel. Structural design absolutely does.

This calculator can therefore describe the outside triangular, gambrel or mono profile of a proposed steel truss, but it is not a steel roof truss design calculator. Steel member sections, connections, buckling, loads and fabrication details require an appropriate steel structural design process.

Metric Roof Truss Calculator

Switch the calculator to Metric to enter span and building length in metres, overhang and heel reference in millimetres, and spacing using 400, 450, 600 mm or a custom value.

The underlying geometry is identical:

The x:12 pitch format remains available because it is a dimensionless slope ratio, while all linear results are converted to the selected measurement system.

Worked Example: 30 ft Span, 6:12 Pitch, 48 ft Building

Use these inputs:

• common gable;
• 30 ft truss span;
• 48 ft building length;
• 6:12 roof pitch;
• 24 in maximum O.C. spacing;
• 12 in horizontal overhang per side.

First calculate the run and rise:

Top chord from bearing to peak:

A 6:12 pitch factor is approximately 1.1180, so a 1 ft horizontal overhang adds approximately 1.118 ft along the slope:

Now calculate the repeated positions:

Why the Calculator Shows Actual Even Spacing

Suppose a building is 35 ft long and the selected maximum spacing is 24 in, or 2 ft.

You cannot lay out half a space, so the calculator rounds up:

If all bays are distributed evenly:

The result stays below the selected 24 in maximum rather than leaving one bay longer than 24 in.

What This Roof Truss Calculator Does Not Design

Common Roof Truss Calculation Mistakes

Using the full gable span as the run

For a centred symmetrical gable, the run is half the span. Using the full span doubles the calculated rise and gives the wrong top-chord geometry.

Halving a mono-truss span

A mono or shed truss normally uses the full bearing span as the horizontal run, so it is the opposite of the common gable case.

Forgetting the truss at the far end

Dividing building length by spacing gives the number of spaces, not the number of truss positions. Add one for the opposite end.

Calling a geometry result an allowable span

A calculated triangle can be drawn at any span. Structural capacity is a separate engineering problem.

Assuming the outer triangle defines the web layout

Two trusses with similar roof outlines can have very different web arrangements, member sizes and load capacities.

Using a generic price for every truss

Use an actual supplier quote. Truss type, span, load, material and fabrication requirements can change the unit price substantially.

Roof Truss Calculator FAQs

How do I calculate the height of a roof truss?

For a centred gable, divide the span by two to get the run, then multiply the run by the roof slope. A 30 ft span at 6:12 has a 15 ft run and a 7.5 ft geometric rise.

How do I calculate roof truss length?

If by “truss length” you mean the top chord of a simple gable, calculate the square root of run² + rise². The bottom chord is approximately the entered bearing span in the simplified geometry.

How many trusses do I need for a 40 ft building?

At a maximum 24 in O.C. repeated spacing, 40 ft contains 20 spaces, so the basic repeated layout has 21 truss positions. Project-specific special trusses can change the actual order.

How many roof trusses do I need at 24 inches on centre?

Divide the building length in feet by 2, round the number of spaces up, then add one. For exact multiples, a 48 ft building gives 24 spaces and 25 positions.

Can this calculate a gambrel roof truss?

Yes, for the outside gambrel geometry. Enter lower pitch, upper pitch and the break-point position. It calculates the runs, rises, chord segments and repeated quantity, but not structural webs or connector plates.

Can this calculate a shed roof truss?

Yes. Use Mono / Shed mode. The full span becomes the horizontal run, and the calculator returns high-side rise, sloping chord length, angle and quantity.

Can this calculate a Fink roof truss?

It can calculate the outer common-gable geometry that may be associated with a Fink truss, but it does not design the characteristic Fink web system.

Can this calculate an attic truss?

It can calculate the outside roof outline, but it does not calculate the room opening, floor/bottom-chord design or structural webs required for an attic truss.

Can this calculate a scissor truss?

Not as a complete truss design. A scissor truss has sloping bottom chords and additional interior geometry that are outside the current calculator's geometry modes.

Does the calculator determine maximum roof truss span?

No. Span is an input. Maximum safe span depends on the engineered truss design, materials, connector plates and project loads.

Does it calculate roof truss load capacity?

No. The tool does not calculate dead, live, snow, wind or uplift capacity and should not be used as a structural load calculator.

Does it calculate roof truss weight?

No. Reliable weight requires the actual engineered member sizes, web layout, lumber/material properties and connector plates.

Can I calculate roof truss cost?

Yes, once you enter a real quoted cost per truss. The calculator multiplies that supplier rate by the order quantity and adds any delivery/setup allowance you enter.

Can I use metric measurements?

Yes. Metric mode accepts metres for the main building dimensions and millimetres for overhang, heel reference and spacing.

Can I use this for a steel roof truss?

Only for basic outside geometry and repeated quantity. It does not design steel members, connections or structural capacity.

Technical References

The geometry in this calculator is elementary roof mathematics. Structural truss design is governed by a much wider set of material, loading, connection and manufacturing requirements, which is why the structural boundaries are stated clearly throughout this page.

• Truss Plate Institute — ANSI/TPI 1-2022 — current national design standard for metal-plate-connected wood truss construction; TPI states that it is referenced in the 2024 International Building Codes.
• Truss Plate Institute — ANSI/TPI 1-2022 publication overview — describes the standard's scope for truss members, joints, connector plates, design responsibilities and manufacturing quality assurance.
• American Wood Council — Trusses — explains that metal-plate-connected wood trusses are designed using structural software for the applicable load conditions and manufactured under ANSI/TPI 1.
• American Wood Council — 2024 Wood Frame Construction Manual — structural wood-frame design provisions covering wind, snow and other loading considerations.
• American Wood Council — roof pitch and snow-load span guidance — illustrates why roof snow loading requires specific load inputs rather than being inferred from pitch alone.