Roof Pitch Calculator
Pitch, angle and rafter length from rise and run.
Roofing & Siding
Roof Truss Calculator
Enter the roof span, pitch, overhang, building length and truss spacing to estimate how many trusses you need, the roof rise and the sloping rafter length for early planning.
Your result
Sample values21
Trusses
6.00
Rise, ft
14.53
Top chord length, ft
26.57
Pitch angle, degrees
24 ft span at 6:12, 12 in overhang, 24 in centres over 40 ft.
Geometry summary
| Measure | Value |
|---|---|
| Half span | 12.00 ft |
| Rise at ridge | 6.00 ft |
| Top chord, including overhang | 14.53 ft |
| Overall width with overhangs | 26.00 ft |
| Pitch angle | 26.57° |
Roof Truss Calculator: Estimate Truss Quantity, Rise, and Rafter Length
The deep4sol roof truss calculator estimates roof geometry and truss quantity using pitch, overhang, building length, and spacing.
Homeowners, builders, and estimators can use the outputs for early plans and supplier discussions.
What Is a Roof Truss Calculator?
A roof truss transfers roof loads to supporting walls or beams. Wood trusses typically contain top chords, bottom chords, and triangular web members.
This truss roof calculator estimates geometry and positions, not chord sizes, connector plates, reactions, or bracing. A prefabricated truss is different from individual site-cut rafters: its internal web pattern and connection design distribute forces through the entire assembly. Geometry alone cannot determine that arrangement.
A roof truss span calculator describes a span but cannot verify capacity for snow, wind, roof, or ceiling loads. That requires engineered drawings.
Roof Truss Calculator Inputs and Outputs
Enter the following five measurements:
| Input | Meaning | Why It Matters |
|---|---|---|
| Roof span | Horizontal distance between the main truss support points | Sets the width used in rise and rafter calculations |
| Roof pitch | Vertical rise for each 12 inches of horizontal run, such as 6/12 | Determines slope and roof rise |
| Overhang | Horizontal eave extension beyond each support line | Adds to overall roof width and sloping length |
| Building length | Distance along the building in the truss-placement direction | Determines how many truss positions are needed |
| Truss spacing | Center-to-center distance between adjacent trusses | Controls the estimated count |
What Results Does the Tool Provide?
Estimated truss quantity
Approximate number of truss positions across the building length.
Roof rise
Vertical rise associated with the span and selected pitch.
Rafter length
Geometric sloping length from ridge to outer eave for a simple symmetrical roof.
Total roof span
Horizontal width including the assumed overhang on each side.
It does not output member sizes, structural capacity, connectors, or engineered drawings.
How to Use the Roof Truss Calculator
Step 1: Measure the Roof Span
Measure horizontally between the roof’s main bearing points.
Enter overhang separately from structural span. When plans show several widths, confirm which dimension represents the distance between bearings. Measuring exterior fascia-to-fascia width as the structural span and then adding the overhang again would overstate the total width.
Step 2: Choose the Roof Pitch
A 6/12 pitch rises 6 inches per 12 inches of horizontal run.
Steeper pitches increase roof rise and sloping length.
Step 3: Enter the Overhang
Enter the horizontal eave extension beyond the bearing line, assumed equal on both sides.
Unequal overhangs need separate geometry. This input generally describes eave extensions across the span; a rake extension at a gable end runs in the building-length direction and should not automatically be added to the roof span.
Step 4: Add Building Length and Truss Spacing
Measure building length along the truss-placement direction; enter center-to-center spacing.
The truss designer must approve spacing for actual loads.
Step 5: Review the Estimate
Compare the results with building plans and the supplier’s layout.
Roof Truss Calculation Formulas
These formulas assume a symmetrical gable roof with constant pitch.
1. Calculate Roof Rise
Let S = roof span, in feet, and P = pitch rise per 12 inches of run. The horizontal run from one bearing line to the ridge is half the span.
Roof rise = (S ÷ 2) × (P ÷ 12)
For a 24-foot span with 6/12 pitch:
(24 ÷ 2) × (6 ÷ 12) = 6 ft of rise
Actual truss height may include additional heel height. In a symmetrical 6/12 example, each horizontal foot toward the ridge adds half a foot of rise. The simplified six-foot result measures the slope rise over the half-span, not necessarily the exact peak elevation above the finished ceiling.
2. Calculate Total Roof Span With Overhangs
Let O be the horizontal overhang on each side.
Total roof span = S + (2 × O)
For a 24-foot structural span with 1-foot overhangs:
24 + (2 × 1) = 26 ft overall width
Overall width remains a horizontal measurement.
3. Calculate Rafter Length
The horizontal distance from ridge to outer eave is:
Extended run = (S ÷ 2) + O
Using the Pythagorean theorem:
Rafter length = Extended run × √[1 + (P ÷ 12)²]
This is geometric length, not a truss fabrication cut list. Including the overhang increases the sloping distance because the roof plane continues past the bearing line. Manufacturer drawings may show multiple top-chord segments and different end details.
4. Estimate Truss Quantity
For trusses placed at the building ends and repeated at a specified maximum center-to-center spacing:
Estimated truss quantity = Round up (Building length ÷ Truss spacing) + 1
The last bay can be shorter if spacing does not divide evenly.
Actual plans may need special end or girder trusses. Count intervals, not just whole sections of building length: for example, five equally spaced trusses create four gaps. This is why the regular-layout equation adds one position.
Worked Example: Roof Trusses for a 24-Foot Span
Consider a simple rectangular building with the following proposed framing:
| Measurement | Value |
|---|---|
| Roof span | 24 ft |
| Roof pitch | 6/12 |
| Overhang on each side | 1 ft |
| Building length | 48 ft |
| Truss spacing | 24 in (2 ft) |
Step 1: Estimate Roof Rise
(24 ÷ 2) × (6 ÷ 12) = 6 ft
Step 2: Calculate Overall Roof Width
24 + (2 × 1) = 26 ft
Step 3: Calculate Sloping Rafter Length
Extended run = 12 + 1 = 13 ft
13 × √[1 + (6 ÷ 12)²] = 13 × 1.118 ≈ 14.53 ft
Step 4: Calculate Truss Quantity
48 ÷ 2 = 24 spacing intervals
24 + 1 = 25 truss positions
The preliminary result is 25 truss positions, 6 feet of roof rise, 14.53 feet of sloping length per side, and 26 feet of overall horizontal roof width.
This does not authorize fabrication of 25 identical trusses.
How Truss Spacing Changes the Quantity
For a 48-foot building with trusses at both ends:
| On-Center Spacing | Spacing in Feet | Estimated Positions |
|---|---|---|
| 12 in | 1 ft | 49 |
| 16 in | 1.333 ft | 37 |
| 24 in | 2 ft | 25 |
| 32 in | 2.667 ft | 19 |
Confirm structural spacing against sheathing, loads, and bracing requirements. Closer spacing can increase material quantity and installation handling, but it should not be selected solely to increase a perceived safety margin. The engineered roof system determines acceptable spacing.
Factors That Affect Roof Truss Estimate Accuracy
Roof Type and Symmetry
Hip, mono-pitch, scissor, and attic trusses need different geometry.
Bearing Locations and Heel Height
Raised heels and unusual bearing details alter fabrication dimensions.
Overhang and Eave Configuration
Horizontal overhang differs from sloping eave length.
Truss Spacing and End Conditions
Vaulted sections, girders, and intersections can alter positions.
Common Mistakes When Calculating Roof Trusses
- Confusing span with overall roof width: Overhangs extend beyond the bearing points.
- Treating pitch as degrees: A 6/12 roof is a rise-to-run ratio, not a 6-degree slope.
- Using inches and feet without conversion: Convert truss spacing before dividing building length.
- Forgetting an end position: Count spaces between trusses, then account for both ends.
- Treating rafter length as a cutting dimension: Manufactured trusses require engineered member details.
- Assuming quantity confirms safety: Geometry does not determine allowable loading or required bracing.
Roof Truss Safety and Engineering Requirements
A truss is a structural system, not simply a set of rafters connected into a triangle. Its performance depends on the selected lumber, web arrangement, connector plates, bearing conditions, permanent bracing, and environmental loads.
The Structural Building Components Association explains that truss design drawings identify critical information such as slope, span, spacing, loading, bearing, and connection requirements. The MiTek guidance on truss design errors also highlights the importance of checking overhang and chord-panel dimensions.
Use the calculator to prepare initial measurements, then have the truss manufacturer or qualified design professional confirm the structural design and installation requirements. Do not cut, modify, or relocate prefabricated truss members without an approved repair or redesign.
Plan Roof Framing With Confidence
The deep4sol roof truss calculator supports preliminary quantity and geometry estimates.
Verify all results against approved structural and placement drawings. They govern truss type, roof loads, support reactions, permanent restraints, and installation sequence; the calculator serves the earlier estimating stage.
Questions
Frequently Asked Questions
At 24-inch (2-foot) on-center spacing, a simple 40-foot building needs approximately 21 truss positions, including both ends. The final quantity depends on the approved placement plan.
Multiply half the roof span by the pitch ratio. For example, a 24-foot span at 6/12 pitch produces a simplified geometric rise of 6 feet, excluding additional heel-height details.
Yes. Overhang extends the horizontal run beyond the truss bearing point, increasing the sloping distance. The calculator accounts for this when estimating total roof width and rafter length.
No. It estimates dimensions for an entered span but does not calculate structural capacity. Safe span depends on loads, materials, truss configuration, bearings, and engineered design requirements.
Sixteen-inch spacing places trusses closer together and generally requires more trusses along the same building length. The appropriate spacing must be confirmed by the structural design rather than quantity alone.
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Estimates, not specifications.
Results are based on the values you enter and on typical material
properties. Densities, coverage rates and product sizes vary between
suppliers, so confirm quantities against your supplier's data before
ordering. Structural work must be signed off by a qualified
professional and meet your local building code.