Input Parameters
Summary & downloads
Total cases evaluated: 0
Maximum design moment: –
Critical deflection ratio: –
Generate shareable reports for documentation, coordination, and design reviews.
Example flat roof truss scenario
The following example demonstrates typical inputs and outputs for a flat roof bay.
| Case | Span (m) | Truss spacing (m) | Dead load (kN/m²) | Live load (kN/m²) | Additional load (kN/m²) | Line load (kN/m) | Max moment (kN·m) | Max shear (kN) |
|---|---|---|---|---|---|---|---|---|
| Example 1 | 8.0 | 3.0 | 0.8 | 0.6 | 0.2 | 4.8 | 38.4 | 19.2 |
Here, the uniform design line load is the sum of area loads multiplied by truss spacing, then used to derive moment and shear.
Formulas used in the flat roof truss calculator
The calculator models each flat roof truss as a simply supported member carrying a uniform design load per unit length.
- Service area load = dead load + live load + additional load.
- Factored area load = dead load × dead factor + live load × live factor + additional load.
- Line load per truss (w) = design area load × truss spacing.
- Maximum moment M_max = w × L² / 8.
- Maximum shear V_max = w × L / 2.
- Estimated midspan deflection δ = 5 × w × L⁴ / (384 × EI) when an effective stiffness EI is provided.
- Deflection check compares δ with the allowable limit L / x, where x is the chosen deflection ratio.
- Required section modulus S_req = M_max / F_b if the allowable bending stress F_b is specified.
Units must remain self-consistent for all inputs. Always verify results against governing building codes and project-specific design requirements before use in practice.
How to use this flat roof truss calculator
- Select the unit system that matches your project documents.
- Enter effective stiffness, deflection limit, and allowable stress if required.
- For each case, specify span, truss spacing, and roof area loads.
- Use additional load for mechanical units, ceiling loads, or finishes.
- Adjust dead and live load factors to match applicable design standards.
- Click Calculate to generate moments, shears, and deflections.
- Review the summary for the most critical truss and deflection ratio.
- Download CSV or PDF to archive calculations or share with collaborators.
- Use results as a screening tool and refine with detailed structural analysis.
This tool supports early-stage sizing and comparison of flat roof truss options but does not replace full structural design and independent engineering review.
Typical flat roof design loads
These example design loads illustrate how different occupancies influence truss demand. Always check the governing building code for project-specific values.
| Roof use | Dead load (kN/m²) | Live load (kN/m²) | Notes |
|---|---|---|---|
| Lightweight residential roof | 0.4 – 0.7 | 0.6 – 0.8 | Insulation, membrane, light ceiling finishes. |
| Commercial office roof | 0.6 – 1.0 | 0.8 – 1.5 | Allow for services, suspended ceilings, maintenance. |
| Roof with heavy plant | 1.0 – 2.0 | 1.5 – 3.0 | Localized heavy equipment, plinths, access walkways. |
Common spans and truss spacing combinations
Use these span and spacing pairs as starting points before refining member sizes.
| Span | Truss spacing | Typical application |
|---|---|---|
| 6 m | 2.5 – 3.0 m | Small residential or light commercial bays. |
| 8 m | 3.0 – 3.5 m | Standard office or retail flat roofs. |
| 10 m | 3.5 – 4.0 m | Wider bays where deflection control is critical. |
Example deflection limits for flat roof trusses
The table compares common span lengths with corresponding midspan deflection limits for different L/x criteria.
| Span | L/180 limit | L/240 limit | L/360 limit |
|---|---|---|---|
| 6 m | 33.3 mm | 25.0 mm | 16.7 mm |
| 8 m | 44.4 mm | 33.3 mm | 22.2 mm |
| 10 m | 55.6 mm | 41.7 mm | 27.8 mm |
Enter your preferred deflection ratio in the inputs to align results with these example limits.
Effect of span length on bending moment
Bending moment increases with the square of span for uniform load, so modest span changes can significantly alter member demand.
- Doubling the span roughly quadruples the maximum bending moment.
- Increasing span while keeping spacing constant increases tributary load per truss.
- Shorter spans often permit lighter sections and more relaxed deflection limits.
Use multiple cases with different spans to compare how sensitive the design is to layout changes.
Using calculator results within a full design workflow
The calculator is most effective when combined with a structured roof design process.
- Screen different spans, spacings, and roof layouts at concept stage.
- Select promising options based on moment, shear, and deflection results.
- Size chords and webs using section modulus and local design rules.
- Check connections, supports, and secondary framing under critical load cases.
- Verify final design with detailed structural analysis software and code checks.
Save CSV and PDF outputs as part of the project calculation record for traceability.
Frequently asked questions
What assumptions does this flat roof truss calculator use?
Each truss is treated as a simply supported beam carrying uniform load from roofing, finishes, and live actions. It does not model three-dimensional behavior, bracing, connection slip, or secondary framing effects.
Can I use the results for final structural design?
No, the calculator is intended for preliminary sizing and comparison of options. Always perform full code-compliant design, detailed analysis, and independent checking before issuing drawings or approving construction.
How should I choose span and truss spacing values?
Start with typical spans and spacings from project standards or manufacturer guidance, then test several combinations in the calculator. Check moments, shears, and deflections, and coordinate choices with architectural layout, services routing, and economical bay sizes.
What values should I use for dead and live loads?
Use loads from the governing building code, structural design basis, or manufacturer data sheets. Include roofing, insulation, ceilings, permanent equipment, snow, occupancy loads, and any future allowance required by the project brief or specification.
What is effective EI and how do I estimate it?
Effective EI represents flexural stiffness of the truss considering member properties, shear deformations, and connection behavior. You can approximate it from design manuals, section property calculations, or calibration against deflections from detailed frame or finite element models.
Why are my deflection results shown as N/A in the table?
Deflection is only calculated when you enter a positive effective EI value. If EI is blank or zero, the calculator cannot estimate midspan deflection, so those cells display N/A and no deflection ratio comparison is performed.
Can this calculator handle point loads or asymmetric loading?
No, the underlying formulas assume uniform loading along the span. For significant point loads, asymmetric layouts, openings, or discontinuities, use dedicated structural analysis software or hand calculations tailored to the actual loading configuration.