Calculator Inputs
Example Data Table
These rows are illustrative only. Final sizing should match project conditions.
| Case | Support | Roof load | Spacing | Section | Safe span |
|---|---|---|---|---|---|
| Timber light roof | Simply supported | 1.60 kN/m² | 3.00 m | 75 x 225 mm | 2.80 m |
| Glulam moderate roof | Simply supported | 2.00 kN/m² | 3.60 m | 90 x 315 mm | 4.20 m |
| Steel roof beam | Fixed at both ends | 2.20 kN/m² | 4.00 m | Custom section | 5.60 m |
| Canopy cantilever | Cantilever | 1.20 kN/m² | 2.50 m | 100 x 250 mm | 1.75 m |
Formula Used
This calculator checks span against bending, shear, and deflection limits for a uniform roof load.
Load conversion
w = (q × s) + g
w= line load on the beam in kN/mq= total roof area load in kN/m²s= tributary width or beam spacing in mg= beam self weight in kN/m
Rectangular section properties
S = b d² / 6 and I = b d³ / 12
Bending check
M = cₘ w L² and allowable span L = √(M_allow / (cₘ w))
Shear check
V = cᵥ w L and allowable span L = V_allow / (cᵥ w)
Deflection check
δ = c_d w L⁴ / (E I) with serviceability limit L / n
Solving for span gives L = ((E I) / (c_d w n))^(1/3)
Support coefficients
- Simply supported:
cₘ = 1/8,cᵥ = 1/2,c_d = 5/384 - Fixed at both ends:
cₘ = 1/12,cᵥ = 1/2,c_d = 1/384 - Cantilever:
cₘ = 1/2,cᵥ = 1,c_d = 1/8
How to Use This Calculator
- Select a material preset or choose custom values.
- Pick the support condition that matches the beam layout.
- Enter dead, live, snow, and self weight loads.
- Enter tributary width or beam spacing.
- Choose rectangular dimensions or custom section properties.
- Review modulus, stress values, design factor, and deflection limit.
- Submit the form to see bending, shear, and deflection span limits.
- Use the recommended safe span as the governing result.
- Download the summary as CSV or PDF for project records.
Roof Beam Span Guide
Why span checks matter
Roof beams carry gravity loads from decking, purlins, finishes, and weather. A longer span increases bending moment very quickly. It also increases deflection even faster. That is why a beam that looks strong enough in stress may still feel too flexible in service. This calculator helps you compare those checks in one place. It gives a practical estimate for early planning, budgeting, and preliminary coordination.
Loads drive the answer
The most important input is the total roof load. Dead load usually covers sheathing, ceiling materials, and permanent layers. Live or maintenance load covers temporary access. Snow load can become dominant in cold climates. The tributary width converts area load into beam line load. A small change in spacing can noticeably reduce safe span. That makes framing layout just as important as member size during concept design.
Strength and stiffness both matter
Bending stress controls when the section modulus is too small. Shear stress becomes important for short deep beams, heavy reactions, or cantilevers. Deflection depends on stiffness, which comes from modulus of elasticity and moment of inertia. Timber members often hit deflection limits before bending limits. Steel beams can be strong but still require stiffness checks for roof serviceability. Reviewing all three limits gives a more realistic span recommendation.
Use results with good judgment
This tool is ideal for schematic design and quick comparison studies. It does not replace a full code review, connection design, vibration review, or local loading rules. Uplift, unbraced length, lateral torsional buckling, notches, holes, and fire requirements may reduce real capacity. Use the result as a screening value, then confirm the final member with project standards and a qualified engineer.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates a practical safe roof beam span using bending, shear, and deflection checks under uniform roof loading.
2. Can I use it for steel and timber?
Yes. You can use presets or enter custom stiffness and allowable stresses. Custom section properties also let you model non rectangular members.
3. Why is tributary width important?
Tributary width converts roof area load into line load on one beam. A wider spacing increases beam demand and usually reduces safe span.
4. What if deflection controls first?
That means the beam may feel too flexible before stress limits are reached. Increase depth, improve stiffness, reduce spacing, or shorten the span.
5. Is snow load required?
No. Enter zero if snow does not apply. In snowy regions, snow load often becomes the governing roof action and should be included carefully.
6. Does the tool include local code factors?
No. It is a preliminary design tool. Final design should use local code combinations, adjustment factors, and project specific engineering checks.
7. What does the design factor do?
It reduces the entered allowable stresses. A larger factor gives a more conservative span result and can help early stage screening.
8. Can I use it for cantilevers?
Yes. Select the cantilever option. Cantilever spans are usually much shorter because moment and deflection rise sharply near the support.