Quick Notes
This tool sizes a simply supported deck beam for uniform load. It checks bending, shear screening, and deflection against a selected limit.
Inputs
Example Data Table
| Span (ft) | Tributary (ft) | Dead (psf) | Live (psf) | Wood option | Deflection limit | Typical recommendation |
|---|---|---|---|---|---|---|
| 10 | 6 | 10 | 40 | SPF #2 (typical) | L/360 (common) | Often 2-ply 2x10 or similar |
| 12 | 7 | 12 | 40 | DF-L #2 (typical) | L/360 (common) | Often 3-ply 2x10 or 2-ply 2x12 |
| 14 | 6 | 10 | 50 | LVL (engineered) | L/480 (stiffer) | Engineered beam commonly needed |
These are examples only. Use the calculator for your exact span, post spacing, and local load requirements.
Formula Used
The calculator evaluates common rectangular beam sizes and built-up plies, then selects the smallest option that passes bending, shear screening, and deflection.
How to Use This Calculator
- Enter the beam span between supports in feet.
- Set tributary width based on joist layout and post spacing.
- Provide dead and live loads using your project requirements.
- Select a wood option, or choose Custom and enter design values.
- Pick a deflection limit and a safety factor for conservatism.
- Click Calculate to see the recommended size and check table.
- Export results using the CSV or PDF buttons if needed.
Professional Notes and Field Guidance
1. Purpose of deck beam sizing
Deck beams collect joist reactions and deliver them to posts and footings. This calculator estimates a practical beam size for typical, simply supported spans with uniform loading. It reports bending demand, a conservative shear screen, and service deflection so you can compare options quickly before detailing connections and permits.
2. Why tributary width drives beam load
Tributary width represents the deck area feeding a beam line. For a beam supporting joists on one side, it is usually half the joist span. If the beam supports joists from both sides, tributary width can be the sum of half-spans. Line load is computed as psf × tributary width (ft) to obtain plf.
3. Typical load ranges used on decks
Many residential decks use live loads around 40 psf, while dead loads often fall between 8–15 psf depending on decking, finishes, and railing systems. Heavier surfaces, planters, and hot tubs can dramatically raise live loads. When in doubt, use project-specific criteria and local requirements rather than generic defaults.
4. Bending check and section modulus
For a uniformly loaded, simply supported beam, maximum moment occurs at midspan and is calculated as M = wL²/8. Required section modulus is Sreq = (M×12)/Fbadj, where the ×12 converts ft-lb to in-lb. The tool compares Sreq to the provided S of each candidate.
5. Deflection matters for serviceability
People notice bounce long before strength limits are reached. Deflection is computed using Δ = 5wL⁴/(384EI) with consistent inch units. Choosing L/360 provides a common balance of comfort and economy, while L/480 tightens vibration and sag control. If finishes or tile exist, stricter limits are often preferred.
6. Built-up beams and material selection
Built-up beams increase width by combining plies, which boosts both section modulus and stiffness. Higher E values reduce deflection, while higher Fb values raise bending capacity. Engineered products often provide stronger, more consistent properties, but require correct fastening schedules, bearing lengths, and moisture protection. Always follow manufacturer data for engineered members.
7. Reading the candidate check table
The table shows required S, provided S, allowable deflection, and calculated deflection for each size and ply count. A PASS means the candidate meets bending, the shear screen, and the chosen deflection limit for your inputs. If all candidates fail, reduce span or tributary width, increase plies, or select stronger material properties.
8. Detailing checks beyond the calculator
Beam sizing is only one part of a safe deck. Confirm post spacing, footing capacity, lateral bracing, and connection hardware. Bearing length at posts, notch limits, and ledger attachment can govern final details. This calculator supports early sizing decisions; final design should reflect your jurisdiction’s code provisions and material specifications.
FAQs
1) What is tributary width for a deck beam?
It is the deck width that “feeds” load into the beam line. For one-sided joists, it is usually half the joist span. For joists on both sides, add both half-spans.
2) Which deflection limit should I choose?
L/360 is common for comfort. Use L/480 when you want a stiffer deck or when finishes are sensitive to movement. L/240 may feel bouncy and is project-dependent.
3) Why does a built-up beam help?
Adding plies increases beam width, raising section modulus and moment of inertia. That improves bending capacity and reduces deflection, provided plies are fastened properly to act together.
4) Can I rely on the shear result shown?
The shear check here is a conservative screening method for quick sizing. For final design, use proper allowable shear values for your grade/species or engineered member documentation.
5) Do I need to include beam self-weight?
For typical residential decks, beam self-weight is small compared with area loads, but including it is safer. The tool adds a modest allowance per ply to keep early sizing conservative.
6) What if no candidate passes?
Shorten the span with more posts, reduce tributary width by changing layout, increase ply count, select a stronger material option, or relax deflection only if permitted for your use-case.
7) Does this cover connections and footings?
No. It checks member sizing for uniform load only. Connections, bearing length, post/footing capacity, and lateral bracing must be designed separately using code-approved hardware and details.
Choose safer beam sizes and build decks with confidence.