Deck Beam Calculator
Calculate deck beam size and support requirements
Joist span in feet
Width supported by beam
Tables vary by species
Beam Size
—
ply x dimension
Max Post Spacing
—
ft
Posts Needed
—
posts
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Deck beams support the entire deck structure. Our calculator determines correct beam size and post spacing based on IRC tables.
- Determine joist span
- Choose lumber species
- Use calculator for beam size
- Plan posts and footings
Expert Insights
Smart analysis for your Decking project
Wood vs Composite TCO
Pressure-treated: $15-25/sqft, lasts 15 years. Composite: $30-60/sqft, lasts 25-50 years with zero annual sealing. AI compares true lifetime cost.
Joist Spacing Intelligence
16" OC for wood decking, 12" OC required for most composite. AI checks your material choice and adjusts joist count automatically.
Ledger Board Safety
#1 deck failure source: improper house attachment. AI flags critical flashing and lag bolt requirements for code-compliant installation.
Calculation Formula
IRC Table R507.5: Beam size based on joist span and species
Post spacing from IRC table
Decking Material Comparison
| Material | Cost/Sq Ft | Lifespan | Annual Maintenance | Joist Spacing |
|---|---|---|---|---|
| Pressure-Treated Pine | $15-25 | 15-20 years | Seal yearly | 16" OC |
| Cedar/Redwood | $25-40 | 20-30 years | Seal every 2-3 yrs | 16" OC |
| Composite (Basic) | $30-45 | 25-30 years | Wash only | 12" OC |
| Composite (Premium) | $45-60 | 30-50 years | Wash only | 12" OC |
| PVC/Vinyl | $50-70 | 50+ years | Wash only | 12-16" OC |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- American Wood Council (AWC) — DCA6 — Prescriptive Residential Wood Deck Construction Guide. Learn more
- International Residential Code (IRC) — Section R507 — Decks. Learn more
- North American Deck and Railing Association (NADRA) — Deck Safety Guidelines. Learn more
Frequently Asked Questions
What size beam for 10 ft joist span?
For 10 ft joist span with SP lumber, use (2) 2x8 beam with posts max 6 ft apart.
How far apart should deck beam posts be?
4-8 ft depending on beam size. (2)2x8 beams max 6 ft, (3)2x8 max 10 ft per IRC.