Deck Calculators
Every calculator and guide you need to design a deck, in the order an engineer checks it. Start at the top (joists) and work down to the footings: each step sets the load for the next one. All tools follow the deck rules in the International Residential Code (IRC Section R507) and the AWC DCA 6 deck guide.
Step 1: Joists and decking
- Deck Joist Span Calculator – maximum joist span by size, spacing and wood species
- Joist Spacing for Composite Decking – 12, 16 or 24 inch spacing for composite and PVC boards
- Deck Joist Cantilever – how far joists can overhang the beam
- Deck Board Calculator – boards, screws and hidden clips
Step 2: Footings and posts
- Deck Footing Size Calculator – footing diameter from the load each post carries
- Deck Footing Requirements – size, depth and spacing rules explained
- Sonotube Concrete Calculator – bags of concrete per tube
- Concrete for Post Holes Calculator – bags per post hole
Step 3: Attaching to the house
- Deck Ledger Board – bolts, flashing and the lateral load connection
- Freestanding vs Attached Deck – when a deck must stand on its own beams
Step 4: Railings and stairs
- Deck Railing Height Code – 36 inch guards and the 4 inch rule
- Stair Stringer Calculator – risers, treads and stringer length
- Stair Code Requirements – risers, treads, handrails and landings
Step 5: Loads and safety
- Live Load vs Dead Load – the 40 + 10 psf loads every deck table uses
- Snow Load on a Deck – how much snow a deck can hold
- Why Is My Deck Bouncy – causes and fixes for a springy deck
- Why Decks Collapse – the failures engineers see most often
Key deck numbers (IRC R507)
| Item | Typical requirement |
|---|---|
| Design loads | 40 psf live load + 10 psf dead load |
| Joist deflection limit | Span ÷ 360 |
| Guard (railing) | Required where the deck is more than 30 in above grade; 36 in minimum height |
| Baluster openings | A 4 in ball must not pass through |
| Footings | Below the frost line and on undisturbed soil |
| Ledger fasteners | 1/2 in lag screws or through-bolts, staggered, never nails alone |
Engineer's note: most deck failures happen at connections, not in the middle of a board or joist. If you only have time to check one thing on an old deck, check the ledger: its bolts, its flashing and the wood of the house rim joist behind it. Sizes from the tables here assume new, sound lumber and the standard 40 psf live load; a hot tub, a roof or heavy snow needs its own design.
Everything in this section
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Maintained by: Engr. Abdul Latif, civil and structural engineer. Every calculator here shows its formula and the code table it follows. For planning only; confirm with your local building department.