Deck Calculator
A 12 by 16 ft pressure-treated deck with 16-inch joist spacing needs 13 joists, 3 beams, and approximately 44 pieces of 2x6 decking at 12 feet long — or 35 pieces at 16 feet long. That footprint is 192 sq ft, which falls in the typical residential sweet spot where a permit is required but no engineer stamp is needed.
Worked example: A 10 by 20 ft deck at 16-inch on-center joist spacing. Joists: (20 ft / 1.33 ft) + 1 = 16 joists. Decking: (10 ft / 0.458 ft actual board width) = 22 board runs x 20 ft long = 440 linear ft. With 10% waste: 484 linear ft. In 16-ft boards: 31 pieces.
Beam sizing: a two-ply 2x10 handles up to 8 ft spans for most residential loads. Consult the American Wood Council DCA-6 span tables for your specific joist and post spacing.
Fasteners: use 3-inch deck screws (stainless or coated for pressure-treated lumber). Plan 2 screws per board per joist = about 32 screws per 8-ft board run.
Composite note: composite decking (Trex, TimberTech) eliminates painting and staining but costs 2 to 3 times more than pressure-treated lumber. Over a 25-year lifespan, composite is often cheaper when you factor in zero resealing and minimal maintenance.
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Frequently Asked Questions
How many footings does a 12x16 deck need?
A 12 by 16 ft deck typically needs 6 footings — 3 rows of 2, spaced 8 ft apart along the 16 ft length. Each footing should be 12 to 16 inches in diameter and below the local frost line.
Should I use 12-inch or 16-inch joist spacing on a deck?
Use 16-inch on-center joist spacing for standard 5/4x6 deck boards running perpendicular. Use 12-inch OC if boards are running at a 45-degree diagonal, or if you are using thinner composite decking. Check the decking manufacturer spec for the maximum allowed joist spacing.
What is the cost difference between composite and pressure-treated lumber decking?
Pressure-treated lumber decking costs $2 to $4 per linear foot. Composite decking (Trex, TimberTech) costs $5 to $12 per linear foot for materials only. Over 25 years, composite is often cheaper when you factor in no resealing, no staining, and minimal maintenance.
Do I need a permit to build a deck?
Most jurisdictions require a permit for decks over 200 sq ft or attached to the house. Rules vary by city — check your local building department before starting. A permit typically requires a site plan showing footing locations and beam spans.
How many screws do I need for a 12x16 deck?
A 12 by 16 ft deck has about 13 joists and 44 board runs at 12 ft length. At 2 screws per board per joist crossing: 44 x 13 x 2 = 1,144 screws minimum. Buy a box of 350-count 3-inch coated deck screws — you will need 4 boxes with some left over.