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Deck Material Calculator

Estimate joists, beams, decking, fasteners and footings for a deck frame.

Deck size

Joists run across the width and land on a beam; decking runs the length.

ft

Along the house, parallel to the ledger.

ft

Out from the house — this is the joist span.

Framing

Checked against the IRC R507 span table for treated southern pine.

Support
ft

Posts get laid out at or under this spacing.

ft

Finished deck surface to grade. Drives post length and guard requirements.

Decking
Decking product

Butt joints have to land on a joist, so longer boards cut waste.

Fastening
10%

10% covers a rectangular deck. Angles, picture framing and stairs need more.

Unit pricing
$/LF
$/ea
$/ea

Concrete pier, post base and the 6×6 post itself.

$/ea
$/100

Per hundred clips or screws.

Estimated material cost

$2,773

192 sq ft of deck, or $14 per square foot in material alone.
Decking boards26 rows at 5.75 in per course
29 × 16 ft
Joists2x10 at 16 in on center
13 × 12 ft
Beam & rim lumber32 LF beam, 56 LF rim and ledger
88 LF
Posts & footingsPosts land 8.0 ft apart
3
Joist hangersOne per joist at the ledger
13
Bill of materials
ItemQuantityUnitUnit costExtended
Joists — 2x10 × 12.0 ft13pieces$23.40$304
Ledger, rim & end joists — 2x1056linear ft$1.95$109
Beam — 2-ply 2x1032linear ft$1.95$62
Composite / PVC — 16 ft boards29boards$62.00$1,798
Posts, bases & footings — 2.0 ft posts3each$145.00$435
Joist hangers13each$1.65$21
Hidden fastener clips390each$0.11$43
Material total$14.44/sq ft$2,773

2x10 at 16 in OC carries the 12.0 ft span

The IRC R507 table allows 14.0 ft for a 2x10 at 16 in on center in No. 2 treated southern pine. Other species and grades span less, so confirm the grade stamp on what you actually buy.

Joist spacing suits the decking product

Composite / PVC is normally rated to 16 in on center for a perpendicular layout. Composite and PVC boards drop to 12 in on center for a diagonal pattern, and most manufacturers void the warranty if the framing is wider than their instructions allow. This is the one decision you cannot retrofit after the frame is built — check the decking installation instructions before you set joists.

The ledger is where decks fail

Deck collapses begin at the ledger connection far more often than anywhere else. Nails are not acceptable. Use through-bolts or approved structural screws in the pattern given in IRC R507.9, flash the ledger properly, and never fasten to brick veneer or to a cantilevered rim joist. At 3.0 ft above grade this deck also needs a guard, and above 30 in a lateral load connection per R507.9.2 applies.

What this assumes

  • Joist spans follow common pressure-treated southern pine values; verify against your local span tables.
  • Composite decking generally requires 16 inch joist spacing, or 12 inch for diagonal patterns.
  • Footing count assumes a standard beam-and-post layout with posts at or under 8 foot centers.

Important

Decks require a permit almost everywhere, and ledger attachment is the single most common cause of deck collapse. Follow the IRC prescriptive deck provisions in R507 or have the connection engineered.

Understanding the result

Joist spacing depends on the decking

Pressure-treated 5/4 decking spans 16 inches comfortably. Most composite decking also requires 16 inch spacing when laid perpendicular, but drops to 12 inches for a diagonal pattern. Check the decking manufacturer's instructions before you frame — it is a cost difference you cannot retrofit.

The ledger is the critical connection

Deck collapses overwhelmingly begin at the ledger. Nails are not acceptable; use through-bolts or approved structural screws in the pattern the IRC specifies, with proper flashing above. Never attach a ledger to a brick veneer or a cantilevered rim.

Footing depth follows frost line

Deck footings must extend below the local frost line — 36 inches in many northern states, 12 inches in the South. A footing that heaves with frost pushes the post and twists the frame. Check your local building department for the required depth before you dig.

Beam span determines post spacing

A doubled 2×10 beam spanning 8 feet between posts supports joists that are 12 to 14 feet long depending on species and load. Wider decks need intermediate posts and beams — a 16-foot-wide deck typically has three beam lines with posts every 8 feet.

Common questions

How much decking do I need for a 12x12 deck?
A 12×12 deck is 144 square feet. Five-quarter (5/4) decking boards 5.5 inches wide cover 0.46 square feet per linear foot. You need 314 linear feet of decking before waste. With 10% waste and 12-foot boards, order 29 boards.
How far apart should deck joists be?
Pressure-treated 5/4 decking on perpendicular layout spans 16 inches on center. Most composite decking requires 16 inches on center perpendicular and 12 inches for diagonal layouts. Check the manufacturer's span rating before framing — it cannot be changed after the joists are set.
How many deck boards for a 10x10 deck?
A 10×10 deck (100 square feet) with 5.5-inch-wide boards needs 218 linear feet before waste. With 10% waste and 16-foot boards, order 15 boards. Composite boards in 12-foot or 16-foot lengths may reduce the board count but not the total linear footage.
What size joists for a 12 foot deck span?
A 12-foot joist span with pressure-treated southern pine at 16 inches on center requires 2×10 joists. A 10-foot span needs 2×8. A 14-foot span needs 2×12 or engineered joists. These are common prescriptive values — verify against your local span tables.
How many footings for a 12x16 deck?
A 12×16 deck with a beam at the outer edge and ledger attachment to the house typically needs 6 footings: two rows of three posts at 8-foot spacing along the 16-foot length. Each footing is 12 inches diameter minimum, extending below the frost line.
How much does it cost to build a 12x12 deck?
A 12×12 pressure-treated wood deck at grade costs $2,500 to $5,000 in materials and $3,000 to $6,000 installed. Composite decking adds $1,500 to $3,000 to material cost. Elevated decks with stairs and railings run $8,000 to $15,000 installed.
What size beam for a 12 foot deck?
A 12-foot span between posts with joists up to 12 feet long typically uses a doubled 2×10 beam of pressure-treated southern pine. A 14-foot joist span needs a doubled 2×12. The beam size is set by the joist span it supports, not the deck width.
How many screws per deck board?
Use two screws per joist crossing — one at each joist the board spans. A 12-foot board crossing 10 joists at 16 inches on center needs 20 screws. A 144-square-foot deck with 29 boards needs roughly 580 deck screws. Use coated or stainless screws rated for treated lumber.
How deep should deck footings be?
Deck footings must extend below the local frost line. Common depths: 12 inches in the deep South, 36 inches in the Midwest, 48 inches in northern Minnesota and Maine. A 12-inch diameter concrete footing at 36 inches deep uses 0.12 cubic yards of concrete per footing.
Can I use 2x6 for deck joists?
2×6 joists span only 9 feet 6 inches at 16 inches on center in southern pine — adequate for a small landing but not a full deck. A 12-foot span needs 2×10. Undersized joists bounce, sag, and fail inspection. Frame to span tables, not to the lumber you have on hand.

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