Deck Calculator

How a Deck Is Framed

Most attached decks hang one end of the joists on a ledger bolted to the house and rest the other end on a beam carried by posts and footings. The joists can run past the beam as a short cantilever. Decking boards run across the joists, usually parallel to the house.

Two numbers decide whether the framing works: the joist span, the clear distance from the face of the ledger to the face of the beam, and the cantilever past the beam. Both depend on species, joist size and spacing.

The American Wood Council’s DCA 6, the Prescriptive Residential Wood Deck Construction Guide, turns the IRC deck provisions into tables for single-level wood decks, and building departments widely use it for permits. This calculator uses its joist table.

Key DCA 6 Deck Rules

A few requirements shape almost every deck, before you get to beams and footings.

• Joist spans come from Table 2 and assume No. 2 grade lumber, wet service, 40 psf live load and 10 psf dead load. Spans are capped at 18 ft.
• Cantilevers are limited to the table value or one-fourth of the joist span, whichever is less.
• The ledger must be at least a 2x8 and at least as deep as the joists, attached with 1/2" lag screws or bolts. Never attach a ledger to brick, stone or masonry veneer.
• Decking must be 2x lumber or span-rated decking, gapped about 1/8", fastened with two 8d threaded nails or two #8 screws at every joist, and each piece must bear on at least three joists.
• Fasteners and hangers in treated lumber must be hot-dip galvanized or stainless steel.

Deck Formulas

W is the deck width along the house and P is the projection out from the house, both in feet.

LJ

Allowable joist span from DCA 6 Table 2

LO

Allowable cantilever from DCA 6 Table 2

N

Number of joists, including both ends

Boards per row = deck width ÷ board length, rounded up. Waste is added to the deck board count only.

Worked Example: 12 x 16 Attached Deck

A deck 16 ft along the house and 12 ft out, with Southern Pine joists at 16" on center bearing on a beam at the outer edge. Decking is 5-1/2" boards, 16 ft long, with 1/8" gaps and 10% waste.

1. Joist span: 12 ft. A 2x8 at 16" allows 11' 10", so it fails. A 2x10 allows 14' 0", so it works.
2. Joists: 192 ÷ 16 = 12 spaces, so 13 joists, 12 ft long.
3. Decking: 144 ÷ 5.625 = 25.6, so 26 rows of one 16 ft board, 29 boards with waste.
4. Fasteners: 26 rows × 13 joists × 2 = 676. Posts at 8 ft along the beam: 3.

Result: 13 2x10 joists, a 16 ft ledger and rim, 29 deck boards, 3 posts and footings, and about 676 deck screws.

Worked Example: Cantilevering Past the Beam

A deck 14 ft out from the house with the beam set back 2-1/2 ft, using Douglas Fir 2x10 joists at 16" on center.

1. Joist span: 14 − 2.5 = 11.5 ft (11' 6"), under the 13' 7" allowed.
2. Cantilever limit: the table allows 3' 2", but one-fourth of the span is only 2' 10-1/2".
3. The 2' 6" cantilever is under both limits.

Result: the framing works. Pulling the beam in shortens the span, but the cantilever can never exceed a quarter of the span behind it.

Worked Example: Joists at 24" on Center

The same 12 x 16 deck framed at 24" on center to save joists.

1. A Southern Pine 2x10 at 24" allows only 11' 5", short of 12 ft. A 2x12 allows 13' 6".
2. Joists: 192 ÷ 24 = 8 spaces, so 9 joists instead of 13.

Result: nine 2x12 joists, but the decking must be rated for a 24" span. Span-rated decking is usually rated for 16" or 24", so check the board’s rating before you widen the spacing.

Deck Joist Span Chart

Maximum clear joist span by species, size and spacing.

Field Tips

Practical requirements from the American Wood Council’s DCA 6 deck guide and IRC Section R507.

1. 1

   Flash the ledger

   Remove the siding before installing the ledger, and use continuous corrosion-resistant flashing with a drip edge over it. Flash and caulk the door threshold too, since snow and splash collect there.

2. 2

   No ledgers on veneer

   Never attach a ledger to brick, stone or masonry veneer. Build a freestanding deck with its own beam and posts along the house instead.

3. 3

   Size the ledger to the joists

   The ledger must be at least a 2x8, at least as deep as the joists and no deeper than the house rim. Fasten it with 1/2" lag screws or through-bolts, staggered top and bottom.

4. 4

   Use the right fasteners

   Treated lumber eats plain steel. Use hot-dip galvanized or stainless fasteners and hangers, and keep fasteners and connectors the same material. Within 300 ft of salt water, use 304 or 316 stainless.

5. 5

   Three joists per board

   Every piece of decking has to bear on at least three joists. Plan board lengths and butt joints so short offcuts don’t end up spanning only two.

Sources: American Wood Council: DCA 6 deck construction guide, International Residential Code, Section R507: Exterior Decks