Deck joist span calculator

Tell us the joist span and spacing and we name the smallest joist that works, and how far it may overhang the beam, straight from the AWC DCA 6 span table.

At 10'-0" and 16 in on center in Southern Pine, the smallest joist DCA 6 allows is a 2x8. It spans up to 11'-10" and may overhang the beam by up to 2'-0".

Table 2 for Southern Pine, 40 psf live and 10 psf dead load, No. 2 grade, wet service. Your row is highlighted.

Allowable joist span and overhang, DCA 6 Table 2
Joist	Span 12 in	Span 16 in	Span 24 in	Overhang 12 in	Overhang 16 in	Overhang 24 in
2x6	9'-11"	9'-0"	7'-7"	1'-0"	1'-1"	1'-3"
2x8	13'-1"	11'-10"	9'-8"	1'-10"	2'-0"	2'-4"
2x10	16'-2"	14'-0"	11'-5"	3'-1"	3'-5"	2'-10"
2x12	18'-0"	16'-6"	13'-6"	4'-6"	4'-2"	3'-4"

Design this deck in 3D, free

Carries over: species, spacing, and a deck depth of 13 ft, which gives a joist span of about 10 ft. Not carried: deck width, height, and the joist size, which the designer works out for the whole deck. This table sizes joists only, not the beam, posts or footings.

Planning figures from the AWC DCA 6-2015 guide, not an engineer's design or a permit. Your building department has the final word, and local rules may be stricter.

How to read the result

The span is the clear distance between the two supports a joist rests on, for example the house ledger and the beam. Pick your species, the spacing you plan to use and the span. The calculator names the smallest joist in DCA 6 Table 2 that reaches it, and highlights that cell in the table below.

Spacing, 12, 16 or 24 inches

Closer joists carry more span, but the decking sets the limit first. One inch thick wood decking is normally laid on joists no more than 16 in apart, and 2 in thick wood on joists up to 24 in. Composite boards have their own limit from the maker, often 16 in. Check the box before you choose 24 in.

Overhang past the beam

Joists may stick out past the beam. Table 2 gives a maximum overhang for each size and spacing, and the overhang may not pass a quarter of the span. The result above shows the smaller of the two.

The longest joist allowed

DCA 6 limits joist spans to 18'-0" for the footing sizes it tables. Past that, add a beam in the middle to cut the span in two.

Joists that hold railing posts

Joists and rim joists that carry guard posts must be 2x8 or deeper. Switch on the railing option and the calculator skips smaller sizes.

Species groups

Southern Pine has its own row in Table 2. Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir share one row, which follows the Hem-Fir values, so any of the three meets it.

Where the numbers come from

Every limit on this page is read from the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), 2015 edition, a guide written for the 2015 International Residential Code. The deck designer, the builder's plan and these tools all read the same tables. The span tables assume 40 psf live load, 10 psf dead load, No. 2 grade lumber in wet service, and footings on soil that bears at least 1,500 psf.

A number that is our own choice and not in the guide, such as the 24 in joist overhang we draw by default, is never presented as a code requirement. Where a value comes from somewhere other than DCA 6, the page says so.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

As a deck joist at 16 in on center, a 2x8 spans 11'-10" in Southern Pine and 11'-1" in Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir, by DCA 6 Table 2. Closer spacing reaches further: at 12 in it spans 13'-1" in Southern Pine, and at 24 in only 9'-8". Those figures are for decks at 40 psf live load. House floor joists follow different tables.

For a deck, DCA 6 Table 2 gives a 2x6 joist at 16 in on center 9'-0" in Southern Pine and 8'-4" in Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir. A 2x6 inside a house floor is sized from other tables, so use this one only for a deck.

For a deck 12 ft out from the house and 16 ft wide, with the joists overhanging the beam by 24 in, each joist spans 9'-10 1/2". At 16 in on center that takes 2x8 joists in Southern Pine and 2x8 in Hem-Fir. Your answer shifts with where the beam sits, which is why the calculator above takes the span and not the deck size.