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Deck Beam Span Calculator

The lightest beam that spans, on the fewest posts that carry it.

Spans from a prescriptive guide. A licensed contractor or engineer, and your local building department, decide what is built.

24' · 288 in · 24 ft · 7.3152 m

12' · 144 in · 12 ft · 3.6576 m

3' · 36 in · 3 ft · 0.9144 m

288 ft²

10% covers decking cut to length and culled, the figure Decks.com adds for a straight lay (15% for a diagonal one). It is added once, to the deck boards — the joists, beam, posts and footings are counted off the frame and read off the DCA 6 tables.

This page recalculates as you type. There is no Calculate button.

Answer

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Beam

2-2×12 nominal

288 ft² of deck · 19 2×8 joists · 4 footings

Deck boards 58 boards
Deck area 288 ft²
Joists 19 joists
Posts 4 posts
Footings 4 at 20" footings
Footing concrete 5.82 ft³
Decking fasteners 988 screws
More numbers
Joist size 2×8 nominal
Ledger bolts 17 screws
Joists on center 16"
Joist overhang past the beam 24"
Guard to build 48 ft
Materials $2,771.86
DrawingThe project drawn to scale. Deck 1: 24' × 12'.24'12'Deck 1

Buying noteThe concrete is the footing pads only. A pier or a tube form from the pad up to the ground is extra, and so is anything the frost line adds below it.
Buying noteDCA 6 gives a maximum spacing for guard posts on a stair, not for ones on the deck surface. The 6 ft used here is that stair figure; your building department decides.

Breakdown

Deck Measurements Qty Area Share
Deck 1 24' × 12' × 3' Add ×1 288 ft²

What to buy

Buying noteThe concrete is the footing pads only. A pier or a tube form from the pad up to the ground is extra, and so is anything the frost line adds below it.
Buying noteDCA 6 gives a maximum spacing for guard posts on a stair, not for ones on the deck surface. The 6 ft used here is that stair figure; your building department decides.

Ways to buy the footing concrete

The footings only. The lumber and the connectors are priced by their own rows below.

9 × 80 lb + 1 × 60 lb Bags 10 Left over 0.03 ft³ Weight 780 lb Cost $56.10
9 × 80 lb + 1 × 60 lb Bags 10 Left over 0.03 ft³ Weight 780 lb Cost $56.10
10 × 80 lb Bags 10 Left over 0.18 ft³ Weight 800 lb Cost $56.80 No reference price. Type what your store charges.
13 × 60 lb Bags 13 Left over 0.03 ft³ Weight 780 lb Cost $64.74 No reference price. Type what your store charges.
20 × 40 lb Bags 20 Left over 0.18 ft³ Weight 800 lb Cost $79.60 No reference price. Type what your store charges.

Shopping list

58 58 × 12 ft $939.60 Reference price $1.35. Type your store's.
4 boxes $131.88 Reference price $32.97. Type your store's.
19 19 × 12 ft $387.60 Reference price $1.70. Type your store's.
1 1 × 24 ft $40.80 Reference price $1.70. Type your store's.
1 1 × 24 ft $40.80 Reference price $1.70. Type your store's.
17 screws $32.30 Reference price $1.90. Type your store's.
24 ft $50.40 Reference price $2.10. Type your store's.
2 2 × 24 ft $81.60 Reference price $1.70. Type your store's.
4 4 × 8 ft $158.08 Reference price $4.94. Type your store's.
4 post bases $96.00 Reference price $24.00. Type your store's.
19 hangers $45.60 Reference price $2.40. Type your store's.
19 ties $20.90 Reference price $1.10. Type your store's.
10 10 × 80 lb $56.80
9 posts $288.00 Reference price $32.00. Type your store's.
5 5 × 20 ft $160.00 Reference price $1.60. Type your store's.
105 balusters $241.50 Reference price $2.30. Type your store's.
Ticked items $2,771.86

Untick what you already have. Lumber is priced by the foot beside its row; the concrete's bags are priced in the table above.

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Standards

We hold the criteria and the clause, not the wording of the standard.

Meets DCA 6-15 · Table 2

Joists span 10', inside the 11' 10" the guide allows a 2×8 at this spacing.

Meets DCA 6-15 · Table 2

The joists pass the beam by 2', inside the 2' allowed.

Meets DCA 6-15 · Table 3A

A 2-2x12 beam spans 8' between posts, inside the 9' 2" allowed.

Meets DCA 6-15 · Table 4

The posts stand 1' 4 1/2", inside the 14' the table allows.

Meets DCA 6-15 · Table 5

The ledger bolts go in at 18" on center.

Meets IRC 2021 · R507.7

Joists at 16" are inside the 16" this decking spans.

Meets IRC 2021 · R312.1

At 36" the deck is over 30", and the list carries a guard.

How this was worked out

Every number below is yours. Each constant links to the row where you can change it.

Deck 1 24' × 12' 288 ft²
Joist spacing 5/4 × 6 treated decking laid Square to the joists allows joists up to 16" apart 16"
Rows of decking 144" deep ÷ (5 1/2" board + 1/8" gap) 26

Change:Deck board widthGap between screwed boards

Deck boards 26 rows × 24 ft run, cut from 12 ft boards 52
After waste 52 boards + 10% 58

Change:Decking waste

Joists 24 ft along the house, none more than 16" apart 19
Joist size from the table spans 10 ft at 16", so 2×8 11.83 ft
Beam from the table 24 ft on 4 posts is 8 ft a span 2-2×12
Posts 4 under each of 1 beams 4
Footing from the table beam 8 ft, joists 10 ft 20" round, 8" thick

Change:Footing below groundYour frost depth

Footing concrete π × (20" ÷ 2)² × 8" × 4 footings 5.818 ft³
Deck screws 26 rows × 19 joists × 2 988

Change:Screws a board a joist

Ledger fasteners 24 ft of ledger, none more than 18" apart 17
Guard 48 ft of guard on 9 posts 105

Change:Most between guard postsWidest opening in a guard

Bags 5.818 ft³ as 10 × 80 lb 10

Change:Yield, 80 lb bagYield, 60 lb bagYield, 40 lb bag

Cheapest combination 58 × 12 ft × $1.35/ft + 4 × $32.97 + 19 × 12 ft × $1.70/ft + 1 × 24 ft × $1.70/ft + 1 × 24 ft × $1.70/ft + 17 × $1.90 + 24 × $2.10/ft + 2 × 24 ft × $1.70/ft + 4 × 8 ft × $4.94/ft + 4 × $24.00 + 19 × $2.40 + 19 × $1.10 + 10 × $5.68 + 9 × $32.00 + 5 × 20 ft × $1.60/ft + 105 × $2.30 $2,771.86

Assumptions

Every one of these is editable. Change one and the answer changes with it.

Assumption Value Confidence
R507.7 decking, square layIRC R507.7, maximum joist spacing for wood decking. A single span is decking bearing on two joists; continuous is three or more. DCA 6 requires every segment of decking to bear on at least three supports, so the continuous column is the one this page reads. IRC 2021 and IRC 2024, R507.7 From a data sheet or an agency document
DCA 6 T2 joist span, Southern PineDCA 6-15 Table 2, allowable span L_J: how far a joist of this size may span at this spacing, face of support to face of support. Assumes 40 psf live load, 10 psf dead load, No. 2 grade, wet service and L/360 deflection. Species: Southern Pine. DCA 6-15 From a data sheet or an agency document
DCA 6 T2 overhang, Southern PineDCA 6-15 Table 2, allowable overhang L_O: how far a joist of this size may pass the beam. The overhang used is the lesser of this and a quarter of the actual span. The column does not always fall as the spacing widens - the guide shades those cells as deflection controlled - and the figures are carried as printed. DCA 6-15 From a data sheet or an agency document
DCA 6 T3A beam, 2-ply, Southern PineDCA 6-15 Table 3A, beam span L_B: how far a beam of this make-up may span between posts, by the span of the joists it carries. The column is read as "joist spans less than or equal to", so a joist span is rounded up to the next column before the table is read. Assumes 40 psf live, 10 psf dead, No. 2 grade and wet service. DCA 6-15 From a data sheet or an agency document
DCA 6 T3A beam, 3-ply, Southern PineDCA 6-15 Table 3A, beam span L_B: how far a beam of this make-up may span between posts, by the span of the joists it carries. The column is read as "joist spans less than or equal to", so a joist span is rounded up to the next column before the table is read. Assumes 40 psf live, 10 psf dead, No. 2 grade and wet service. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 footing diameterDCA 6-15 Table 4. Assumes 1,500 psf soil bearing and 150 pcf concrete; the guide allows the value to be multiplied by 0.9 for a corner post, which this page does not do. Both axes are read as "less than or equal to", so a span is rounded up to the next column before the table is read. The diameter of a round footing under one post. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 footing thicknessDCA 6-15 Table 4. Assumes 1,500 psf soil bearing and 150 pcf concrete; the guide allows the value to be multiplied by 0.9 for a corner post, which this page does not do. Both axes are read as "less than or equal to", so a span is rounded up to the next column before the table is read. The thickness of the footing pad. Assumes 2,500 psi concrete. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 post height, Southern PineDCA 6-15 Table 4, post heights for a 6x6: how tall a post may stand, measured from grade or the top of the foundation, whichever is higher, to the underside of the beam. An 8x8 may stand anywhere in the table up to 14 ft. Species: Southern Pine. A blank cell is the guide's "NP" - not permitted - and the page says so rather than fill it in. DCA 6-15 From a data sheet or an agency document
DCA 6 T5 lag spacingDCA 6-15 Table 5: the on-center spacing of ledger fasteners, by what the house's band or rim joist is and how far the joists span. Deck live load 40 psf, dead load 10 psf; 15/32 in maximum sheathing; fasteners staggered. The 1 in and 1 1/8 in rows are LVL rim joists, the 1 1/2 in row solid-sawn lumber. Half-inch lag screws, tips fully past the inside face of the band. DCA 6-15 From a data sheet or an agency document
Deck board widthThe width a nominal 6 in deck board really is: 5 1/2 in. Trex publishes its boards as 0.94 in x 5.5 in, and PS 20 dresses a nominal 6 in face to 5 1/2 in, so wood and composite are the same width. Decks.com counts rows at the nominal 6 in and so leaves three rows out of a 16 ft depth. in From a data sheet or an agency document
Gap between screwed boardsAWC DCA 6-15, p. 3: space decking boards approximately 1/8 in apart. in From a data sheet or an agency document
The wide allowance fromNIST PS 20-20, Table 3: the dressed allowance goes from ½ in to ¾ in at 8 in nominal width. in From a data sheet or an agency document
Dressed, boards 1 to 1½ inNIST PS 20-20, Table 3 (p. 16): a board from 1 in and under 2 in nominal is dressed dry ¼ in under it - 1 to ¾, 1¼ to 1, 1½ to 1¼. in From a data sheet or an agency document
A guard is needed aboveAWC DCA 6-15, p. 19: a deck more than 30 in above grade has to have a guard [IRC R312.1]. in From a data sheet or an agency document
Widest opening in a guardAWC DCA 6-15, Figure 24 and its commentary on p. 41: no opening in a guard may let a 4 in diameter sphere pass. The commentary extends it to the openings under a fixed bench used in place of a guard, which Figure 24 itself leaves out. in From a data sheet or an agency document
Most between guard postsMost between guard posts. AWC DCA 6-15, Figure 30, gives 6 ft for the guard on a deck STAIR. The guide gives no maximum for a guard on the deck surface, so this figure is carried across to it and the page says so beside the count; your building department decides, and some ask for 4 ft. in From a data sheet or an agency document
Screws a board a joistAWC DCA 6-15, p. 3: attach decking to each joist with 2-8d threaded nails or 2-#8 screws. Omni uses a flat 3.5 screws a square foot, which only matches at 16 in on center. screws From a data sheet or an agency document
Hangers a joistAWC DCA 6-15, p. 9 and Figure 7: one joist hanger at the ledger for each joist. At the beam the joists bear over it and take a hurricane tie instead (Figure 6, Option 2), so they are not hung twice. hangers From a data sheet or an agency document
Ties a joist at the beamAWC DCA 6-15, p. 9, Figure 6 Option 2: one mechanical fastener or hurricane clip per joist at the beam, with at least 100 lb capacity in uplift and in lateral load. ties From a data sheet or an agency document
Smallest ledger and guard joistAWC DCA 6-15, Table 2 footnote 6 and p. 19: the ledger is a minimum 2x8 nominal, and so is any joist or rim joist a guard post is attached to. in nominal From a data sheet or an agency document
Post face, 6×6 dressedAWC DCA 6-15, p. 10: every deck post is 6x6 nominal or larger. 5 1/2 in is what a nominal 6 in face dresses to under PS 20. in From a data sheet or an agency document
Brace a corner post overAWC DCA 6-15, p. 10: diagonal bracing parallel to the beam at every corner post more than 2 ft 0 in tall. The guide prohibits it on center posts. in From a data sheet or an agency document
Footing below groundAWC DCA 6-15, p. 11 [R403]: footings bear on undisturbed soil at least 12 in below the undisturbed ground surface, or below the frost line, whichever is deeper. in From a data sheet or an agency document
Your frost depthHow far down the ground freezes where you are building. AWC DCA 6-15, p. 11 [R403]: a footing bears 12 in below the undisturbed surface or below the frost line, whichever is deeper, and the guide says to ask the authority having jurisdiction for the depth - no document publishes it for your town. Zero is no frost allowance, which is what the guide's own 12 in base case assumes; type your depth and every hole follows it. in From a data sheet or an agency document
Dressed, 8 in and widerNIST PS 20-20, Table 3: 8 in nominal and wider is dressed dry ¾ in under it - 8 to 7¼, 10 to 9¼, 12 to 11¼. in From a data sheet or an agency document
Dressed, 2 in up to 8 inNIST PS 20-20, Table 3: from 2 in nominal and under 8 in wide, lumber is dressed dry ½ in under it - 2 to 1½, 4 to 3½, 6 to 5½. in From a data sheet or an agency document
Yield, 80 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document
Yield, 60 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document
Yield, 40 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document

How far a deck beam spans, and how many posts

A 24 foot deck carrying joists that span 10 feet takes 4 posts and a doubled 2×12. DCA 6-15 Table 3A gives a 2-2×12 in Southern Pine 9' 2" between posts at this joist span; the beam is split into three 8 foot runs, which fits. Three posts would mean 12 foot spans and nothing in the table reaches that doubled, so the page adds the fourth.

The page looks for the fewest posts that any beam in the table will carry, because fewest posts is fewest holes to dig. It tries two plies before three at each count, so you get the lighter beam where the lighter beam works.

Reading the column, not interpolating it

Table 3A's columns are joist spans "less than or equal to" 6, 8, 10, 12, 14, 16 and 18 feet. A joist spanning 11' 2" is read in the 12 foot column, not between the 10 and the 12. Rounding up to the next column is part of reading the guide, and the working on this page prints which column it used.

Species matters here too, but the guide groups them differently than it does for joists: Table 3A has two blocks, Southern Pine and everything else. In the second block the two-ply row is printed "3x6 or 2-2x6" — a solid 3× and a doubled 2× read the same line. A doubled 2×12 that spans 8' 4" in Southern Pine spans 6' 7" in the other group, so the same deck gains a post.

What comes with the beam

The beam span sets the footing. Table 4 reads beam span against joist span and gives a round footing diameter, a square size and a pad thickness; widen the post spacing and the footings grow. It also sets how tall a 6×6 post may stand, and some combinations of long beam and long joist are left blank — the guide does not permit a prescriptive post there at all, and this page says so rather than filling the blank in.

Joists must not frame in from opposite sides of the same beam, and a beam attached to the side of a post without notching is prohibited. Notch the post or use an approved post cap; a three-ply beam always takes a cap.