Deck joist spacing: how far apart deck joists should be, by the decking
Sixteen inches on center is the most a joist can be spaced under 5/4 wood decking laid straight across it, by Table R507.7 of the 2021 International Residential Code. On center means from the middle of one joist to the middle of the next. Under the same boards laid diagonally it is 12 inches. The table drops both to 12 and 8 inches when a board rests on only two joists. The decking sets the spacing. The joist's size and species then set how far it can span between the ledger and the beam.
The decking calculator: joists, boards and screws for your deck size and spacing
In brief
- Code (residential code Table R507.7, 2021): 5/4 wood decking 16 inches straight and 12 diagonal on multiple spans, 12 and 8 on a single span; 2 inch wood decking 24 straight, 18 to 24 diagonal.
- Composite and PVC: 16 inches for most boards, 12 at an angle or for commercial loads; Trex 2x6 boards and TimberTech MAX and DockSider boards allow 24.
- Every board must bear on at least three joists and be fastened to each with two screws or nails.
- Joist span by species at 16 inches: southern pine 2x8 11 ft 10 in, 2x10 14 ft, 2x12 16 ft 6 in; spruce-pine-fir and hem-fir 5 to 9 inches less.
- A 20 ft deck has 16 joists at 16 inches on center, 21 at 12, 11 at 24.
The decking sets the spacing
Joist spacing is a decking question first. The residential code's Table R507.7, read in the Town of Denton's 2021 deck guide, gives the limits for wood boards. It sends composite and plastic decking to the maker's installation instructions.
| Decking | Straight, on 3 or more joists | Straight, on 2 joists | Diagonal, 3 or more | Diagonal, 2 |
|---|---|---|---|---|
| 5/4 wood (1 1/4 in nominal) | 16 in | 12 in | 12 in | 8 in |
| 2 inch wood (2x4, 2x6) | 24 in | 24 in | 24 in | 18 in |
| Composite and PVC | per the maker, 16 in for most | 12 in for most |
The single-span columns are the 2021 edition's addition: a short board that rests on only two joists bends more, so its joists must be closer. The 2015 edition, read in the City of Muscatine's guide, has no such split. It gives the same 16 and 12 for 5/4 boards and 24 and 16 for 2 inch boards. Cedar follows its own trade table. The Western Red Cedar Lumber Association gives 24 inches for 2 inch decking. For 1 1/4 by 4 inch boards it gives 16 inches for clear grades and 12 for knotty.
What the composite makers allow
Every maker's guide read puts standard boards at 16 inches and angled or commercial installations at 12, and each names its own exceptions. Trex's 2024 guide spans 1 inch boards 16 inches for residential and 100 psf commercial loads, and 12 inches at 200 psf. Its 2x6 board spans 24 inches. At a 45 degree angle the span drops 4 inches, at 60 degrees 2 inches, and at 30 degrees to half. TimberTech's PRO and EDGE boards are 16 inches straight, 14 at 30 degrees and 12 at 45, with the DockSider board at 24.
Advanced PVC's standard board is 16 inches, its MAX board up to 24, and 12 for angled or commercial work. The same guides ask for a row of solid blocking between joists every 4 to 6 ft. They suggest 12 inches wherever a more rigid feel matters. Fiberon's Promenade board is 16 straight and 12 up to 45 degrees. Two rules repeat in every guide and in the wood council's. A board must bear on at least three joists. It is fastened to each joist it crosses with two screws or nails.
Spacing is not span
Spacing is how far apart the joists sit; span is how far each one reaches from the ledger at the house to the beam. The decking limits the first, and the joist's size, species and spacing limit the second. The wood council's prescriptive guide prints the spans, the same values the 2018 code adopted in its Table R507.6.
| Joist | Southern pine, 12 / 16 / 24 in | Douglas fir, hem-fir, spruce-pine-fir | Redwood, cedar, ponderosa and red pine |
|---|---|---|---|
| 2x6 | 9'11" / 9'0" / 7'7" | 9'6" / 8'4" / 6'10" | 8'10" / 8'0" / 6'10" |
| 2x8 | 13'1" / 11'10" / 9'8" | 12'6" / 11'1" / 9'1" | 11'8" / 10'7" / 8'8" |
| 2x10 | 16'2" / 14'0" / 11'5" | 15'8" / 13'7" / 11'1" | 14'11" / 13'0" / 10'7" |
| 2x12 | 18'0" / 16'6" / 13'6" | 18'0" / 15'9" / 12'10" | 17'5" / 15'1" / 12'4" |
The table assumes No. 2 grade, wet service, a 40 psf live load and 10 psf dead load. The guide caps any joist at 18 ft for its footing tables. Read it both ways. A 2x8 southern pine joist spans 11 ft 10 in at 16 inches. Moving to 12 inches stretches the same joist to 13 ft 1 in, so closer joists can replace a size step. A 16 ft span in spruce-pine-fir needs 2x12 joists at 12 inches, where southern pine makes it at 16.
Counting and laying them out
A deck's joist count is its length along the house divided by the spacing, rounded up, plus one for the end. A 16 ft deck has 13 joists at 16 inches, 17 at 12 and 9 at 24. The same spacings give a 20 ft deck 16, 21 and 11. The first joist sits at the end, and each next one is marked at the spacing from the last. So the final bay is short rather than every bay being stretched. A bay wider than the limit anywhere means an extra joist, never a wider spacing. Rim joists close the ends, and the ledger carries the house side. The guide wants the ledger and any joist that carries a guard post to be at least 2x8. The decking calculator takes the deck size, board width and spacing, and returns joists, boards and the two-screws-per-crossing fastener count together.
When to go to 12 inches anyway
Diagonal decking, a commercial or rental deck and a composite board whose maker asks for it all push to 12 inches or to an engineer. A span that runs out in a lighter species does as well. So does a deck that will carry a hot tub, a crowd or a heavy planter. The last of those is outside every prescriptive table here. For a plain rectangular deck with 5/4 or standard composite boards laid straight, 16 inches is the figure the code and the makers agree on. The frame can be built once and re-decked in either material later.
Questions about deck joist spacing
Can deck joists be 24 inches apart?
Under 2 inch thick wood decking laid straight, yes, by the code table. Under 5/4 wood boards, no: 16 inches is the limit. Among composites, Trex allows 24 inches for its 2x6 board only, and TimberTech for its MAX and DockSider boards. Every standard 1 inch composite board is 16. A 24 inch frame under standard boards is a frame that will need a joist added between each pair.
Is 12 inch joist spacing better than 16?
It is stiffer, and it is required for diagonal decking and for commercial loads on most composite boards. It is also required for a 16 ft span in spruce-pine-fir or hem-fir 2x12 joists, where 16 inches runs out of span. TimberTech's guides suggest 12 inches for a more rigid feel. The cost is a third more joists: 21 instead of 16 on a 20 ft deck.
Is joist spacing the same as joist span?
No. Spacing is the distance between joists, set by the decking on top. Span is how far each joist reaches from the ledger to the beam, set by the joist's size and species. A 2x8 southern pine joist spans 13 ft 1 in at 12 inch spacing and 11 ft 10 in at 16. At 24, it spans 9 ft 8 in. So closer spacing also lets the same joist reach farther.
How many joists do I need?
Divide the deck's length along the house by the spacing in feet, round up and add one for the end joist. A 20 ft deck is 20 ÷ 1.333 = 15, plus one, which is 16 joists at 16 inches. It is 21 at 12 inches and 11 at 24. The decking calculator counts them with the rim joists and the screws.
10 source documents
- Town of Denton, Maryland, Planning and Codes, Deck Construction Guide, 2021 International Residential Code: Table R507.7 maximum joist spacing for wood decking, single and multiple spans, straight and diagonal. Read 2 November 2026.
- City of Muscatine, Iowa, Community Development, Deck Construction Guidelines, 2015 International Residential Code: Table R507.4 joist spacing (16 and 12 inches for 1 1/4 inch wood; 24 and 16 for 2 inch wood). Read 2 November 2026.
- Penn State Housing Research Center, Decked Out: the 2021 IRC residential deck provisions, webinar handout, January 2025: R507.7 wording, composite decking per the manufacturer. Read 2 November 2026.
- American Wood Council, DCA 6, Prescriptive Residential Wood Deck Construction Guide (2015 IRC), Table 2 maximum joist spans; decking on at least three joists, two fasteners per joist, 1/8 inch gap. Read 2 November 2026.
- Town of Ocean City, Maryland, Typical Deck Details, 2020: Table 1 maximum joist spans, southern pine, from the 2018 IRC Table R507.6. Read 2 November 2026.
- Trex Company, 2024 Decking Installation Guide, joist spanning chart: 1 inch boards 16 inches (12 at 200 psf), 2x6 boards 24 inches; 4 inches less at 45 degrees, 2 less at 60, half at 30; boards span at least three joists. Read 2 November 2026.
- TimberTech (AZEK), PRO, EDGE and Specialty Decking Installation Guide: never over 16 inches, 12 preferred for rigidity; 16, 14 and 12 inches at 90, 30 and 45 degrees; DockSider 24. Read 2 November 2026.
- TimberTech (AZEK), Advanced PVC Decking Installation Guide, July 2026: 16 inches, up to 24 for MAX boards, 12 for angled or commercial; blocking rows every 4 to 6 ft. Read 2 November 2026.
- Fiberon, Promenade decking product page: 16 inches perpendicular, 12 inches up to 45 degrees. Read 2 November 2026.
- Western Red Cedar Lumber Association, Western red cedar decking span tables: 2 inch decking 24 inches; 1 1/4 by 4 inch clear grades 16, knotty grades 12. Read 2 November 2026.