What joist spacing does composite decking need?

Whatever its manufacturer publishes, which is 16 in on centre for most brands run perpendicular, 12 in on a 45 degree diagonal, and as tight as 8 or 9 in for stair stringers. MoistureShield's Stratos and 2x6 boards are rated to 24 in perpendicular. The IRC does not set a figure: R507.7 sends plastic composite to R507.2, and R507.2.2.5 requires installation in accordance with the manufacturer's instructions.

Does the IRC have a table for composite decking joist spacing?

No. IRC Table R507.7 is titled "Maximum Joist Spacing for Wood Decking" and applies to nominal 2 in decking, 5/4 in decking and wood structural panels. R507.7 states that spacing for plastic composite decking shall be in accordance with Section R507.2, which routes to ASTM D7032 and the manufacturer's instructions.

Why does diagonal decking need closer joists?

A board laid at 45 degrees crosses the joists at 45 degrees, so the unsupported distance between bearing points is about 1.41 times the joist spacing. Manufacturers absorb that by publishing a tighter figure: 12 in where the perpendicular figure is 16 in, and Trex expresses it as the chart value minus 4 in.

Can I space joists at 24 in for Trex?

Only Transcend and Select 2x6 boards, and only at the 100 psf rating in Trex's 2026 guide. Trex's 1x6 profiles are rated 16 in at 100 psf. Trex publishes a load chart rather than a single number, so the answer depends on the load you are designing for.

How many composite boards do I need for a 16x12 deck?

With 5 1/2 in boards at a 1/4 in gap, each row covers 5.75 in, so 192 sq ft takes about 401 linear feet of board. At 16 ft lengths with a 10% waste allowance that is 28 boards. Change the gap to 3/16 in and it drops to 27.

What does ASTM D7032 actually do?

It is the standard specification for wood-plastic composite and plastic lumber decking. It sets how a board's span rating, structural performance and durability are established by test, which is why each product line ends up with its own published spacing rather than a shared one.

Why does this page not show composite board prices?

Because no manufacturer publishes one and retail pricing varies too much between regions and retailers to quote a figure honestly. Enter what your supplier quotes and the calculator will use it. Every other number on the page traces to a document you can open.

Home › Decking › Composite Decking Calculator

Composite Decking Calculator

Spacing read from each manufacturer's own guide. The IRC sets none.

Ask what joist spacing composite decking needs and almost everything you find says 16 in and credits the building code. The code does not say it. The IRC hands plastic composite decking to the manufacturer's instructions, and those differ by brand, by product line, and by whether your boards run square or on the diagonal. Pick your board and this checks your framing against the figure that brand actually published.

Composite decking check and takeoff

Board (brand and product line)

How the boards run

Application

Your joist spacing (on centre)

Deck length (boards run this way)

Deck width

Board width (actual, not nominal)

Gap between boards

Board length sold

Waste allowance

Your price per board (optional, leave 0)

The spacing comes from the manufacturer's current installation guide, named in the result. The quantities are arithmetic from your dimensions.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Your joist spacing against what the board requires

Worked example

A 16 ft by 12 ft deck in Trex Enhance, boards running square across joists framed at 16 in on centre, 5 1/2 in boards at a 1/4 in gap, bought in 16 ft lengths.

The spacing. Trex publishes 16 in on centre for Enhance 1x6 run perpendicular, in its 2026 guide. Framing at 16 in passes, with nothing to spare.
The trap. Turn those same boards 45 degrees and the figure drops to 12 in. The framing that passed a moment ago now fails, and no joist moved.
The other trap. Enhance stair stringers go to 9 in, the tightest figure Trex publishes. A stair framed at the deck's own 16 in is wrong even though the deck is right.
Coverage. 5.5 in of board plus a 1/4 in gap covers 5.75 in per row, so 192 sq ft needs 401 linear ft of board.
Boards. Add 10% waste and that is 441 ft, or 28 boards at 16 ft.

Change the board at the top and watch the required spacing move without touching a single dimension. That is the whole point: the number is a property of what you bought, not of your deck.

The formula

There is no formula for the spacing. It is a published figure, looked up. The quantities are arithmetic:

coverage = w + g linear ft = 12 A / coverage boards = ⌈ linear ft × (1 + waste) / ℓ ⌉

w: actual board width, in. A 1x6 composite board is 5 1/2 in, not 6
g: gap between boards, in. Composite expands more than wood, so follow the guide
A: deck area, sq ft
ℓ: the length the boards are sold in, ft
waste: your allowance. Higher on a diagonal, where every row is cut twice on an angle

Coverage is where the money leaks. A 1/4 in gap instead of a 3/16 in gap over a 16 ft by 12 ft deck is one extra board, and getting the actual board width wrong by using the nominal 6 in is nearly three.

Why this page asks which board you bought

Wood decking has a code table. IRC Table R507.7 gives a maximum joist spacing for nominal 2 in decking, 5/4 in decking and wood structural panels, and any calculator can print it. Composite has nothing equivalent, because a composite board's span is a tested property of that specific formulation and profile, established under ASTM D7032 and printed on the board's own label.

That is why the figures below are not all 16 in. MoistureShield's Stratos and 2x6 boards carry 24 in perpendicular, the widest here. Trex Enhance stair stringers go to 9 in, the tightest. TimberTech's Advanced PVC and Composite lines are published in two separate guides with two separate revision dates. Averaging those into one number would be convenient and wrong.

Every figure on this page names the document it came from and that document's revision code, because these get revised. Trex's 2024 guide is still what most search results surface, and its stair figures no longer match the 2026 one.

Where a manufacturer's own instructions could not be obtained, this page says so and publishes nothing in that cell rather than borrowing a neighbour's number.

Maximum joist spacing on centre, from each manufacturer's own instructions
Brand	Product line	Perpendicular	45° diagonal	Stair stringers	Document
Trex	Signature, Transcend, Lineage or Select 1x6	16 in	12 in	12 in	2026 edition
Trex	Enhance 1x6	16 in	12 in	9 in	2026 edition
Trex	Transcend or Select 2x6	16 in	12 in	16 in	2026 edition
Trex	Transcend Lineage 1x4	16 in	12 in	12 in	2026 edition
TimberTech	Advanced PVC	16 in	12 in	10 in	REV 07/26
TimberTech	Composite and ReliaBoard	16 in	12 in	9 in	REV 09/23
Fiberon	PE composite (Concordia, Sanctuary, Good Life, ArmorGuard)	16 in	12 in	12 in	FIB-0354-LIT 10/25
Fiberon	PVC (Promenade, Paramount)	16 in	12 in	24 in	FIB-0354-LIT 11/22
MoistureShield	1x5.4 and 2x4	16 in	12 in	9 in	MS21-130, March 2021
MoistureShield	Stratos, 2x6 and 2x8	24 in	16 in	16 in	MS21-130, March 2021
Deckorators	Excursion, Frontier, Summit, Vault, Voyage	16 in	not published	9 in	Reissued June 2026

Read 2026-09-07 from the documents listed below. Manufacturers revise these; check yours before building.

Download this table as CSV. Same cells as printed above, nothing added.

The building code sets no joist spacing for composite decking

Almost every calculator and article on this subject prints a single number, usually 16 in, and attributes it to the IRC. The IRC does not say it. The relevant table is titled “TABLE R507.7 MAXIMUM JOIST SPACING FOR WOOD DECKING”, and the sentence that follows it in R507.7 reads:

Maximum allowable spacing for joists supporting wood decking, excluding stairways, shall be in accordance with Table R507.7. ... Maximum allowable spacing for joists supporting plastic composite decking shall be in accordance with Section R507.2.

R507.2 sends plastic composite boards to ASTM D7032, the standard that establishes a board’s span rating in the first place, and R507.2.2.5 closes the loop:

Plastic composite deck boards, stair treads, guards and handrails shall be installed in accordance with this code and the manufacturer's instructions.

So the number that governs your deck is the one printed on the board’s own label and in that manufacturer’s current installation guide. It is not a code figure and it is not the same between brands, between product lines within a brand, or between a square and a diagonal layout. An inspector checking composite decking is checking it against the manufacturer’s instructions, which is why this page asks which board you are buying before it answers.

Code text read from the Massachusetts Residential Code, 10th edition, based on the 2021 IRC. Verified through a municipal reproduction of the code text because codes.iccsafe.org returns HTTP 403 and cannot be fetched.

Gapping, and the mistake that costs a board

Composite expands and contracts along its length far more than wood does, and the end gap a manufacturer specifies is often a function of the temperature on the day you install. Both the side gap and the end gap come from the same installation guide as the spacing, and both matter: too tight and the boards buckle in summer, too loose and the deck looks unfinished and drops small items through.

The arithmetic trap is separate. Composite boards are sold as 1x6 and measure 5 1/2 in, exactly like lumber. Entering 6 in above will under-count your order by roughly 8%, which on a 16 ft by 12 ft deck is two boards you will discover you need on the last row.

Grooved boards, hidden clips and stair treads

Most brands sell each board in a square-edge and a grooved profile, and the grooved one is the only one that takes hidden clips. Several manufacturers specifically prohibit grooved boards as stair treads, Fiberon among them, because the groove removes material from exactly the region a tread loads. If you are buying grooved boards for the deck, check whether you also need square-edge boards for the stairs before the delivery arrives.

Clip counts are a fastener question rather than a spacing one. The deck fastener calculator counts board-to-joist crossings, which is the number a clip system charges you for.

What still comes from the code

Deferring the decking to the manufacturer changes nothing else about the frame. The joists still have to span their distance under IRC Table R507.6, the beam under Table R507.5, the posts and footings under DCA 6, and the guard under R312. A board rated to span 24 in does not permit joists at 24 in if those joists cannot carry their own span at that spacing, and on most decks the joist table is the tighter of the two.

Size the frame first with the joist calculator, then bring the spacing it gives you back here to check the boards against it. If the board needs tighter framing than the joist table does, the board wins.

Which document each figure came from

Every spacing on this page was read out of the document named beside it on 2026-09-07. Nothing is averaged across brands, and no figure here comes from a retailer listing, a contractor blog or another calculator.

Brand	Document	Revision
Trex	2026 Trex Decking Installation Guide	2026 edition
TimberTech	TimberTech Advanced PVC Decking Installation Guide	REV 07/26
TimberTech	TimberTech Composite and Specialty Decking Installation Guide	REV 09/23
Fiberon	Fiberon PE Decking and Fastener Instructions	FIB-0354-LIT 10/25
Fiberon	Fiberon PVC Decking and Fastener Installation Instructions	FIB-0354-LIT 11/22
MoistureShield	MoistureShield Installation and Maintenance Guide	MS21-130, March 2021
Deckorators	ICC-ES Evaluation Report ESR-5672, EOVATIONS LLC	Reissued June 2026

Trex’s 2024 guide is superseded and still ranks. Trex now publishes a 2026 guide and the stair figures changed between the two: the Transcend and Select 2x6 stair tread went from 12 in to 16 in. If a page quotes Trex stair spacing without saying which guide it read, it may be quoting the old one.

Deckorators could not be sourced from the manufacturer. deckorators.com refused automated access, so the Deckorators row above comes from ICC-ES evaluation report ESR-5672 rather than the installation guide, and is labelled that way in the revision column. The report states no separate diagonal figure, so this page publishes none: a missing figure is honest, an invented one is not. Read your own guide before building a diagonal deck in Deckorators.

MoistureShield’s guide misprints its metric conversions. The inch values are internally consistent and are what is carried here.

Frequently asked questions

What joist spacing does composite decking need?: Whatever its manufacturer publishes, which is 16 in on centre for most brands run perpendicular, 12 in on a 45 degree diagonal, and as tight as 8 or 9 in for stair stringers. MoistureShield's Stratos and 2x6 boards are rated to 24 in perpendicular. The IRC does not set a figure: R507.7 sends plastic composite to R507.2, and R507.2.2.5 requires installation in accordance with the manufacturer's instructions.
Does the IRC have a table for composite decking joist spacing?: No. IRC Table R507.7 is titled "Maximum Joist Spacing for Wood Decking" and applies to nominal 2 in decking, 5/4 in decking and wood structural panels. R507.7 states that spacing for plastic composite decking shall be in accordance with Section R507.2, which routes to ASTM D7032 and the manufacturer's instructions.
Why does diagonal decking need closer joists?: A board laid at 45 degrees crosses the joists at 45 degrees, so the unsupported distance between bearing points is about 1.41 times the joist spacing. Manufacturers absorb that by publishing a tighter figure: 12 in where the perpendicular figure is 16 in, and Trex expresses it as the chart value minus 4 in.
Can I space joists at 24 in for Trex?: Only Transcend and Select 2x6 boards, and only at the 100 psf rating in Trex's 2026 guide. Trex's 1x6 profiles are rated 16 in at 100 psf. Trex publishes a load chart rather than a single number, so the answer depends on the load you are designing for.
How many composite boards do I need for a 16x12 deck?: With 5 1/2 in boards at a 1/4 in gap, each row covers 5.75 in, so 192 sq ft takes about 401 linear feet of board. At 16 ft lengths with a 10% waste allowance that is 28 boards. Change the gap to 3/16 in and it drops to 27.
What does ASTM D7032 actually do?: It is the standard specification for wood-plastic composite and plastic lumber decking. It sets how a board's span rating, structural performance and durability are established by test, which is why each product line ends up with its own published spacing rather than a shared one.
Why does this page not show composite board prices?: Because no manufacturer publishes one and retail pricing varies too much between regions and retailers to quote a figure honestly. Enter what your supplier quotes and the calculator will use it. Every other number on the page traces to a document you can open.

Check these numbers yourself

IRC R507.7 and R507.2: decking and plastic compositeMassachusetts Residential Code 10th edition, based on the 2021 IRC. Used because codes.iccsafe.org returns HTTP 403 and cannot be fetched.
Trex installation literatureTrex Company. The 2026 guide is current; the 2024 guide still circulating has different stair figures.
TimberTech installation guidesAZEK. Advanced PVC and Composite are separate documents with separate revisions.
Fiberon installation instructionsFiberon. The clearest published matrix: residential and commercial, perpendicular, diagonal and stairs.
MoistureShield installation instructionsMoistureShield. Note the metric conversions in this guide are misprinted; the inch values govern.
ASTM D7032 standard specificationASTM International. The standard the IRC routes composite decking to. Purchase required to read in full.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-07. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Check your own guide. Manufacturers revise these documents and the figures here were read on 7 September 2026 from the revisions named on the page. The installation guide packed with your boards, and the span rating printed on the board itself, govern over anything published here. This page does not size joists, beams, posts or footings.