Composite Decking Spacing Calculator

How to use the composite decking spacing calculator

1. Pick your deck boards from the list: 27 board lines from Trex, TimberTech (composite and Advanced PVC) and Fiberon (composite and PVC). The board line matters: two boards from the same maker can have different joist and stair spacings.
2. How the boards cross the joists: square (90°), 60°, a 45° diagonal or 30°. A board laid at an angle spans farther from joist to joist, so the joists come closer.
3. The temperature when you lay them, because composite boards grow and shrink with heat and the end gaps allow for it. Then the deck’s width along the house, to count the joists, and whether it is a home or a commercial deck.
4. You get the joist spacing and how many joists, the stair stringer spacing, and the three gaps (side to side, end to end, at the house), each with the maker’s own words. Copy link keeps your board, and the green button hands the spacing to the deck span calculator to check the joists themselves.

Joist spacing for composite decking, maker by maker

A deck board has to span from one joist to the next without sagging, so its maker sets the widest joist spacing it allows. These are the numbers in each maker’s current installation guide, read on October 16, 2026:

TimberTech says it in one sentence: “Joist spacing should never exceed 16″ on center. For a more rigid feel, 12″ may be preferred. However, all angled or commercial installations of decking require 12″ oc max.” Fiberon writes the same for its PVC boards: “Promenade and Paramount decking are approved for use over joists spaced a maximum of 16 in. (406.4 mm) on-center. However, for a more rigid feel, Fiberon recommends spacing the joists at 12 in. (304.8 mm) on-center.”

Can composite decking span 24 inches?

Some boards can. Trex’s span chart rates Transcend 1×6, Transcend Lineage 1×6 and Select 1×6 for joists 24 inches apart, and both 2×6 boards too, each with the load it carries at that spacing (75 to 100 psf). The building codes ask for 40: “The IBC International Building Code and IRC International Residential Code require a 40 psf rating for most walking surface installations.” Trex still recommends the closer spacing: “For a stiffer installation, a 16″ on-center span is recommended for the decking surface.” So the calculator answers 16 inches for every Trex board and shows the 24-inch rating beside it when your board has one.

TimberTech allows 24 inches only for its thicker MAX boards (“Joist spacing should never exceed 16″ on center (up to 24″ on center for TimberTech MAX boards).”). Fiberon allows 16 inches at most for all its boards.

Boards on a diagonal: why the joists come closer

A board laid at 45° crosses each joist on a slant, so the distance it spans between two joists is about 1.4 times the joist spacing. Trex gives a rule for each angle: “At a 60° angle, maximum joist spanning is 2″ (51 mm) less than listed in the chart below.” “At a 45° angle, maximum joist spanning is 4″ (102 mm) less than listed in the chart below.” “At a 30° angle, maximum joist spanning is 1/2 of the distance listed in the chart below.” From 16 inches, that is 14, 12 and 8 inches.

TimberTech and Fiberon give one diagonal number (12 inches; Fiberon 8 inches on a commercial deck) and stop there. TimberTech adds: “More severe angles may require closer joist spacing.” For their boards at 30°, the calculator does not make up a number: it tells you to ask the maker.

The gaps between composite deck boards

Composite and PVC boards move with heat, mostly along their length. The gaps let them move and let water and leaves through. Every maker prints three:

Trex: “The minimum required width-to-width gapping is 3/16″ (5 mm). This is allowed for both hot and cold weather installations.” Fiberon ties the gaps to its warranty: “Spacing requirements must be met for full Fiberon warranty coverage.” TimberTech’s PVC boards are the exception at the ends: “Install PVC deck boards ends tight together. DO NOT leave a gap at butt joints, splices or miters”.

A cold day means a wider end gap: boards laid when they are shrunk will grow into it in summer. On a day that falls on the edge of two rows, the calculator takes the colder row, with the wider gap.

How many joists you need

Divide the deck’s width along the house, in inches, by the joist spacing, round up, and add one for the first joist: a 16 ft deck at 16 inches is 192 ÷ 16 = 12, plus 1 = 13 joists. At 12 inches it is 17. Trex adds a floor for short decks: “Trex decking must span at least three joists.”

Worked examples

• Trex Enhance, square to the joists, 70°F, 16 ft deck: joists 16″ on center, 13 of them; gaps 3/16″ side to side, 1/8″ end to end, 1/4″ at the house; stair stringers 9″ apart.
• The same deck on a 45° diagonal: joists 12″ on center (16″ less 4″), 17 of them.
• Fiberon Concordia Novus at 95°F: joists 16″, ends 1/16″ apart, 3/16″ or more side to side, 1/4″ or more at the house.
• TimberTech PRO Terrain at 30°F on a commercial deck: joists 12″, ends 3/16″ apart, stair stringers 8″.

Common mistakes

• Using wood decking’s spacing. Thick wood boards are often laid on 24-inch joists; most composite boards need 16.
• Keeping 16 inches on a diagonal. Every maker asks for 12 inches or less at 45°: a herringbone or picture-frame deck usually needs extra joists or blocking.
• Reading the stair spacing from the deck. Stair treads are short and walked on hard; Trex Enhance treads need stringers 9 inches apart, closer than the joists under the deck.
• One end gap for the whole job. If the day warms up by 30 degrees, Fiberon’s table can move a row. Check the temperature when you start each run.
• Forgetting the joists themselves. The board’s spacing says nothing about how far each joist can reach. Check that with the deck span calculator.

Composite decking spacing: questions people ask

What is the joist spacing for composite decking?

16 inches on center for Trex, TimberTech and Fiberon boards laid square to the joists, and 12 inches on a 45° diagonal. TimberTech and Fiberon ask for 12 inches on a commercial deck; TimberTech’s thicker Advanced PVC MAX boards may go 24 inches.

How much gap between composite deck boards?

Side to side, 3/16 inch for Trex and Fiberon (Fiberon: at least that), 1/8 to 3/16 inch for TimberTech composite and 1/8 to 1/4 inch for TimberTech PVC. End to end, it depends on the temperature on the day you lay them; the calculator reads the maker’s own table.

How much gap at the ends of Trex boards?

3/16 inch at 40°F and below, 1/8 inch above. Where the boards meet a wall, Trex asks for 1/2 inch at 40°F and below and 1/4 inch above.

Can Trex span 24 inches?

Trex’s span chart rates Transcend 1×6, Lineage 1×6 and Select 1×6, and the Transcend and Select 2×6 boards, for 24 inches on center. Signature, Enhance and Lineage 1×4 stay at 16 inches. Trex recommends 16 inches for a stiffer deck.

What joist spacing for diagonal composite decking?

12 inches on center at 45° for all three makers (Fiberon: 8 inches on a commercial deck). Trex also gives 14 inches at 60° and 8 inches at 30°; TimberTech and Fiberon give no number past 45°.

How far apart should stair stringers be for composite treads?

It depends on the board: 9 inches for Trex Enhance, 12 inches for the other Trex 1×6 boards and Fiberon composite, 16 inches for Trex’s 2×6 boards, 8 to 10 inches for TimberTech composite, 10 inches for TimberTech PVC (14 for MAX), and 24 inches for Fiberon PVC over a 2×4 on edge.

Read next

• Low deck ideas at ground level
• Patio deck designs
• Deck railing designs
• Pool deck ideas
• Deck lighting ideas

Choosing the boards’ color? The free composite decking color picker shows them against your house. With the joists spaced, the deck span calculator checks how far they and the beam can reach, the deck footing calculator sizes the footings, the stair rise and run calculator lays out the steps, and the 2D planner places the furniture on it.

Sources, read October 16, 2026: the 2026 Trex Decking Installation Guide, the TimberTech Composite and Specialty Decking Installation & Maintenance Guide, the TimberTech Advanced PVC Decking Installation & Maintenance Guide, and Fiberon’s PE and PVC decking installation instructions. Makers revise their guides: the one with your boards has the last word, and this page is re-checked every six months. Your local building code may ask for more.