# Deck board spacing and layout: gaps, direction, and joints Side and end gaps, board direction, joint placement, and last-board planning from Trex and American Wood Council guides. Guide page: (published 2026-10-05, updated 2026-10-05). This Markdown version has the guide's key points, outline, FAQs and sources; the full article, with its worked examples and diagrams, is on the page. ## Key takeaways - Gaps are set by the board maker: Trex requires at least 3/16 in side to side; AWC's wood deck guide says about 1/8 in for lumber decking. - Ends need gaps too: Trex specifies 1/8 in above 40°F and 3/16 in below, and 1/4–1/2 in where boards meet a wall. - Run boards from perpendicular up to 45° to the joists, with every piece bearing on at least 3 joists; angled boards need closer joists. - Plan the layout across the whole width first so the last row is not a thin sliver. ## In this guide 1. [Why gaps matter](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#why-gaps-matter) 2. [Side-to-side gaps](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#side-gaps) 3. [End gaps and gaps at walls](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#end-gaps) 4. [Board direction and joist spacing](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#board-direction) 5. [Butt joints and breaker boards](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#joints) 6. [Planning the last board](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#planning-the-last-board) 7. [How many boards to order](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/#how-many-boards) ## Frequently asked questions ### How far apart should composite deck boards be? Follow your board's installation guide. Trex requires a minimum 3/16 in width-to-width gap in all temperatures, recommends 3/8 in for docks and heavily wooded areas, and says gaps should never exceed 1/2 in. ### Do deck boards need a gap at the ends? Yes. Trex specifies end-to-end gaps of 1/8 in when installing above 40°F and 3/16 in below 40°F, and 1/4 to 1/2 in where decking meets a wall or other solid object. ### Can deck boards run diagonally? Yes, within limits. AWC's DCA 6 allows decking from perpendicular up to 45 degrees to the joists. Trex reduces the joist spacing for angled boards: 4 in less than the chart at 45 degrees, and half the chart spacing at 30 degrees. ### How far can deck boards overhang the rim joist? Trex limits the perpendicular overhang of its decking to 3/4 in. Other products set their own limits, so check the installation guide for yours. ### Where should deck board joints go? Over framing. Trex says boards that meet over a single joist should use its butt joint clip, or a 2x4 nailer added at the joist. Its guide also shows breaker-board layouts, where a board runs across the deck over a single or doubled joist and the field boards end against it. ## Related calculators and guides ### Calculators - [Deck Calculator](https://www.howmanyyardsdoineed.com/deck-calculator/) — Decking board count from deck size, actual board width, gap, and board length, with the cutting assumptions shown. - [Concrete Calculator](https://www.howmanyyardsdoineed.com/concrete-calculator/) — Slab volume in cubic feet, cubic yards, and cubic meters, with ready-mix order size and bag counts for documented products. ### Guides - [Nominal vs. Actual Lumber Sizes](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/) — A chart of real lumber dimensions from the national standard, dry versus green sizes, lengths, and board feet. - [How to Measure an Irregular Area](https://www.howmanyyardsdoineed.com/guides/measuring-irregular-areas/) — Split odd shapes into rectangles, triangles, circles, and trapezoids, then average width and depth. - [How to Plan a Bulk Material Delivery](https://www.howmanyyardsdoineed.com/guides/planning-material-delivery/) — Rounding to supplier increments, questions to ask, and a checklist before bulk material arrives. ## Sources - [Trex Decking Installation Guide (2024)](https://images.trex.com/is/content/trexcompany/2024_Trex_Decking_Installation_Guide(update)pdf) — Trex Company. Used for: Side gap ≥ 3/16 in (3/8 in for docks and heavily wooded areas, never over 1/2 in); end gap 1/8 in above 40°F, 3/16 in below; 1/4–1/2 in at walls; 16 in joist spacing for residential 1 in boards; max 3/4 in overhang. Checked 2026-10-03. - [DCA 6: Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC)](https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf) — American Wood Council. Used for: Wood decking spaced approximately 1/8 in apart; laid perpendicular to 45° to the joists; each piece on at least 3 joists. Checked 2026-10-05. - [Voluntary Product Standard PS 20-25, American Softwood Lumber Standard](https://www.nist.gov/system/files/documents/2024/12/11/PS%2020-25%20Final.pdf) — National Institute of Standards and Technology. Used for: Nominal 6 in width = 5-1/2 in minimum dressed (dry). Checked 2026-10-03. **Estimate only.** Results are planning estimates computed from the numbers you enter. They are not engineering, structural, or code advice. Confirm quantities, product yields, and installation requirements with your supplier, the product label, and your local building authority before you order or build.