Composite Deck Spacing: How to Get It Right

Trex waterproofing products protect wood and composite decks alike, extending their service life. But if your wood decking is tattered, I suggest you look at composite decking to replace it. Manufactured in a controlled environment from wood fibers and polymers, composite deck boards eliminate natural variation. That makes them straighter, more stable, and longer lasting than wood decking.

However, like wood, composite and PVC deck boards need spacing between them. These spaces allow the decking material to expand and contract. The gaps also drain away water and provide other benefits that extend deck service life. Thankfully, getting the deck spacing right is easier with composite decking materials because they all run straight. I don’t think any deck builder misses having to wrench bowed or twisted wood boards into place.

In this article, I discuss why deck board spacing is critical, how much space to leave, and what tools and techniques you need to get the spacing right. Whether you’re upgrading a wood deck surface or building a DIY deck from scratch, these pointers will help you nail the spacing.

Why Is Decking Board Spacing Necessary?

By leaving space between deck boards, you’re allowing nature to take its course, but with a minimal effect on the deck boards. They remain free to expand and contract without pushing against one another. Water and debris pass through instead of accumulating, and air circulates better, discouraging moisture buildup.

1. Material Expansion and Contraction

Composite decking boards are less prone to expansion and contraction than wood but still undergo slight dimensional changes with fluctuations in temperature and/or moisture levels. The deck’s wood substructure, typically pressure-treated fir, can also “move” as the temperature and/or humidity change. By leaving space between the deck boards, you allow them to accommodate these movements, preventing warping, buckling, and fastener failure.

2. Water Drainage 

Allowing water to drain between the boards prevents moisture buildup that could lead to mold, mildew, and rot growth. Proper drainage is also essential to the longevity of the joists. You can further protect the deck joists by covering them with flashing tape. This waterproof membrane adds little to the decking installation cost and can extend the joist and fastener service life significantly. Use this materials estimator to find out how much flashing tape your project needs.

3. Air Circulation

Adequate spacing also promotes airflow around the boards, which helps prevent moisture buildup, fungal growth, and decay. Ground-level decks functioning as patios are especially prone to trapping moisture. In such cases, some builders make the spacing wider than normal to provide more air circulation and faster drying.

4. Cleanliness / Debris Clearance

Spacing between the boards also prevents debris such as leaves, twigs, seeds, and other organic matter from accumulating on the deck surface. Instead, they fall through the spaces. Proper spacing between the deck boards not only maintains the aesthetic appeal of the deck but also simplifies maintenance because there is less sweeping and cleaning to do.

5. Visual Appeal

Uniform spacing between boards gives your deck project more visual appeal. This enhances the deck’s overall appearance and contributes to the beauty of your outdoor living space.

Follow the Manufacturer’s and the Building Code Specifications

Follow the manufacturer’s installation instructions for spacing and fastening as provided in the installation guide. Homeowners who don’t risk voiding the product’s warranty and possibly undermining the durability of your deck. Additionally, check the local building codes. Some specify the size of deck board spacing to use. A building inspector might require you to justify your joist spacing—12 inches on center or 16 inches?— joist span calculations and footing placements.

Different Ways to Space Composite Deck Boards

Most manufacturers of composite decking boards, including Trex and Timbertech, tell you exactly how to install composite decking. Most manufacturers call for a 1/8- to 1/4-inch gap between their boards, while others say 3/16-inch spacing is the happy medium. Whatever the gap width, several tools and techniques help you get the spacing right.

Using a Deck Board Spacer 

Many items can serve as spacers, including nails, wood shims, and almost any small object of uniform size. But the easiest spacers to use are those made for the job. Often plastic, they have a finger grip for easy placement and retrieval. They also have a flange that ensures consistent vertical placement and a wedge shape that makes them easy to extract after the boards are fastened. Typically, you place two or three spacers between the boards every 24 inches. Where the boards meet end-to-end in what’s known as a butt joint, check what spacing, if any, the manufacturer recommends. Be sure you have enough blocking in the joist bays to support each end of the butt joint.

Or avoid butt joints altogether in favor of “breaker boards,” which can be used to create “picture-frames” and other decking patterns. In these deck designs, the ends of the composite boards don’t meet. Instead, the ends terminate at a board running perpendicular to them. This method reduces or eliminates troublesome butt joints. Plus, dividing the deck into sections diminishes the effects of expansion and contraction on adjacent boards.

Using a Deck Board Spanner

A deck board spanner, or jig, is a handy tool that spans the board width and introduces a pin or some other spacer on each side of the board to create the proper spacing. Most jigs also have guide holes that make it easy to sink fasteners into the board at the correct location, either through the face or at a 45-degree angle through the board’s edge. Spacing jigs, such as this one, can also be used to install hidden fasteners on Trex composite decking.

Using a String Line

Before attaching any deck boards, double-check the deck frame to make sure it’s flat and smooth. Do this by pulling a string line across the deck. Ideally, the joists are in the same plane. For joists that are too low, use shims. For joists that are too high, use a power planer. You can also use this method before installing fascia boards and deck railings.

Creating Chalk Lines

Nothing beats a chalk line for creating a long, straight mark, and it’s very easy to use. With an assistant or nail holding one end of the string, pull it tight between two points and pluck the string. That releases the chalk along the string, giving you a perfectly straight line from which to reference decking board placement and spacing. Remember to factor in any overhang you plan at the deck’s edge.

Tips for Maintaining Your Composite Decking Spacing

If you spaced the boards correctly and fastened them tight to the joists, you should have no trouble preserving your dream deck’s board spacing. Just keep it clean using a brush and water to remove accumulated debris between the deck boards. Otherwise, simply wash the deck surface with a mild detergent, rinse, and enjoy.