Every great outdoor living space starts with a good plan and a solid foundation—which is what our decking series is all about. In this series, we sit down with our good friend Kevin Kunka of Trex Company to cover some of the ins and outs of executing a successful decking project. Trex is the world’s largest manufacturer of wood-alternative decking products and a veteran in the outdoor living industry.

A deck project is dependent upon following installation best practices, which is why today, we’re talking all about spanning and gapping for manufactured decking. Check out the video above or read a recap of what we learned below.

Spanning guidelines

What does span refer to?

Joists are the vertical boards in a deck’s framing system that create the platform for your decking boards, and span is the distance between each of them. The maximum span is determined by the decking manufacturer according to how much weight the installed product can support. Understanding span ratings and installing your joists accordingly are critical elements in creating a solid deck foundation.

How far can manufactured decking typically span?

For most manufactured decking, if you’re running your decking boards perpendicular to the joists, the maximum span is 16 inches on-center. If you have a special case—maybe you're creating a chevron pattern and your boards will run at a 45-degree angle to the joists—you’ll need to adjust the span and/or board size according to the manufacturer’s guidelines.

Are there thicker decking products that can span wider distances?

Yes. If you need to span wider than 16 inches on-center, most manufactured deck boards come in a two-by-six size, which is thicker and can support across a greater span (usually up to 24"). These thicker boards are usually only needed in commercial applications, but there are some special cases where a wider span requires a thicker board in a residential application.

Gapping guidelines

What is gapping?

Whether from side to side or end to end, gapping is the space between each deck board. The width of the gap determines how much air can circulate through the deck and how much debris it will catch. 

What is the minimum gap?

The minimum gap for most decking boards is 3/16”. Depending on the environment your deck will be in, you might choose to expand your gap to provide a lower-maintenance deck (for example, if your deck is surrounded by trees that may frequently drop debris).

What is the minimum gap for butt joints?

A butt joint is where two boards meet end to end. If your deck design includes butt joints, you’ll want to gap 1/8" between the two joining boards to account for expansion and contraction of the materials. If you install your capped-composite deck boards in cold weather, you will want to increase that to 3/16". (Refer to manufacturer’s installation instructions for specifics.)

Is gapping different for PVC products?

PVC decking has unique requirements due to the different ways in which the material behaves when exposed to the elements. Instead of expanding and contracting along the length of the board (like capped composite boards), PVC decking expands and contracts between each screw, so you'll need to fasten it according to the manufacturer's specific instructions.

With PVC decking (and with any manufactured decking, for that matter) it’s very important to read the installation guide and take temperature into consideration. Swings in the temperature at the time of installation can lead to unsightly gaps or boards that expand with no place to go.

Building a beautiful deck is only half of a decking job well done—creating a strong supporting structure is the other half. We hope today’s chat with Kevin helps you understand a bit more about how to properly space your deck joists and gap your manufactured decking boards for the best results. For more decking tips, check out this tutorial on how to build rock-solid bottom deck stair posts or Kevin’s best practices for pre-planning your manufactured deck.