If there is one absolute “Golden Rule” when it comes to fitting composite boards, it is this: You must leave expansion gaps. We see it every single summer. A confident DIYer decides to install their new decking exactly like they would traditional timber—pushing the boards tightly together for a seamless look. As soon as the July heatwave hits, the boards violently warp, buckle, and push each other right off the joists.

In this guide, we explain the science behind why composite moves, the hidden benefits of gapping, and the exact measurements you need to keep your deck strictly within its warranty.