Wooden or Composite Decking boards, all move, but in different directions and for different reasons. Composites decking expands more in length, while wooden decking expands across its width. Composite decking moves mostly because of heat, while wood decking moves as its moisture content changes. Both types of movement are functions of nature and aren’t easily overcome.
Composite decking, cladding or fencing boards change size with temperature, shrink as the temperature drops and expand as it increases. The change happens at a uniform rate that doesn’t depend on direction, except that it’s noticed more over the length simply because the boards are longer than they are wide. In cold weather, miter joints in composite tend to gap. In hot weather, the pressure from expanding boards may tighten the miter joints, or wedge them open.
Wood acts differently. As its moisture content changes, wood expands or shrinks across the width of the board. This causes the miter joint to open at the toe (the pointy end of the cut) or the heel (the not-so-pointy end of the cut). Which part of the miter joint will open is related directly to the moisture content of the boards when they are installed. A wet board will dry out, causing the heel to open up. A dry board will expand as it takes on moisture, causing the toe to open up.