[SkinnyG]

Mar-K says the 1/16" gap between the wood groove and the skid strip is critical.

Trex instructions still require gaps, at ends and at widths because they still expand.

I cut the boards at 77" length instead of wood's 77-1/8", because the composite will expand in every direction, whereas wood will change less in length and more in width. Some say the composite expands more than wood. Some folks here just cut them up like wood, installed them like wood, and they work fine.

The 1/2" between the boards is really only 1/16" of movement until the grooves hit the skid strips, so you're not talking a lot of width change really. I imagine you could close the gap, but it wouldn't make a difference (other then you'd need another skid strip and another board to fill the collectively reduced gap).