utherjorge
Observer
So, I have to contact the company. I won't waste anyone's time by posting the different literature from the company, but I do recall reading their stuff, where it says "virtually no thermal expansion or contraction." But, as I'm not a dummy, I don't think, "Virtually" is a useless word. I had planned on some space overall...but the installation instructions say 1/4 inch side gaps. I need to find out more as their tech specs page would not load. The company's name is Deckorator. I can do a quarter inch...but I'll have to do a bit more work underneath to hide the orange than I wanted to.Composite board will expand when it gets hot--in every direction, not just the width like wood. It is strong enough to buckle itself at butt joints, so you may want to get an idea of what it can do before attaching everything. Looks like your pieces might be small enough to fit in an oven, you might give it a measure then toss it in at 150º and see how much it grows...
And of course, wood would have been no better...I think.