If it is not heresy to talk about composite decking here, I have a question. We are building a multi-level deck, where the largest section is 7' x 8' , the installation guide calls for quite large spacings against solid objects , at the picture frame miters and on the fascia corner bevels. We are just starting now and doing mostly layout. I am sure I will get used to it, but right now those gaps just make it look like shoddy construction. My question is, on a smaller sized deck can I cheat a bit and reduce the joint spacings?
Unlike wood, composite decking expands significantly along the length with temperature change, so it needs room to expand and contract. When I resurfaced both of our decks, I used a fastener system that allowed the pieces to slide lengthwise. My layouts weren't constrained on both ends by solid objects. I have had one problem where a decking board was mostly wrapped around the railing posts from one side of the deck to the other. In the winter when the material shrinks, the piece cracked right next to one post from the stress.
When I built my decks there was no information on the product regarding expansion. I don't know if the current manufacturers provide any guidance regarding the expansion properties of their materials.
I've only built a couple of decks with composite materials so I lack experience (just followed all the instructions using the recommended spacers across the width. no fancy framing), but I've built many decks with wood. One thing I discovered early - in general I like wider gaps than narrower. With narrow gaps "junk" (leaves, twigs, seeds, pet hair, dirt) more easily gets trapped in the cracks. Fortunately when using wet PT pine as each board dries it shrinks significantly across the width and widens the gap. I guess an extra wide gap would be a problem for those who wear high heeled shoes or tended to drop coins and diamond rings.
But your idea that relatively less end gap is needed for a smaller deck makes sense to me and sure would look better on your miters. (Too bad they don't make flexible gap fillers for such situations.) If there is nothing in the instructions concerning the ratio of end gap to board length that you could use to calculate, perhaps the manufacturer could answer that question.
I wouldn't plan on butted miters. Space them as you would the widths of the planks, but radius the top edge prior to fastening. For that matter......radius the exposed cut edges along the edges of the deck too. That stuff can be sharp !
Our deck was built about 8 years ago, about 6'x8' with mitered corners. The "boards" are spaced with the fasteners that go between the edges, and the miters were made tight fitting. The material is Trex I believe. I have seen no evidence of expansion or contraction in the miters. They are as tight as the day they were made.