Will soon be laying larch decking boards, but I'm having trouble committing to an appropriately sized gap. They're kiln dried so I don't imagine they'll move as much as PT, and I've heard larch is fairly stable... Installing now when it's warm and dry, what's the minimum gap you can get away with to account for contraction and expansion? 28 x 145mm, 13-19% moisture. Cheers!
Water will hang up in a gap of less than 6mm- surface tension and all that. If you leave a 1/4" gap between the boards it will shed water and be much less prone to rot, though if you are going to the expense of using larch, one presumes you will also shell out for a few cans of preserver/sealer to protect the investment
I would also go with 6mm for drainage and allow small debris to pass through and allow a space to clean In between also. Suggest a preservative all round x 2 coats and edge finish coating before any fixing in place