For the treehouse, the gap between the terrace boards can be a little wider than on the ground, as there will be loads of leaves and dirt from the tree. The wider your floor boards, the wider your gap must be, of course, as the boards swell more – we recommend:
Board width 10 – 14 cm (4 – 5.5 in) = joint 10 – 12 mm (0.4 – 0.47 in)
Board width 14 – 18 cm (5.5 – 7 in) = joint 12 – 14 mm (0.47 – 5.5 in)
If the joints are < 8 mm (0.3 in) together, it is possible that the boards will swell and become completely sealed during long periods of rain, making it difficult for water to drain away. A wider joint of 10 – 14 mm has the advantage that you can clean better, so leaves, needles, moss, and other dirt fall down better through the joints!
In my opinion, a joint that is too wide > 14 mm no longer looks aesthetically pleasing.