I'm in the UK and in January had a deck fitted to replace a rotten one that I inherited from the previous owner. The deck installer insisted on fitting it in January even though it was very wet (the weather and the boards). He used a 6 mm spacer (I think) and said this was correct. However, now that it has dried out there are big gaps between the boards. 10 mm to 12 mm in some places, 8 mm in shaded areas of the deck (I've measured).The grooves are at least slightly helping to disguise the gaps but I can easily drop a pen between the boards and I can see the damp proof membrane and ground underneath (it's 1 storey high). Admittedly, I am very fussy, but from what I've read online, 10 mm gaps are on the excessive side. Thoughts? Am I picky or has he miscalculated? Is it likely they'll shrink more? At least it looks clean and tidy I guess.