Had a few deck boards that were completely rotten through. As in step in the wrong spot and the foot is going through the deck. And a few others that were just half rotten.
Previous owner, I swear bought the cheapest deck paint. Or not even deck paint and just bought outdoor paint and gave the deck a coat before selling it. Within a year it was completely worn off. Luckily due to hail damage insurance is paying for a sanding and painting of the deck (and other things too, but that’s irrelivent)
I still have one more I need to do tomorrow.
Recommend minimum 6mm gap between the boards, and potentially some spax airs to separate the boards form the joists, for improved airflow; https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcQIuNmmqhp0usWaHQbIwlma_szxZWdhIMX2j0YnyqAN2St6uiyvRhN-AOE&s=10
6mm is way too much for treated lumber. The wood is wet when new and it shrinks as it dries. If you start with 6mm gaps you end up with 10mm gaps in few months which both look ugly and allow stuff to fall under the deck. The width of a carpenter’s knife blade is plenty which is roughly 3mm. Then you end up with 6mm gaps once it has dried out.
Mine was super wet. Beads of water bubbled out when I was sinking in the screws. They keep the boards outdoors and uncovered.
Some of us dry our PT before building.