Hey Capt Joe, I've grilled and slowcooked on my wooden deck for over 20 yrs and never had a problem
Until I had a problem. While dumping hot coals into my char baskets one of the suckers got caught on the bottom ring and dropped out onto the crack between two deck boards. What you don't see here is that it was also hidden by a Whiskey barrel waiting to be cut up for oak chunks. It was a slow cook and as luck would have it my back was to the slow burn.
Since this happened I've rebuilt the deck with new Western Red Cedar (the one in the pic is over 25 yrs old ... a pretty good run). I still grill/slow-cook on my new wood deck, but I have three of the asbestos like floor covers from Home Despot. It's just the size of a Perf Dlx. When I use one of the daisy wheelers, I USE MY HAPPER COOKER INTERNAL ASH PAN without fail.
I'd much rather cook on the grass but my deck is on the main level of the house well above the ground.
As I see it; the problem areas are transferring hot coals, leaving the ash-sweeps open when pouring hot coals (especially lump, small lump), and brisk wind agitating the fallout when the lid is removed.
At my house (with it's new deck), I do NOT do overnight cooks unless I'm willing to sit and monitor the kettle/WSM.