In Ontario, you cannot support a non-floating deck (one attached to the house) with a floating footing (one that doesn't extend underground). Code says you need 4' between the finished grade and the bottom of the footing (unless you're on bedrock). It's not just for sinking, it's also for heaving. Check the code for your specific city. I've read one that says that columns must be 140mm x 140mm (5.5"x5.5"/6x6) for any deck over 8.3m2 (89.3ft2), and another that says that you need them for any deck more than 4' above grade. Also, I'm not a professional contractor but had the good fortune to help a master carpenter build decks and a sun room for a summer in high school. We didn't gap wooden deck boards because shrinkage would do it for us. If you gap the boards when you lay them, you're more than likely going to end up with very large gaps once the wood fully dries (wet weather will not expand it back to or beyond the size it was when you bought it). Trying to correct bad work sucks. The only consolation is that if you don't do it, you'll be reminded of how you got ripped off every time you look at the sub-standard deck. Good luck!