This depends on meta. I understand IJW as saying 'in my meta, no'. He's also suggesting that he thinks it's a better way to play. In a meta that plays with perfect intent it's possible to get that interaction. Nothing in the rules requires the blast focus to be placed in the centre of a 3mm X 3mm area on the table (so at best 1.5mm from the edge). So in my meta it's entirely possible to throw smoke and have the blast focus be 'at the very edge of the building', so long as you can see 3mm X 3mm of the roof. There is no gameplay issues with this. So I don't actually know why IJW is making the recommendation he does (that the closest a blast focus can be to the edge of a building is 1.5mm). In metas that don't play with perfect intent it's practically impossible to centre a template on the absolute edge of a building, so this is not possible.
I'm very interested in the answer for the smoke question so I created an independant thread about it https://forum.corvusbelli.com/threads/smoke-going-downward.33971/
As per the other thread - I will never make rules judgements that require 'perfect intent' to work, because, in my opinion, 'perfect intent' is a player construct without a proper foundation in the rules.
