Our Builders of Decks and Fences in the Toronto area will likely be outside until some time in January.
Terry Grosjean (photo above), our builder in the Oshawa Whitby area is putting in footings for a 350 square foot deck today. He is drilling the holes, setting the sonotubes while it is near zero this afternoon, then insulating the sonotubes and putting down straw as insulation. So long as the concrete doesn't freeze in the first day or two, the footings will be good to go next week.
Most people don't realize that 3-4' down the ground is warmer. The liquid concrete draws heat from there as it sets, and also generates a little heat of it's own during the curing process--a chemical reaction.
They are busy putting footings in so that they can keep going for a while yet. So long as the ground is fairly warm we can plan footings for decks and fences.