Also, there have been a number of developments in the details of ledger attached decks which you would be well advised to read up on.
The large supporting columns match the size and weight of the
brick building. Anything less would look totally out of place.
For a waterproof deck, of any sort, to actually function properly,
no matter how well the waterproof surface is installed, there has to be
at least a 2% slope.
Water must move off the deck surface naturally.