A 13’ l x 12’ w x 2’ h deck is being installed where 6 of the deck posts will be installed using concrete footings and piers, while the remaining four with be anchored on an existing 4" thick concrete patio slab. 10’ of the 13’ will be over soil, while the remain 3’ will be on the slab. Is this acceptable or must the slab be cut out so footings/piers can be installed? Also, can Sika deck post fill or some sort of expanding backfill foam be used for any of these posts? If so, must they have a concrete footing as well?