There are several factors to consider when picking a foundation type for your deck. Some of those factors include; soil type, climate, load and function of deck, and available products.
In areas where soil density, stability, and load capacity are poor, It will be important to get down beyond that bad soil and into a proper base of either better soil or bed rock bellow.
For this the best option will be a drilled helical pile foundation as it can drill down as far as you need and then set itself in a good base.
If you are unable to reach stable soil, then you can poor an enlarged concrete footing at the base of your pillar to allow the load of the deck to be distributed over a larger surface area.
In areas where we get seasonal freezing and thawing, it is crucial to get the base of your footing bellow the registered frost line to insure that the structure does not heave up or sink down when the soil expands and contracts due to the freeze-thaw cycle.
If you can not achieve these types of footing for reasons related to budget, sub-terrain utilities, or access to required equipment or materials, then there is a final option. You may also use a deck block foundation as long as the deck block sits on a generously excavated base, lined with geotextile fabric and back filled with non compactable drainage stone. The block itself must also be partially buried in the stone to give side to side support. This base will allows the heaving water content to travel in the empty spaces between the stone instead of heaving up the footing or the soil on which it lays.
It is important to always consider the use, and load of your deck when picking the style, type, and spacing of your foundation.
Writen by: Andrew Robic (Espace Unik)