Dwarf wall-suspended timber floor

[oldbutnotdead]

Had a good search, not found an answer.
Library floor- void filled with demolition rubble so clearing it out and digging down into the (very firm) clay to give room for dpm, 50mm concrete oversite, 150mm airgap, 150mm joists.
Planning to sit the joists on dwarf walls (max span is 4100 so 3 walls will be nice and stiff)- do i need to dig extra footings for the dwarf walls or can i just mortar them to the oversite?
Ta

 

[Mr Chibs]

I dug a small footing for mine, but I wasn't on clay, just soft virgin earth. Was only around a spade deep from memory.

 

[noseall]

Had a good search, not found an answer.
Library floor- void filled with demolition rubble so clearing it out and digging down into the (very firm) clay to give room for dpm, 50mm concrete oversite, 150mm airgap, 150mm joists.
Planning to sit the joists on dwarf walls (max span is 4100 so 3 walls will be nice and stiff)- do i need to dig extra footings for the dwarf walls or can i just mortar them to the oversite?
Ta

Foundations.