Subsiding Porch

[gumdrop]

The sliding door in my front door porch would no longer latch, there was a big gap at the top (3/4")
The front corner had sunk, removing the cladding from inside revealed that there were no footings it was resting on two rotten pieces of wood!
I have jacked and wedged the porch level the door closes but how do I support it? Does it need DPC between the woodwork and anything I pack it up with e.g. steel box section and fill with concrete or mortar mix
Then to raise the floor level would mortar and brick piers with 19mm MDF floor be ok?

 

[noseall]

Does it need DPC between the woodwork and anything I pack it up with e.g. steel box section and fill with concrete or mortar mix
Then to raise the floor level would mortar and brick piers with 19mm MDF floor be ok?

However which way you repair, use plywood instead of mdf.