We live in mid-Michigan in a 3,000 sq ft ranch. It's unique for the area as it has a wooden foundation (WF).

The front porch is a wooden deck. Approximately 5 X 40, connected to the house on 3 sides. We want to replace the deck with flag stone. Everything I have read indicates that the best foundation for the flag stone is a concrete foundation (CF) and slab.

The new porch will be approximately 40% - 50% larger. For the front edge of the porch (not connected to the house) I plan on a traditional footer / foundation.

My question is supporting the 40' span of the porch without connecting it to the wood foundation? I am assuming that any connection between the CF and WF would be a bad idea. Likewise I don't want the weight of the fill under the slab to bear on the wood foundation while being compacted.

Should I pour a concrete foundation all the way around the porch, staying 12" - 18" inches from the WF? Could I just pour 3 or 4 sections of foundation, at a right angle to the front, stopping before it gets to the house?