What is this rotting wood 'ledge' around my house? Is replacing it something I can tackle? The outdoor trim too?
If anyone has a similar odd situation this is what I did:


tore out the skirt board and this ledge
the underlying osb sheathing was rotting in parts, cut that out 
some of the underlying framing was even rotting in parts.  Treated this with hardener and filled with bondo wood filler, sprayed down anit-mold for good measure
replaced old sheating with plywood, sealed with r-guard fast flash
to meet the thickness of 5/4 old skirtboard I put a layer of zip-system board and sealed it with r-guard flast flash
put pvc trim board longer than the previous skirt board to make up for this 'ledge'
sealed with osi guard caulking


issues that Im not happy with:


the height of the gap changed around the house so it was hard to cut the different boards to the right height and this created gaps that were larger than I wanted in areas required to be filled with caulking more than it should be
the z channel drip edge was bent a lot during the removal and replacement, probably should have tried to replace it, but I just tried to bang it back into place