I'm having an attached garage addition built and the new slab will cover the sill plate that rests on the existing foundation wall. Do I need some sort of flashing to cover the exposed sill and rim joist so the concrete isn't poured directly against it?

 

I would find a way to protect the existing sill and rim joist from direct contact. I had a somewhat similar problem long ago when I had to pour a new structural slab in my garage. I held the 1x treated board about an inch above the surface of the new slab. I moved out of the house about 12 years later, so I don't know how well my solution withstood the test of time. This is what I did

 

As shown in Bruce's drawing you will need something solid around the perimeter to act as a form. I would also flash with galvanized steel flashing.

 

There is a solid wall because of the beam and rim joist that runs the entire length. That being said, could I just do flashing?

 

As long as it's solid, I think just flashing will work. Just have as few joints as possible (lap them) and stop the flashing at least an inch above the top of the slab and run all the way down to below the existing plate. Also, get galvanized flashing as thick as you can find, at least 26 ga. if you can. Lap the joints and and seal them; for the life of me, I can't recall what we used to spec for sealing the joints. Edit: If fact, if you could put some ice and water seal over the existing wood, then the flashing, that would really be better. It's a second line of defense.

 

This sounds like it would do the trick:

http://www.homedepot.com/p/Karnak-10...-CTG/100257533

The builder mentioned something about ice and water shield. I put that over the wood, then flashing (attached with galvanized roofing nails?)

 

The face of the rim joists/beams, etc are flush with the foundation wall. That being said, should I run the ice/water shield down the face of the foundation wall a couple inches? Maybe low enough so it is extends below the surface of the gravel base?

 

I've always tended to over-design, so I would tend to go a good 6" or more onto the old foundation with the ice and water. And still use the galvanized metal over the ice and water. I would tack the galvanized only enough to hold it in place, above the top of the new slab. No nails below the top of the slab; the weight of the backfill and slab will hold everything in place. The roofing cement will probably work as well as anything to seal the joints in the galvanized