The sill beam (bottom plate) sitting on top of my foundation is rotted out for about a four foot reach and needs to be replaced. Although I have not fully opened up the wall yet, I believe it is an 8x8 timber. I am already guessing that I will need to replace the bottom foot or so of the studs that sit on this bad section.
I'm looking for guidance on what to replace the beam with. I believe the bottom plates on new construction are merely 2x4's laid flat. Thus, it would seem that a readily available 6x6 should be plenty large. Would this be adequate? Should I use treated lumber? Home Depot also sells 7"x9" old railroad ties. Would that be a good product (presuming I can find one in relatively good condition that is not soft and not too badly checked.)
Any advice would be appreciated.
Look into your local building codes. The most important thing will be to stop any water infiltration (I'm assuming that's what caused this), and the second most important will be to place a material like Sill Seal (a foam strip similar to packaging material you see on electronics and stuff, meant to form a barrier between wood and concrete to stop capillary action) under whatever you use, but your local building inspector may have other requirements with respect to the rot/insect resistance of whatever you use. They'll probably assume that means pressure treated, but there are some naturally rot-resistant species of wood you can use, depending on what's available to you locally.