I have water damaged underlay that runs under an exterior wall. the damaged area is about 2 feet wide. I want to cut this piece out, but am unsure what to use to cut the nails that are through the base plate and how best to support the wall when I do. Any do's and don'ts that come with replacing underlay under an exterior wall would be helpful. Thank you to one and all.
By underlayment I assume you mean either plywood, OSB or chipboard. If it is only two feet, and you have a conventionally built stud wall, you are unlikely to need any temporary support for the 3/4 inch piece you are taking out. As for the nails, you can cut right through them with a carbide toothed blade specifically designed for rough lumber, or you can use a Sawzall (my tool of choice for this project) with a nail cutting blade like the Axe. Once you have all the wet underlayment cut out, you should be able to insert a replacement piece. I would use exterior rated plywood for this application.