As mentioned, you really need to understand the root cause, which should be done by a qualified person, such as a licensed engineer. Why? Liability.
Could Bill from work fix it? Maybe. If all goes well, you'll have a few beers and laugh at the quotes and money you saved beating the system. Now, what if it doesn't. Bill is a handy guy, but is not licensed for this work I am guessing. What does that contact look like? Does he promise there will be no issues, how long is the guarantee? Here is the kicker, it doesn't matter. For a contract to be enforceable in court, there are 5 requirements. One of those is capacity. A chemical engineer has no business stamping a bridge, nor does Bill likely have actual qualifications for this work. Anything you sign won't matter.
If it does go south, you could end off worse off than when you started and pay a heavy price for doing this correctly with no legal route to go. It always looks cheaper up front, but shortcuts rarely are. There is a reason this is a first world country with building standards, codes, and license requirements. At the very least, have a qualified person help you understand the problem, they do this for a living.