How much steel do I need to contain a magnetic field?

[John Ramazzini]

I am helping in a project in Guatemala, C.A., and we are installing an MRI and the 5 Gauss exclusion zone is outside of the room by 3 feet. I need to contain the Magnetic field inside of the room. How much and what kind of steel is use for this. Sorry if this is not the right place to ask.

Thank you in advance.
Johnny