Generally speaking I find Rhododendron root systems are relatively shallow and fibrous. with older plants you should avoid damage to any woody roots you may encounter. the less damage overall, the better.
Jimmyq is right, they have very shallow root systems. I also suspect that even though this rhody is large there may not be much in the way of roots under the concrete. Surficial roots are generally reliant upon good air exchange. It also depends how close the steps are to the rhody. Often it is safe to say that the roots do not extend much beyond the dripline of the plant where this does not apply is when the plant has been pruned heavily. One thing I have had the opportunity to see is that even when a large rhody is moved as long as 50-60% of the root mass is preserved the plant will survive well; maybe losing some of the branchlets and vigor the first season.
I agree with all here as well -- roots are not deeper then about 6 inches in a thick mat that can spread out to drip line, although often less -- and can survive on less as Tennyo says. This may be a myth, but root trimming was once considered a healthy practice as it promoted root growths. It was once believed that Rhodies should not be used as foundation plants due to lime leaching from motor -- this may be another myth as I've seen plenty around foundations showing negitable ill effects but there may be some concern with new cement -- sure someone here knows somthing about this.
