I can tell you that (at least in my local) the calculation for determining how many custodians to assign to a school is base on 'square footage' and student population. However 'square footage' is strictly instructional use rooms, and ignores things like hallways, administrative space, and outside yard. While maintaining the property is our job, the union has chosen to withdraw services from areas that have never been factored into our staffing before.

You will also find that custodians are used to fill any role or complete any task in the building which no one else wants to do or which is not part of a job description. All of these little extras add up to quite a bit.