The general rule is that workers on public projects of more than $1,000 use be paid at least the prevailing rate of per diem wages. This is not limited to some subset of workers to whom the contractor or subcontractor has agreed to pay prevailing wages, but extends to all workers employed on public works construction projects. Accordingly, the question is two-fold: 1) who qualifies as a worker; and 2) what qualifies as a public works construction project?
The first question is pretty easy to answer. The definition of worker is broad, and includes any individual who performs skilled or unskilled labor in construction projects - whether or not that individual is an employee. This is interpreted broadly and is meant to be inclusive to provide a large range of prevailing wage protection.
The second question requires a bit more examination. Maintenance work can be subject to prevailing wage regulation, subject to some exceptions. Generally, maintenance work subject to these rules is defined by 8 CCR § 16000, in part, as: "Routine, recurring and usual work for the preservation, protection and keeping of any publicly owned or publicly operated facility (plant, building, structure, ground facility, utility system or any real property) for its intended purposes in a safe and continually usable condition for which it has been designed, improved, constructed, altered or repaired."
However, "[j]anitorial or custodial services of a routine, recurring or usual nature is excluded." So, to the extent the only work being done with respect to the maintenance work is actually cleaning bathrooms, it likely falls under the janitorial/custodial services exception to the prevailing wage rules, and is to subject to those regulations.