Parts of the Grade I listed building, which hosts the majority of the local council's meetings and hundreds of its staff, will be "at risk of collapse" if wear and tear to its roof is not fixed quickly, a Wakefield Council report states.
It has revealed that the whole of County Hall's third floor has now closed as a result of damp and that the roof's disrepair has now become visibly obvious from several parts of the building.
And council employees have complained that the venue has become a "dark and dingy" place to work in an internal survey.
Meetings were moved out of County Hall for several weeks as a precaution at the start of 2018 after water leaks damaged some of the plasterwork.
But problems have continued and the report suggests that the roof now needs to be almost completely replaced.
This is expected to cost the council up to £6.5m.
The report, which will go before the authority's Cabinet next week, said: "Over the last eight years, there have been a number of significant roof and patch repairs as an ongoing planned maintenance programme.
"It is now evident moving forward that the roof is at the end of its serviceable life and requires replacement for the most part."
"Over the last 24 months, there have been multiple significant roof leaks.
"This has been due to chronic roof fabric decay, which has formed over the decades. As a result, internal ceiling damage is prevalent.
"In (the worst) cases, this has caused collapse of masonry, plasterwork, and damage to electrical infrastructure."
The issue may be complicated by the possible need to gain Historic England approval for repairs, as a result of the building's Grade I status.
Repairs would take an estimated two years to complete, with work not expected to begin until the autumn of this year at the earliest.
The report added that "doing nothing" would mean County Hall will deteriorate further.
It said: " In the medium term, this will mean part room closures due to water ingress and or unsafe ceilings collapsing or at risk of collapse.
"Postponing works for future years, will increase overall repair costs and cause additional damage to the fabric of the building, and loss of heritage aspects."
Local Democracy Reporting Service