Work costing £70,000 is needed to stop flooding in Norwich's war memorial gardens - because a seal was not put in place during £2.6m restoration work.
The Grade II* listed gardens, at the top of Norwich Market, were repaired, revamped and refurbished in 2011, after six years of being off limits to the public.
But it has emerged that Norwich City Council needs to spend £70,000 to prevent flooding in the gardens - and into the undercroft below - because of oversights during that revamp.
The lack of a seal or weepholes to allow water to escape from the grassed area in the gardens has caused water damage to the structure of the 1938-built terrace.
And there has been "severe water ingress" into the undercroft, with water running down the back wall and through electrical conduits.
The council had intended for the undercroft to be used as a space for art exhibitions, but there have been occasions when artwork has suffered water damage due to the leaks.
The council says although the space can be used, that "exhibitors must accommodate the potential leaks when there is heavy rainfall into their plans, which is certainly not ideal".
The council has applied to its own planning committee for permission to add a liquid plastic/membrane and flashing system on the back wall of the terrace, to resolve the problem.
They said the £70,000 work would be colour matched as closely as possible to the stonework.
The city council was unable to explain why a seal had not been installed when the memorial gardens were revamped a decade ago.
The council said "historic information" was not available, but that they were not planning to take any action against the original contractor.
The council has spent £200 on an internal gutter to try to direct water to a drain.
A council spokesperson said: "The new memorial undercroft project will seek to maximise the potential of the site as an inviting exhibition space in the heart of Norwich.
“Around £70k of funding has been secured through the Towns Deal accelerated fund and work on the undercroft is due to start late this summer.”
James Wright, leader of the Liberal Democrat group at the city council, said: "It is disappointing to learn the council has to spend some £70,000 from the Towns Fund to make good something which you might have expected to have been done at the time."
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