A report to the Assembly Government by Welsh Health Estates shows that almost half the £468 million backlog is needed for essential work.
The report said a "radical modernisation programme" is needed in order to make significant improvements in infrastructure.
At first glance, it seems the bill is being adequately managed, with the total falling £12 million last year. But this hides the fact that some sites have further deteriorated.
A total of 52 sites cut their bills by about £31 million, with 90 sites seeing increases in backlogs of more than £19 million.
More than half of the NHS estate is over 30 years old and about a quarter of it is more than 50 years old.
Only one out of 14 trusts met national performance targets for 2008, with three others performing strongly.
The Welsh Health Estates said that despite a few examples of excellent performance: "Overall, the report paints a less favourable picture in which a significant number of trusts failed to meet the 2008 targets, some by a wide margin."
Complying with statutory and safety requirements alone, including fire safety, is estimated to cost £174 million.
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