" The material is not the root cause, rather inadequate roof maintenance, which permits water infiltration, and decisions by building owners as to repair or the replacement of existing roofs, which is a part of cost-benefit analysis."

quote from:

https://en.wikipedia.org/wiki/Autoclaved_aerated_concrete#Reinforced_autoclaved_aerated_concrete

This is the best summary I could come up with:

The same lightweight concrete roofing material blamed for safety risks at the Ontario Science Centre is used in hundreds of other aging public buildings across the province, yet officials haven’t pointed to any others that have been ordered closed.

The revelation comes as critics question whether the findings of an engineering report on the science centre’s roof justify the Ford government’s move to close the 55-year-old Toronto facility to the public with less than two hours notice.

An engineering report delivered last week to the province found a small percentage of the centre’s RAAC roof panels have degraded to the extent that they’re at high risk of collapse come winter.

CBC News asked provincial ministries that oversee the largest portfolios of real estate for the number of buildings with RAAC roofing, their state of repair and whether any have been closed because of safety risks.

Trevor Hrynyk, an assistant civil and environmental engineering professor at the University of Waterloo who specializes in reinforced concrete structures, says RAAC is about 80 per cent air, making it vulnerable to water penetration.

The roof isn’t the only part of the Ontario Science Centre that needs fixing, according to a business case published last year by the province that cited hundreds of millions of dollars in necessary maintenance work.

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