THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN
12 
ONTARIO ROOFING NEWS – ISSUE 2 2026
Collapsed U.K. roof deck.2
to “implement an investigation, 
assessment and management 
strategy for RAAC within their 
buildings”1 in response to the 2018 
partial collapse of a U.K. school roof 
constructed with RAAC panels, 
prompting the closure of 104 similar 
schools. Thankfully, the school was 
empty during the collapse.
More recently, the Ministry of 
Infrastructure announced that it 
would be closing the Ontario Sci-
ence Centre as of June 2024, with 
one of the major justifications for 
the closure being the condition of 
the RAAC roof panels.3 Some have 
criticized the decision to close the 
Ontario landmark, pointing out 
that the engineering report pre-
pared by Rimkus identifies only one 
RAAC roof panel as being in critical 
condition and an additional six per 
cent as high risk.4 Regardless of your 
opinion on the closure, RAAC is an 
important topic that merits further 
discussion and awareness.
To assist building owners and 
restoration 
professionals 
work-
ing with RAAC, this article will 
provide a brief history of RAAC 
construction in Ontario, including 
commonly used products, describe 
the primary durability concerns as-
sociated with RAAC, explain how 
to identify RAAC structures in ex-
isting buildings and provide some 
recommendations on the mainte-
nance, repair and replacement of 
RAAC structures.
What Is RAAC?
The composition of RAAC 
varies slightly by product, but it is 
typically comprised of a mixture 
of sand, gypsum, lime, Portland 
cement, water and aluminum 
powder.5 It is fabricated as panels 
in factories and cut to the desired 
structural shape, typically as ma-
sonry blocks, roof planks, wall 
panels or floor units. Once the 
From Roof Collapse 
to Remediation:
Understanding RAAC in Ontario’s buildings
By Brandon Gemme, Leading Edge 
Building Engineers
Roof Remediation 
The following article first ap-
peared in the fall 2024 issue of 
Pushing the Envelope magazine, 
published by the Building Science 
Association of Ontario, as well as 
the spring issue of the IIBEC SOC 
Newsletter. It’s reprinted here 
with permission.
Top-Line Roofing & Sheet-Metal Inc. specializes 
in the installation, repair, and maintenance of 
industrial roofs throughout Southern Ontario. 
OUR SERVICES:
Commercial roofing
Roof maintenance
Roof inspection, 
evaluation & report
Historical restoration
Emergency roof repairs
Re-roofing
Skylight installation
Sheet metal flashing, 
fabrication & installation
Retro-fit curb installation
(905) 602-0760
1325 Fewster Dr  
Mississauga, ON  L4W 1A2
toplineroofing.ca
Think Safe,  
Work Safe,  
since 1988
T
he use of Reinforced 
Autoclaved 
Aerated 
Concrete (RAAC) has been 
a contentious topic in Ontario. In 
November 2023, the Ministry of 
Education issued a memorandum 
to the Directors of Education 
requesting all school boards 
View of Reinforced Autoclaved Aerated Concrete (RAAC).8

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