Reinforced Autoclaved Aerated Concrete roof planks (RAAC), were widely used in flat roof construction between the 1960’s to 1980’s.
Unfortunately, the methods employed to construct some of these planks resulted in a relatively weak structure. One with a low capacity for developing a bond with embedded reinforcement. Recent roof failures have identified rusting of this embedded reinforcement and cracking of the planks. Whilst not definitive, this cracking is believed to be associated with moisture and temperature-related movements in the planks.
Many of the earlier RAAC roof planks have been replaced or had spans shortened by the introduction of secondary supports. However, buildings constructed with RAAC planks in the 1980s are past their estimated 30-year life and pose a serious risk. In May 2019 the Standing Committee on Structural Safety (SCOSS) issued a formal alert;
Sight must not be lost of the fact that recent collapses have been sudden with very little noticeable warning. This is indicative of shear failure in cementitious materials and can only be protected against by knowingStanding Committee on Structural Safety, Alert | May 2019
that there is sufficient shear resistance in the material, the reinforcement, or both.
If there is any suspicion that RAAC planks are present in a building’s roof, SCOSS recommends the appointment of an appropriately experienced Chartered Building Surveyor. An NHS Foundation Trust did just that. We have recently surveyed nearly 30 NHS Foundation Trust sites to establish if there is a potential for RAAC planks to be present.
SCOSS warned of the problem in the Twelfth Report of SCOSS in 1999. Since then, there will have been deterioration, possibly effects from maintenance or refurbishment, or a change in environment, all of which can adversely affect long-term performance.Standing Committee on Structural Safety, Alert | May 2019
Please Contact Us if you own or manage a building with a flat roof from the 1980’s or earlier. Perhaps you have noticed one of the following warning signs;
- The roof has been resurfaced since its original construction. This is particularly an issue if the load has been increased or the new surface has a black finish
- There is significant ponding on the roof
- The roof is leaking or has leaked in the past