Since such additional spaces may not be visible or accessible, the concealed void presents a particular hazard in firefighting operations, as a fire may develop or persist in such a space undetected.
Rain roofs are commonly found in North America over an older flat or minimally sloped roof that has developed leaks, or in a building that has been added to in a manner that requires revised drainage patterns.
[1] While rain roofs are commonly found in large, older commercial buildings, they may also be built over single and double-wide mobile homes to improve drainage.
A concealed space, potentially unknown to firefighters, and difficult or impossible to access, may allow a fire to grow or persist in spite of firefighting operations inside and outside the building.
[1] A rain roof was a significant hazard that contributed to the deaths of six New York City Fire Department firefighters in the 1978 Waldbaum's supermarket fire.