Ventilation of Low Slope Roof Systems in Northern Climates

Abstract

For over generations, multi residential (flats) buildings were constructed in North American with low slope roofs. The roof design included an interstial space of 1.5 to 2 meters between the ceiling and roof deck. This ‘attic’ space was typically vented via static air vents. The ceilings were typically constructed of plaster with little or no insulation. The roofing was typically asphalt or coal tar built up roofing applied directly to the roof deck. This simple ceiling to roof arrangement worked for centuries. In circa 1998, with the advent of mandated reflective roofs in several large North American cities, a simply workable design was changed, often with disastrous results. So that greater floor to ceiling heights could be attained, the 1.5 to 2 meter attic was reduced to .33 m or smaller. Plaster ceilings were changed to gypsum board. Fiberglass insulation with an ineffectual vapor barrier was installed at the ceiling, and often punctured by ‘can’ lights which allowed air transport of warm air, often containing moisture, into the attic cavity. With no attic ventilation, insulation on the roof deck, condensation accumulation on the attic structural elements to such a degree that, in many situations, resulted in the deterioration of the wood structural elements.This paper will review the importance of proper design with ‘modern day’ low slope roof systems with ceilings on the roof structure, and the role that venting, vapor retarders and roof color have on the success or failure of these roof systems. The author will review historical examples of recent failures and provide designers with recommendations for appropriate low slope residential roof system design.