Roof ventilation to prevent problematic icings at eaves.

Attic ventilation 1/150 and 1/300 rules of thumb were established to avoid problems from indoor moisture. In cold regions another strong reason to ventilate roofs that slope to cold eaves is to prevent the formation of problematic icicles and ice dams. Building heat, not the sun, is responsible for the large icings that cause such problems, and roof ventilation is a direct and effective way of solving them. The authors have instrumented buildings to determine attic ventilation needs to minimize icings and have developed design guidelines for natural and mechanical ventilation systems. These guidelines have been applied to roofs with cathedral ceilings, two of which are discussed in detail. Head losses in narrow airways complicate the ventilation of cathedral ceilings. It is particularly difficult to ventilate the valley areas of such roofs. However, as these case studies indicate, ventilation systems can be developed to eliminate problematic icings on complex roofs. Details often determine the success or failure of ventilation systems.