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It is impossible to completely prevent the passage of water vapor through the ceiling and into the roof structure. Therefore, it is important to try to remove the vapor and deal with the condensation. This is done by providing an unobstructed vent area of not less than 1/300 of the insulated ceiling area.

Locate intake vents/screens at the soffit openings, making sure that the ventilation area is evenly distributed and as high as possible at the ridge. This method takes advantage of the chimney stack effect in promoting good air movement.

Ventilation at the soffit and ridge >>>

Ice damming is the build-up of ice at the edge of the roof, caused by heat escaping from the house and melting snow. This melted water flows down underneath the snow covering the roof. As it reaches the outer edge of the eave over the unheated soffit, it freezes, creating an ice dam. The ice dam stops the flow of melted water from reaching the eave. Ice then builds up in the form of icicles and slabs, and the melted water can back up through the shingles and leak into the attic (see drawing). Ice build-up also causes structural damage to the eave and the roof and can be a potential hazard to passers-by.

Water leakage caused by ice dams >>>

• If the roof is insulated between the rafters, don't forget the ventilation gap above the rafters, which is created by the contra-batten, and sufficient intakes at the eaves.
  
   
• With a cold roof system, keep the attic space cold by insulating it from the warm house interior. This will reduce or eliminate snow melting.
  
   
• In case there are no snow stoppers mounted on the roof and the snow slides freely and falls from the roof, ensure that the outer edge of the gutters is lower than the slope line to allow snow and ice to slide clear (see drawing). It must always be safe, to avoid danger for passers-by.

Place gutters below the slope line to allow snow and ice to glide clear. >>>