How do I handle mansard roofs? What are the challenges with a mansard roof?

Here is some general guidance for venting a mansard roof.

The best way to handle a mansard roof is with a ridge vent and continuous soffit vent system. Use as much as possible of continuous soffit vent and a ridge vent along the entire peak of the roof making sure that the upper portion of the mansard roof is at least 3/12 pitch. One of the challenges with the mansard roof is at the point where the roof pitch changes from the steep eave area to the shallow top. This elbow must be open so that there is a path for the air to move from the continuous soffit vents under the steep slope through the break into the upper shallow slope. In situations where there is not an air path through the break, The Edge Vent may be used at the elbow assuming that it can be installed so that the louvers will be horizontal and the vent can be installed properly to prevent leaking. Another option on mansard roofs is to use gable louvers -- if the roof line has gables or it may have gable vent areas designed into the peak of the roof, similar to dormers, except in line with the ridge. Under these cases it may be possible to vent the upper shallow slope portion through the gables and use undereave vents to ventilate the smaller steep slope areas. There is more information about tricky roofs such as mansard roofs inside Air Vent University on airvent.com.