How to properly ventilate a Mansard Roof?
I own a mansard and we are battling this issue.  We had our roof replaced last year and the roofer that did it didn't even install 1/10th of the recommended ventilation.  Last winter was brutal in the north and we sustained substantial water damage from condensation inside of our home.  We are working with another roofer to get the proper edge venting installed.  Think of a cold glass on a hot summer day, that is what the inside of my roof decking looked like last winter.  

Do not skimp or you will be sorry and sustain a substantially decreased lifespan of your roof, not to mention you could incur damage to the interior of your home if you live in the north (hot inside of the roof + extreme cold outside = large amounts of condensation).  

Make sure you have a proper vapor barrier as well.  Loose fill insulation = more required ventilation.  I would also consider a power vent with both thermostat and a humidistat.  I am not a professional, but I am a Mechanical Engineer by education and since our issue, I have researched proper ventilation of mansards extensively.  Go with a very reputable roofer that will stand behind their work if you have condensation issues.