Attic is an unvented box

[Soparklion]

I recently purchased a two story home with a pitched slate roof.

http://photos2.zillow.com/p_d/ISpps0wvzbdowo0000000000.jpg

There is NO soffit or other ventilation to the attic.

How should I ventilate it? I plan to place a gable vent on one end with a gable fan in the other end, but I have no idea how to size them. I've heard 1 cu ft/min per 150 sqft of roof space, which in my case is ~600 sq ft. How do I size the passive gable vent? Given that it is a slate roof, I don't plan to poke holes and add roof vents.

All replies appreciated.
Brian

 

[nealtw]

Are you sure you haven't got drip edge vents just above the gutters for intake and some tile roofs have built in ridgevent at the top.

 

[Jungle]

It's strange because in the city i grew up none of the houses have roof twirlers, while the small town most of the houses do.