SDF-20SF (Self Flashed Base) is designed for sloped low profile roof installations such as composite shingle.  Minimum slope should be greater than 2:12 and maximum slope should be less than 6:12.  (Use a curb mount base if the slope is less or greater than these slope ratios.)

The self flashed base is designed to slide under an existing roof.  The based is attached to the roof decking with either roofing nails or screws.  Care should be taken to waterproof the based to the roof decking and roofing materials.  Use the highest quality roofing silicon.  Always determine the amount of run off water at the head of the base.  If the location of the fan is an area where there may be an excessive amount of run-off water, then use a curb-mount base.