[Greenbuilding] Ridge Vent on Corugated Metal Roof

[conservationarchitect at rockbridge.net]

Hi List

Fishing again for great ideas from our list.  I have a gable end roof framed 
with truss's at 4'oc with 2x4 purlins at 1'oc.  I am planning on installing 
Fabral Grandrib 3 corrugated prefinished metal roofing.  It is a simple roof 
with no hips or valleys.  The top chord overhangs the wall and are left 
exposed with a 2x fascia installed to keep them from twisting at the end. 
The truss has a vertical raised heel at the end sufficient for additional 
cellulose over the 8" eps foam that runs below the bottom chord.  I am also 
installing a continuous soffit vent installed vertically just below the top 
chord and above the cellulose.

My plan is to install a ridge vent.  I like the grand rib, because the 
profile of the ribs offers significant free space so that boient driven air 
flow has a vertical path to follow to the ridge.  One contractor describe 
his ridge vent using a ridge cap over foam rubber compressible material that 
can work around the shape of the ridges.  My concern is that this foam 
rubber may reduce the free space I would like for encouraging the air-vapor 
flow.

I have thoughts of adding blocking and a screen to lift up the ridge cap 
(standard or fabricated) to greatly increase the free space.  The roof pitch 
is 9/12.  Therefore, I would expect a decent amount of overlap (4"-6"?)over 
the top edge of the corrugated roof will protect  the roof from wind driven 
rain driven water.  I could also get my installer to turn the top edge up to 
block flow going up hill.

I hope people on our list are willing to share your ideas for ridge vents 
over corrugated metal roofs.  I am sure they are out there.

Thanks in advance.

Eli