[Grumpy]

If it is a cold deck design, meaning that the ceiling is insulated and the roof is not, simple install http://www.commercialproductsgroup.com/Libraries/Product_Literature/VENTS_and_BREATHERS.sflb.ashx a two way breather.

If it is a warm deck design do not install a 2 way breather. And do not install ridge vent on roofs less than 3/12 in pitch.

 

[Grumpy]

The smaller size are for venting lay overs, the larger are for venting "Attic" space. They work just fine when you install adequate ammount. Just like any ventilation, do the math first.

 

[Grumpy]

I've read specs saying on lay-overs to use 1 of the smaller 1 way breathers per 10 squares. That's a modofied bitumen spec though, which honestly never made sense to me since mod bit should be fully adhered, but what ever.

I know this is not scientific, but generally speaking on a 1,000 sq ft foot print, I would see 2 of the larger 6" 2 way vents. That doesn't mean it's right or wrong, just that is what is common practice (from the roofers that actually ventilate) in my area.

Contact the vent manufacturer and see if they have a spec sheet on their products with NFA, and contact the single ply manufacturers you work with and ask if they have a NFA recomendation.