Can a roof with a ridge vent to have two shingle layers?

[David S.]

Hey all, I’m looking to put an offer on a home that’s part of an estate. The seller has no idea if there’s one or two layers on the roof and I couldn’t tell when walking the property (no ladder). But the house has a ridge vent. Does this make it far more likely it’s one layer, or do some homes with ridge vents still have two layers of shingles? 
Thanks!

[Nathan Grabau]

I am a roofing contractor in Colorado. It should be pretty obvious if a roof has a second layer. I would not consider ridge vent to be that big of a sign either way because a typical residential roofing company, especially one doing a second shingle layer on a house, probably is not looking at the details enough to know what a vent roofing even does. If you do not need to replace the roof though, it venting or not venting and a second layer would not have a significant impact on my analysis of a property. (on a house at least) 

If you google pictures of roofs with multiple layers, you will feel better about it because it is not easy to hide.