Again, Thank you for all the replies and sorry for the long posts!!
To answer a few questions:
The flat roof area receives no foot traffic to speak of, other then myself occasionally getting up there to clear the gutters.
This is in Southern California (which I probably should have stated) So the weather is very moderate. No snow, ice or hail. Generally we have a few months of rain in a good year.
As far as longevity, Here is what I’m trying to reason with…
I understand that adding a 2nd (or middle ply) will increase the lifespan of the roofing system and serve to act as a secondary barrier in the event the top layer fails, but here is where I question the cost/benefit factor.
For discussions sake:
If I get 10 years out of a single ply (base w/mineral surfaced top) and roughly 15 years out of going to a 2 ply (base, smooth, mineral surfaced top)
Wouldn’t it stand to reason though, that after 10 years if the top layer has reached the end of it’s serviceable lifespan and is starting to fail. I’d need to look into replacement, since the overall system is compromised?
Or is the longevity of the top layer extended by adding a middle layer?
If not, aren’t I more or less just buying some extra time. Delaying the inevitable replacement, but adding costs at both ends?
Basically $1500 more up front and XX more at tear off for an additional layer.
Again, I’m not against spending the money.
But if realistically after 10 years I’m going to need to look into replacement, regardless of whether I go with a 1 ply or 2 ply system I don’t see the sense.
Can anyone give me the pro’s and con’s of both options?