R-Values for flat roof insulation?

So possible flat roof coming up and I want to do things as proper as I can. One question that came up was if there was a code mandating flat roofs to meet a certain R-Value. I live in Texas, specifically climate zone 3. I’ve heard numbers as high as 20 R value. Is this a requirement or a recommendation? Also if it helps answer my question my city still uses 2015 IECC as well as 2012 IBC. Though I couldn’t find an exact number in either codebooks, maybe I’m not looking hard enough, or maybe not looking in the right place? What organization mandates this if any? Are there any consequences for not following it besides a slightly higher energy bill?

The reason I ask is because I see a lot of roofers just slapping 3" polyiso down and calling it a day but all these things I’m reading online it seems like it would take atleast 4 inches of polyiso.

Also I know the numbers seem to change so I would like to use onto what’s required if any, not more. Also its a roof replacement, not new construction.

Any help to guide me in the right direction would be greatly appreciated.


You’re going to use at least 2 offset layers to eliminate thermal breaks.


That is exactly what I needed, thank you so much for the reference. :smiley:

What roof system are you installing?

GAF Everguard 60mil TPO With 5inches of polyiso to meet the r-value requirement.

edit: mechanically attached