Hello all and thanks for taking the time to read this. I have been scouring the internet for the answer to this question and all I have been getting accomplished is like I'm a tennis ball at a game of tennis. Here goes, I have a new addition going onto my house and it has a metal roof on it that is attached to purlins which are screwed down to my cathedral trusses. My question is can I spray Open cell spray foam directly to the interior side of the metal roof. No underlayment, how the contractor put it down. Thank you so much and please any advice is welcomed.

There is rarely a good excuse for open-cell foam.

Even if this was one of them, no one sprays corrugated steel roofs as moisture trapped below the roof could rust through. I fastened rigid foam below mine leaving the corrugations for proper ventilation.

Rigid foam is the standard for flat commercial roofs. Good enough for me.

"There is rarely a good excuse for open-cell foam".

Agree, or closed, or any synthetic foams.. Follow the mfg instructions not that it is always right but any good designer should know the difference otherwise find someone that does and pay them well.

I am putting up a cheap metal shop (carport type), do I understand that rigid foam is better then spray foam of any kind.