Need Advice on Vent Stack

[tmy23]

Greetings and thanks in advance for all help!

I am adding a master bath to my home. All the services are in, regarding plumbing, everything is connected to the waste line, I only have to run the roof vent and install the appliances.
My house is an older home with a Mansard roof, i.e the roof has two pitches, a very steep (probably 80%) side roof and relatively flat (10-15%) top roof. the area under the flat roof is living area, so the vent stack is in the void between living area and the steep roof section.
I don't want to go straight up through the steep section with the vent pipe because the roof is so steep, to get a 2" pipe through I'll end up with a foot long elongated hole, and no vent boot will cover that. I was planning on penetrating adding a 45, penetrating the roof at than angle and then another 45 to get vertical on the outside. Is there any problem with this? I probably will have to install the boot upside down because of the angle, right now I have a generic boot bought at HD...do you have any recommendations for something better?
thank you very much...drawing attached...

 

[Reach4]

Check Lead Roof Boot Flashing

Lead will conform nicely to whatever you want. You fold the top of the lead into the vent pipe, so I don't know what you would do if your vent pipe was too long for that.