Hello, new of the forum, thanks for

I'm assuming this is a pressurized roof?
Increase the roof steepness if practical. Note that if you go over 2:12, it affects rafter design.
Increase the top angle size if required.
Switch from top angle to one of the details with a sloped flat bar/plate section.

Stephen, thanks.
I cannot increase slope I am already at 18 deg against specified 9deg which I had to fight for. Customer wont allow it.
Minimum top angle to reach 4,000mm2 is 150x150x15, which, to me, looks stupid attached to an 8mm shell.
I can try and change detail type, but I have a rigid customer who is not willing to change as there are other old tanks there (probably not to API650) and he gets upset when I propose something that doesn't fit.
I was hoping someone in this forum would suggest I was making some stupid mistake I'd have thought that using atop angle one size higher than min recommended would have given me no probs.
Just to make sure,the area is simply calculated as it should? i.e. Wh*th + Wc*tc plus the top angle area (with Le limited by Fy of material)? I am not missing on some coefficient, maybe, I don't know, just to put a number (randomly...) 4?????

Oh, and yes, sorry, external pressure (vacum) is 0.25kPa, but I added DL, S and Lr (so it comes to 0.78+1+0.4*0.25 (S is 0.5, so less than Lr).