Fixed Cone Roof Slope [API-650 Tank]

Dear All,

i have designed cone roof tank having slope of 1:12[30m ID]. But our client insisting to design with 1:6 slope referring to Bob Long book. Is there any limitations for roof slope. Is the slope of 1: 12 acceptable or not. Has any one did with roof slope of 1:12 [ Internal rafter/External Rafter]. Whether slope 1:12 will affect the roof design.