Hi Jim: 

I would think that mounting the vertical on the tin roof, if you can get permition, would be better, especially if  you don't have sufficient backyard space to A. Able to place the antenna at least 25 feet from your building and B. be able to put down at least 20  25 foot radials, keeping in mind that the good amount of radials to reach should be 30 to 60. 
When I install my vertical next week, I'm placing it about 50 feet from my cottage and putting down 20 radials this year, 20 next spring and maybe 20 the year after that. 
I suggest you email Bencher and they should get back to you  in a couple of days. I know they got back to me in that time frame with the answers to my questions. You might want to read the vertical and radial notes on that site as well. 

73: 
Michael DE VO1AX 

Sent from my iPhone