[TowerTalk] effect of large metal roof

[Gregg Seidl]

I'm sure this has come up before but I don't recall reading it.I want to put 
up the new Bencher Skylark antenna for 12-17 meters and a lower 10 meter 
antenna when the band starts to perk up again.The trouble is all my towers 
have enough on them already and don't want to push my luck.
    I do have a large metal pole shed that I could put an extra 9 foot Glen 
Martin roof tower on.The antenna would be about 10-12 feet above the METAL 
roof.The antenna would be about 30 feet above ground level.Not great but 
better then nothing.
    How much interaction would the large metal roof have on the antenna?The 
shed is roughly 36 by 90.Would a vertical be a better choice,then tie the 
roof into the ground plane?
    Looking for 12,17 and 10 meter coverage.15 would be a great bonus. 
Gregg K9KL