[TowerTalk] questions about installing a mast

[k7wxw]

I live on a small corner lot in Portland and trying to improve my antenna situation. Given my lot layout, powerlines and neighbors, a tower or a guyed mast is not possible so I am considering an unguyed mast to serve as the centerpoint for a dipole. The mast would also host  2M/70CM and GMRS verticals, one under the other. 

Here's the idea: a 50' Rohn telescopic mast on a Rohn base plate attached with wall mounts to my 34' brick chimney.  Can I do this without guying the upper 16' of the mast? If I have to guy the mast, can I do so with two guy points on the roof, assuming I can get enough of an angle?  I am not sure how to figure this out, not having done it before.

Thanks for any help and 73 de bill