I have used chain link fence top rail which comes in a single 10.5' length. The wall thickness is slightly thicker than the 18 ga. product sold as antenna mast.
If mounted on a chimney, you would need to assess the integrity of the masonry. I have installed the top rail on wall mount brackets with about 2' separation between the upper and lower brackets. The last install provided about 7' of mast above the roof peak. I have a tripod on order for an install here, I intend to use a full length of top rail and no guy wire.
As soon as you join two sections of mast, you would want to consider using guy wires.
http://www.homedepot.com/p/YARDGARD-...2#.UhumfH-S-nc (I see the reviews include other antenna applications also.)